Social sustainability tensions in multi-tier supply chain A systematic literature review towards conceptual framework development

As many unethical practices and violations of social norms originate from higher tier suppliers and their sub-suppliers, it is of paramount importance to ensure social sustainability in the entire supply chain, especially for higher tier suppliers. This has led to a considerable increase in interest among the re- searchers and academicians working in the domain of social sustainability. This article reviews 129 research papers published in different journals and attempted to identify drivers, issues, barriers, ten- sions, practices, and performances related to social sustainability in multi-tier supply chains. The prevalent theories and governance mechanisms of social sustainability in multi-tier supply chains have also been presented. The commonly used theories are stakeholder theory, institutional theory, resource based view theory, transaction cost economics theory, grounded theory, and resource dependence theory. It is observed that most of the papers have presented drivers, barriers, issues, and practices in generic ways by conducting exploratory case studies using single case or multiple cases; there is a distinct lack of multi-tier perspectives devoted to the context of emerging economies. The tension, an important aspect of implementation of social sustainability practices, has not been addressed adequately in the extant literature for multi-tier supply chains. This review also proposes a conceptual framework of social sustainability linking drivers, issues, barriers, tensions, practices, and performances. Finally, future avenues of research on social sustainability have been outlined.


Introduction
Environmental, social, and economic dimensions are the three important pillars of sustainable supply chains (Carter and Rogers, 2008;Govindan et al., 2020b). Bruntland (1987) defined sustainability as "development that conforms to the demands of today without compromising the ability to help the future generations." It is argued that environmental and social practices and economic performances are closely interrelated and can be pursued simultaneously to achieve true sustainability (Elkington, 1997;Carter and Rogers, 2008). Hassini et al. (2012) defined sustainable supply chain management as "[…] the management of supply chain operations, resources, information, and funds in order to maximise the supply chain profitability while minimising the environmental impacts and maximising the social well-being." The practices of environmental sustainability include green products and process design (Rusinko, 2007;Heydari et al., 2020); the use of eco-friendly materials and minimisation of pollution, bearing of greater environmental responsibility (Klassen, 2001); adoption of green procurement strategies (Varn€ as et al., 2009); circular procurement ; reverse logistics (Varma et al., 2006); minimisation of carbon footprint through energy efficient logistics, and the use of cross docking and green packaging (Ji et al., 2014), among other approaches. Social sustainability overlaps with corporate social responsibility (CSR) and addresses the well-being of human beings and society through the management of social resources (Sarkis et al., 2010;Panigrahi et al., 2019;Govindan et al., 2021); the alleviation of poverty, respecting justice, human rights, and employee welfare (Krause et al., 2009); and transparency of work (Kumar and Rahman, 2017;Govindan et al., 2019). Carroll (1991) proposed a pyramid of CSR in the form of a hierarchy of economic, legal, ethical, and other voluntary activities and responsibilities to help stakeholders and society in which the organisation is operational. The term "social responsibility" was first used by Howard Bowen in 1953. According to him, it is referred to as "the obligations of businessmen to pursue those policies, to make those decisions, or to follow those lines of action, which are desirable in terms of the objectives and values of our society" (Carroll, 1999).
In the present scenario, the supply chains are very complex and consist of multiple tiers of suppliers spreading across the world (Mena et al., 2013). It has been realised that management of sustainability at different tiers is crucial to achieve sustainability throughout the entire supply chain. There has been little initiative to implement sustainability and CSR activities by multi-tier suppliers. It has been found that the serious violations of environmental and social practices often happen at tier-2 suppliers or further upstream and their sub-suppliers (Meinlschmidt and Schleper, 2018). Several instances of unsustainable behaviours, violations of environmental or labour laws by suppliers or subsuppliers of multi-national firms have been reported (Seuring and Müller, 2008;Wolf, 2014), leading to reputational damage and financial loss to major brands (Seuring and Müller, 2008) and consumers attribute responsibility to the focal firm for such behaviours (Hartmann and Moeller, 2014). In 2013, the collapse of Rana Plaza building in Bangladesh, which killed more than a thousand workers, received international attention and raised serious concerns about lapses from the suppliers' side to maintain safe working conditions for the workers (Huq et al., 2014).
It has been noted by many researchers that social sustainability has not gained as much attention as environmental sustainability (Zorzini et al., 2015;Yawar and Seuring, 2017;Silva et al., 2019). Huq et al. (2014) found that most extant works on social sustainability are from developed countries, and that the situation is even more challenging in developing countries. Managers give more priority to environmental and economic sustainability compared to social sustainability (Kusi-Sarponga et al., 2019). Besides, there is a dearth of research and conceptual frameworks considering sustainability in multi-tier supply chains (Tachizawa and Wong, 2014;Nakamba et al., 2017;K€ oksal et al., 2017;Abbasi, 2017;Mani et al., 2016a). Mena et al. (2013) studied the structure and dynamics of multitier supply chain management (SCM). Tachizawa and Wong (2014) examined existing literature and theories of multi-tier SCM and provided a framework for studying sustainability in these multitier supply chains. Their studies primarily focused on environmental sustainability and did not cover social sustainability. Zorzini et al. (2015) studied social issues in sourcing and pointed that there is a need for research from the suppliers' and multi-stakeholders' perspectives. Abbasi (2017) reviewed the major themes and challenges for socially sustainable supply chains and argued that understanding them would help business practitioners to design a socially sustainable supply chain. K€ oksal et al. (2017) completed an extensive literature review on social sustainability in the textile and apparel industry and developed a conceptual framework to manage social risk. They identified enablers, drivers, and barriers, integrating focal firms and multi-tier suppliers, and their work emphasised the need to further investigate lower tier suppliers.
The current body of supply chain literature demonstrates little attention to the terms of sustainability in multi-tier supply chains; therefore, further research in needed to better understand problems that arise in emerging economies. More specifically, fruitful research needs to incorporate a true focus on the triple bottom line is multi-tier supply chains in diversified sectors (Jabbour et al., 2018). It is agreed that sustainability should be a part of business decisions but its implementation is hindered by barriers that lead to multiple tensions among the actors in the supply chain. The tensions are overcome by practices and strategies through which drivers help organisations to adopt practices that can achieve sustainability goals. As mentioned above, there is a lack of conceptual frameworks and research in social sustainability pertaining to multi-tier supply chains; therefore, an in-depth enquiry in this area is needed. To bridge this gap, a conceptual framework that addresses the following research questions has been proposed incorporating drivers, barriers, tensions, issues, practices, and performances in a multi-tier supply chain.
This work addresses the following research questions to the supply chain fraternity. RQ1: What are the drivers and barriers of social sustainability in multi-tier supply chains? RQ2: What are the social issues in multi-tier supply chains? RQ3: What are the sustainability tensions and social sustainability practices in multi-tier supply chains? RQ4: How do the social sustainability practices impact performance of multi-tier supply chains?
The rest of the paper is arranged as follows. Section 2 provides a review of the literature available in the domain of social sustainability. Section 3 presents the methodology adopted for the systematic literature review. Section 4 outlines the descriptive statistics of research papers. Section 5 introduces social sustainability in multi-tier supply chains. Section 6 presents drivers, issues, barriers, and tensions of social sustainability from a multi-tier perspective and then proposes a conceptual framework linking the aforesaid elements. Section 7 presents some research questions and section 8 targets the future scope of research. Finally, section 9 presents the conclusions.

Review of literature reviews
In this section, an attempt has been made to analyse the existing literature reviews on social sustainability and to figure out the areas that have not been given due attention. This process is essential for content analysis and framework development (Kannan and Hasanagic, 2018).
From the existing literature reviews, it is observed that review papers have been written focusing on social issues (Zorzini et al., 2015), emerging themes, challenges associated with implementation, and assessment of social sustainability (Abbasi, 2017;Nakamba et al., 2017;Sodhi and Tang, 2017). Galuppo et al. (2014) reviewed social sustainability based on multi-stakeholder theory. They suggested that the implementation of social sustainability in any organisation requires participation of all stakeholders. However, the reality of all stakeholders having different priorities, and the fact that sometimes those priorities are by nature opposed, leads to conflicts. Zorzini et al. (2015) reviewed literature on socially responsible sourcing. Their findings suggest that more empirical research and the development of quantitative performance indicators can be used to study financial implications and benefits. Bubicz et al. (2019) reported current trends and identified research gaps on different dimensions of social sustainability in supply chains. According to them, people and product are the key points. People aspects are human rights, decent working conditions, and community engagement, and the product aspect is safety of the consumers from the product use. Gimenez and Tachizawa (2012) identified enablers to make socially responsible supply chains; they proposed a framework that the enablers mediated by governance practice can affect sustainability performances. They argued that assessment of suppliers and collaborations with them are needed to drive sustainability practices and improve performances. K€ oksal et al. (2017) developed a conceptual framework consisting of major actors and associated drivers, barriers, and enablers for the management of social risks after conducting a state-of-the-art literature review pertaining to fashion supply chain. Through the observation that the measuring of social sustainability is not as easy as environmental and economic sustainability, a number of review papers deliberated on developing frameworks for social sustainability measurement Stani skien e and Stankevi ci ut e, 2018;D'Eusanio et al., 2019). D'Eusanio et al. (2019) reviewed tools and methods available to measure social sustainability aspects and performances. It is emphasised that social sustainability should be a part of business decision strategy along with economic and environmental sustainability . It is believed that international certification such as SA8000 would help to mitigate social risks and have been made compulsory for the suppliers. It is sometimes misused by the suppliers as such practices don't help workers (K€ oksal et al., 2017). Suppliers are pressured to lower costs to secure orders and simultaneously practice social management practices. This creates tensions between buyers and the suppliers. The tension is inevitable as conflicts occur among the goals and constraints of buyers and suppliers. Existing literature reviews illustrate the need for an integrated model or conceptual framework that establishes links among different aspects of social sustainability. This gap is addressed in the conceptual model proposed by linking drivers, barriers, issues, tensions, practices, and performance. The summary of the previous literature reviews on social sustainability is given in Table 1.
Although existing reviews on socially sustainable supply chains have covered various aspects including employee perspective, governance mechanism, stakeholder's theory, themes, challenges and measurement, there is a lacuna in terms of a framework relating drivers, barriers, tensions, practices, and performance of socially sustainable supply chain.

Methodology
A systematic literature review methodology has been chosen to discern answers to the aforesaid research gap as this method helps researchers to understand length, breadth, and depth of a particular research area and to synthesise information in unbiased manner by collecting literature from available bibliographic databases (Jaegler et al., 2017;Tranfield et al., 2003;Denyer and Tranfield, 2009). In this paper, we seek to identify drivers, barriers, issues, tensions, and practices pertaining to implementation of social sustainability in multi-tier supply chains and consequent performance.
In the first stage, relevant papers were searched using the library service of EBSCO (ebsco.com), which covers electronic databases of popular publishers, namely Elsevier, Wiley, Springer, Emerald, and Sage. The following search strings were used: "Social Sustainability" OR "Corporate Social Responsibility" OR "CSR" AND "Supply Chain*" AND "Multi-tier" OR "Sub-supplier" OR "Sustainability Tensions". The suffix "*" was used to ensure wider coverage of the literature while searching the databases. The EBSCO produced a list of 4742 articles based on the presence of the search strings in title and abstract of papers. The articles were further filtered by selecting language and year range of publication. English language and articles published from 1989 to 2019 were selected. That filtering resulted in a total of 3487 peer-reviewed articles. The subject filter was then applied to find out articles related to this research. Following subjects were used for the filtering purpose: social sustainability, sustainable development, sustainability, supply chain management, sustainable development e analysis, social responsibility of business, supply chains, corporate social responsibility, economic sustainability, indicators, social capital, governance, social impact assessment, framework, assessment, Table 1 Summary of previous literature reviews.

Title
Authors Summary "Social sustainability and supply chain management: Methods and Tools" D'Eusanio et al.
Reviewed existing tools and methods to assess social impacts or performances of socially responsible supply chain actors in order to achieve competitive advantages. "Incorporating social aspects in sustainable supply chains: Trends and future directions" Bubicz et al. (2019) Reviewed literature on social sustainability dimensions in supply chain management research and identified trends and research gaps. "Social sustainability measurement framework: The case of employee perspective in a CSR-committed organisation" Stani skien e and Stankevi ci ut e Reviewed literature on measurement of social sustainability and developed framework for the same from the employee perspective.
"Conceptualizing and incorporating social sustainability in the business world" Ajmal et al. (2018) Reviewed frameworks for social sustainability indicators from the societal and company perspectives. Findings suggest that social, economic, and environmental sustainability can be practiced simultaneously. "Sustainable supply chain management: A review of literature and implications for future research" Panigrahi et al.
Covered different aspects of sustainable supply chain and indicated the need for more research in the area of social sustainability. "Social sustainable supply chain management in the textile and apparel industry d a literature review" K€ oksal et al. (2017) Reviewed social sustainability in sustainable supply chain in textile/apparel sector as field of application. A conceptual framework related to drivers, barriers, enablers for social risk management was proposed. "Towards Socially sustainable supply chains e Themes and challenges" Abbasi (2017) Reviewed literature on social sustainability and reported emerging themes and challenges. "Corporate social sustainability in supply chains: a thematic analysis of the literature" Sodhi and Tang (2017) Identified eight high-level themes for social sustainability. Proposed 4P model, which implies pressure and partnership influence practice, and, in turn, performance. "How does social sustainability feature in studies of supply chain management? A review and research agenda" Nakamba et al.
Identified key themes pertaining to social sustainability which are important for managers for measuring social performance in practice. "Corporate social responsibility for supply chain management: A literature review and bibliometric analysis" Feng et al. (2017) Reviewed corporate social responsibility in supply chain management.
"Socially responsible sourcing: reviewing the literature and its use of theory" Zorzini et al.
Exclusively focussed on issues in socially responsible sourcing in upstream suppliers.
"Building social sustainability: multi-stakeholder processes and conflict management" Reviewed social sustainability focusing on stakeholder's theory and found that building a socially sustainable organisation requires involvement of multi-stakeholders which often creates conflict among them. "Extending sustainability to suppliers: A systematic literature review" Gimenez and Tachizawa (2012) Reviewed literature on governance structure used to extend sustainability to suppliers. Findings suggest that assessment and collaboration have positive influence on sustainability practices.
performance, social impact, sustainable supply chain management, and socioeconomics. This step resulted in 2180 articles. The articles were further screened to eliminate duplicate results and irrelevant papers. Then, an abstract analysis was performed, and 242 relevant articles were shortlisted. Finally, full papers were analysed to select papers which were written in a multi-tier perspective and linked with any one of the research questions. Finally, 85 articles were selected for analysis. The selection process of articles is summarised in Table 2. Scopus is a widely used bibliographic database; hence, it was used to figure out useful literature. The same keywords were used as in step 1. It resulted in 361 papers from Scopus database. After reading the abstracts and eliminating common papers, 44 relevant papers were identified and selected for full-text analysis.

Descriptive statistics
In this section, the selected papers are examined in detail in terms of year of publication, methodology used, geographic area, industry contexts and coverage in terms of focal company or multitier suppliers.

Distribution of research papers by methodology
When the papers are classified based on research methodology, the distribution according to research methodology are: case study (29%), analytical (25%), literature review (19%), theoretical (10%), survey (10%), interview (5%), and content analysis (2%). The distribution of papers according to research methodology is depicted in Fig. 2. In case study-based approaches, researchers selected cases from either single or multiple firms belonging to the same or different sectors. Semi-structured questions and publicly available secondary data were used to carry out case studies by the researchers. A number of quantitative techniques were used to study various aspects of social sustainability. These techniques include Analytic Hierarchy Process (AHP), fuzzy AHP, hybrid multi-criteria decision making (MCDM), fuzzy Technique for Order Preference by Similarity to Ideal Solutions (TOPSIS), VlseKriterijumska Optimizacija I Kompromisno Resenje (VIKOR), best-worst method, interpretive structural modelling (ISM), decision making trial and evaluation laboratory (DEMATEL), confirmatory factor analysis, regression analysis, structure equation modelling (SEM), etc. A few research papers incorporate mathematical modelling such as game theory, multi-objective optimisation, and so forth. The significant proportion of literature review (19%) in the dataset indicates that the body of knowledge of social sustainability is still emerging and being consolidated by focusing on certain aspects. The papers that categorically specified survey and interview as research methods were included in survey and interview categories, respectively.

Research coverage, industry settings, and geographic contexts
Articles have been classified based on the organisation type, namely focal firm, suppliers, and multi-tier network. Around 46% papers covered focal companies and their suppliers, whereas 24% and 13% covered tier-1 suppliers and multi-tier suppliers, respectively. However, 11% papers did not mention the firm type. Around 50% and 43% of the research papers covered social sustainability and social-environmental sustainability, respectively. Moreover, the issues of social sustainability are more prevalent in manufacturing sectors, largely because they employ a significant number of workers. Therefore, researchers have selected manufacturing sectors, including automobile, chemical, dairy, electronics, food, consumer products, textiles and clothing, etc. for studying various facets of social sustainability. Besides, a sizeable portion of the research papers have originated from China, India, Bangladesh, and Brazil which are manufacturing hubs for lean production. However, a few papers covered social sustainability aspects in developed countries such as the USA, UK, Germany, and Australia. Table 3 shows the number of paper(s) published in a particular journal. The maximum number of publications is reported in "Journal of Cleaner Production" followed by "International Journal of Production Economics," "Journal of Business Ethics," and "Sustainability." From Table 3, it is evident that this area is popular among the researchers of diverse fields.

Theories used
Given the complex nature and the novelty of social sustainability research areas, researchers utilise a number of theories to explain drivers, barriers, governance structures, and performance of multi-tier supply chains. Fig. 3 shows the spectrum and frequency of theories used in this paper. The commonly used theories Table 2 Article selection process for this review paper.
Step Details No. of articles Step 1: Keywords search Keywords used in search space of title and abstract were "Social sustainability" OR "Corporate Social Responsibility" OR "CSR" AND "Supply Chain*" AND "Multi-tier" OR "Sub-supplier" OR "Sustainability Tensions"

4742
Step 2: 1 st stage of filtering of articles are stakeholder theory, transaction cost economics theory, institutional theory, grounded theory, agency theory, paradox theory, resource-based view theory, and resource dependence theory; however, researchers have used other theories as well. Because there is lack of established theories for social sustainability, it should be noted that some researchers posit descriptive insight from their research without giving reference to any theory (Huq et al., 2014;Zorzini et al., 2015). Zorzini et al. (2015) categorically reviewed literature on sourcing and considered social aspects. They critically evaluated socially responsible practices through theoretical lenses. The survival and prosperity of any company depends on direct and indirect influence of stakeholders since they play multifaceted roles. Accordingly, the stakeholders approach has been widely used to assess the social performance of a company. Stakeholder theory is popular because of its theoretical rationality and empirical applicability (Chi, 2011). The relevance of stakeholders has been recognized by many definitions of social sustainability (Egle Staniskiene and Zivile Stankeviciute, 2018). The constructs of transaction of economics fully or partially support the observations on social sustainability in ready-made garment manufacturing sectors of Bangladesh considering buyer-supplier relationships (Huq et al., 2014). Awan et al. (2018) used transaction costs of economics and proposed that cultural intelligence could improve the execution of contracts and could reduce coordination and information exchange costs. Shen et al. (2015) used resource-based theory to identify barriers of CSR. Huq and Stevenson (2018) used institutional theory to demonstrate how horizontal collaboration among the buyers compel suppliers to follow social sustainability practices. Resource dependence theory has been used to study workplace safety compliance at garment factories; meeting rules and regulations of safety compliance has financial implications and is often linked with collaborative longterm partnerships (Akbar and Ahsan, 2019). Stakeholder theory, institutional theory, transaction cost economics theory, and resource-based view theories have been used to explain drivers and barriers of social sustainability. It is noted that a single theory may not be sufficient; hence, researchers commonly employ more than one theory to comprehensively explain their findings and to develop a suitable model (Pagell and Wu, 2009;Zorzini et al., 2015).  used resource-based view, stakeholder resourcebased view, resource dependence theory, and institutional theory to study how supply chain performance is enhanced through a  supplier's social sustainability practices and through collaborations between buyer and suppliers. The agency theory has been used to describe buyer-supplier relationships where buyers use their control to monitor suppliers as agents. On many occasions, tier-1 suppliers act as an agent of buyers to ensure sustainability in tier-2 and higher suppliers (Wilhelm et al., 2016a). In contrast, however, as an alternative to agency theory, the stewardship theory advocates that suppliers are self-motivated and work autonomously as per the contract with buyer for the common goal of the business (Ablander et al., 2016). Multi-tier supply chain sustainability is a challenging task. It is argued that this task can be handled more effectively with supply chain leadership than with supply chain power. As a result, multiple leadership theories have been posited for supply chain leadership. The combined effect of supply chain leadership and governance help multi-tier suppliers to orient to supply chain learning for sustainability (Jia et al., 2019). Four modes of multi-tier supply chain have been identified by Tachizawa and Wong (2014). Sustainability tension is another important aspects of implementation of sustainability in the supply chain. Paradox theory has been widely used to address tensions arising by pressure from the stakeholders to implement sustainability (Bommel, 2018;Brix-asala et al., 2018;Fang et al., 2011).

Social sustainability in supply chain
As stated earlier, social sustainability is one of the pillars of the triple bottom line (TBL) (Kannan, 2018;Rashidi et al., 2020). There are a number of social issues reported in the literature of supply chain management, including violations of human rights and labour rights, child labour, forced labour, discrimination, forced overtime, low wages, poor health and safety, sexual harassment, and the safety of female workers to name a few of the most common social topics. Broadly, social sustainability addresses three points: well-being of human beings, society, and safety of consumers. The human aspect is comprised of skill development (Sarkis et al., 2010); alleviation of poverty and narrowing inequality (Vallance et al., 2011); and respecting human rights, health and safety, welfare, non-discrimination and legitimate wages (Mani et al., 2015;Marshall et al., 2016). The society aspect deals with social values (Sarkis et al., 2010); preserving culture (Vallance et al., 2011); and local community engagement, philanthropy, charity, and hiring local people (Mani et al., 2015). The safety of consumers is also an important part of social sustainbility (Klassen and Vereecke, 2012). Product failure, which may happen due to inadequate testing and inspection at the time of production, may put the safety of consumers at risk. As a consequence, businesses may lose customers and incur huge litigation cost. It goes without saying that the fulcrum of social sustainability is people. Trust and common meaning of social sustainability are the bridging links between the employees and their employers (Missimer and Rob, 2017). Mani et al. (2015) emphasised that housing and living conditions of the workers should be considered a business concern. Eizenberg and Jabareen (2017) proposed a conceptual framework of social sustainability consisting of four broad components such as: equity, safety, eco-presumption, and urban forms. Under this framework, they tried to propose a holistic way to ensure the well-being of people, the planet, responsible production and consumption, and to establish emotional connections among people. Zorzini et al. (2015) presented a list of terminology linked to social aspects and sourcing. The summary of important sources of social sustainability is given in Table 4.

Content analysis
Content analysis method has been used to collate and synthesise information in order to derive valid insights (Krippendorff, 2004). It is advisable to involve more than one researcher in this process for better understanding of circumstances, periods, contexts, and for ensuring reliable results (Kannan and Hasanagic, 2018;Seuring and Gold, 2012). In this review paper, the contents of the selected articles have been analysed considering drivers, barriers, issues, tensions, practices, and performance of multi-tier supply chains in relation to adoption of social sustainability. The same elements have been described in subsequent paragraphs in order to develop the conceptual framework.

Drivers of social sustainability in multi-tier supply chain
Drivers or enablers are factors which help an organisation to achieve sustainability goals (Panigrahi et al., 2019). Extensive literature resources are available on drivers for the implementation of sustainability from the perspective of focal firms. The important drivers for adoption of social sustainability, reported in the literature, are pressure from stakeholders, commitment of top management within the focal firm, middle management pressure, government pressure, collaboration with suppliers, owner's or manager's value, and the competitive advantages of being sustainable (Ehrgott et al., 2011;Goworek, 2011;Meixell and Luoma, 2015;Walker and Jones, 2012). Kumar and Rahman (2017) studied relationships among the enabling factors of sustainable supply chain management. Many researchers have reported the role of stakeholders on adoption of social sustainability. Pressure from stakeholders is a powerful driver as it may make business organisations aware of sustainability concepts that would lead to the adoption of such practices (Meixell and Luoma, 2015). Government pressure is also an important driver. Some researchers opined that government pressures do not significantly influence social sustainability adoption (Ehrgott et al., 2011); whereas others believed that more government pressure, along with consistent community and consumer pressure, drive implementation of social sustainability (Golicic et al., 2019). Sancha et al. (2015) found that the mimetic pressure positively influences the adoption of sustainable practices. Mani et al. (2015) revealed that pressure from stakeholders such as employee unions, customers, and competitors are powerful drivers. Subsequently,  proposed four forces, namely customer, regulatory compliance, sustainability culture, and external stakeholders for adoption of social sustainability. A recent study conducted by Huq and Stevenson (2018) revealed that institutional pressure may not always lead to implementation of social sustainability practices in a true sense.
Incentives such as government tax rebate, assistance from the company's brand, monetary help from the stakeholders, motivational programs from governmental or other agencies, and longterm partnerships generally inspire focal firms to solve any problems with suppliers. Further, trust and commitment among partners, resource sharing, information sharing, monitoring and auditing of supply chain partners, and joint efforts and planning drive adoption of sustainability (Kumar and Rahman, 2017;Panigrahi and Nune, 2018;Govindan et al., 2020a). Incentives and profit sharing have a high impact on diffusion of environmental and social sustainability in the supply chain (Hou et al., 2019).
According to Marshall et al. (2015), the firm's commitment for sustainability culture as part of strategies for competitive advantage drives adoption of social sustainability. Goworek (2011) argued that using competitive marketing strategies, small and medium enterprise (SME) can adopt social and environmental sustainability along with financial sustainability to score well-balanced triple bottom line objectives. Managers' ethics and moral values along with the organisation's ethical culture help to drive social sustainability practices (Zorzini et al., 2015).
Another group of drivers are related to the well-being of employees and the associated community; this group includes health and safety concerns at the workplace, legitimate wages, employee and community welfare, and employment stability (Diabat et al., 2014;Munny et al., 2019). Huq et al. (2014) discovered many new drivers: a unified single code of conduct, shortage of skilled labourers and consideration of socio-economic condition while implementing sustainability code of conduct. According to them, a single code of conduct would bring uniformity in understanding what suppliers must do as part of social compliance and to avoid multiple inspection and audit costs. To retain skilled labourers, especially when their availability is limited and losing them is costly, the suppliers will have to give rightful wages and other benefits. The adoption of social practices considering culture and socio-economic conditions of developing countries (i.e., contextualisation) also drives social sustainability.
Penalty, reward, cost sharing, and long-term commitment by the focal firm drive social sustainability practices. It is suggested that buyers should penalise non-complying suppliers and reward compliant ones by offering big and regular orders. The implementation of social sustainability practices involves huge cost and suppliers are not certain they will reap monetary benefits out of this investment. Cost sharing would inject confidence and motivate suppliers to adopt socially sustainable practices (Huq and Stevenson, 2018). The drivers of social sustainability are presented in Table 5.

Issues of social sustainability in multi-tier supply chain
Many researchers highlighted social issues in the supply chain that include violations of human rights, hazardous working conditions, lack of health and safety measures, low wages and excessive working time, poor community engagement, and lack of safety and privacy of consumers (Giannakis and Papadopoulos, 2016 Morais, 2017; Delai and Takahashi, 2013). Zorzini et al. (2015) studied social sustainability issues in upstream suppliers. They classified social issues into five broad categories, namely human rights, safety, community, diversity, and ethics. According to Klassen and Vereecke (2012), any aspects of product and process, which can harm people and community, put their safety at risk, are also social issues and must be addressed. Yawar and Seuring (2017) identified a number of social issues in the supply chain, including labour conditions, health and safety, human rights, child labour, gender, inclusion of marginalised and disabled people, alleviation of poverty, and minority development. Dubey et al. (2017) emphasised social issues in their framework for world-class sustainable supply chain management. Yawar and Seuring (2017)   Sustainability culture of an organisation reflects that the organisation cares about environment, society, and communities and adopts practices beyond regulatory pressure. Carter and Jennings (2004); Pagell and Wu (2009) Top management commitment and support drives coordination among the supply chain partners and promotes sustainability. Seuring and Muller (2008); Gimenez and Tachizawa, 2012;Kumar and Rahman (2017) Owner's characteristics Owners who received overseas education, worked in MNC, feel that it is important to comply with minimum social standards.
Huq et al. (2014) Skillful policy entrepreneur Ability of entrepreneur to invest in adoption of social sustainability and to institute policy. Mani et al. (2015) Stakeholder related Stakeholders' pressure Primary stakeholders are customers, suppliers, employees, top managers and shareholders. Secondary stakeholders' are government, NGO, community, media, competitors, trade associations and investors. Stakeholders can also be classified as internal and external also. Stakeholders' pressure is a very strong driver.
Carter and Rogers (2008) Hazard management Active management of health hazards promotes social sustainability.
Diabat et al. (2014) Safer working condition Healthy and safer working conditions promote social sustainability.
Diabat et al. (2014) Employee satisfaction Workplace benefits, wellness, compensation benefits, organisational commitment, retirement funds, etc, help to improve morale and employee satisfaction, which drive social sustainability.

International certification
International certification for social sustainability promotes social sustainability. Fayet and Vermeulen (2014) Economic benefits of social sustainability It is perceived that implementation of social sustainability improves productivity by reducing absenteeism and worker's sickness. Huq et al. (2014) Competitive pressure Adoption of social sustainability by a company may force its competitors to do so. Mani et al. (2015); Kumar and Rahman (2017); Panigrahi and Nune (2018) Competition among suppliers for skilled labourers Scarcity of skilled labourers leads to competition among the suppliers to retain them. Huq et al. (2014) Buyer related Buyer's learning in supplier management It is buying firm's ability to interact with suppliers for effective cooperation, coordination, and integration with them.
Ehrgott et al. (2011) Buyer's code of conduct Implementation of buyer's code of conduct leads to improvement in social sustainability.  (2018) Resource sharing Kumar and Rahman (2017) (continued on next page) proposed three broad strategies, namely communication, compliance and supplier development to address social issues. However, they believed that trust, commitment, collaborative efforts, and development strategies are the most important precursors needed to address social issues. Maria-Ariana (2017) highlighted serious social issues of workers in textile and clothing factories and emphasised that there is a need to focus on creating optimal working conditions, respecting human rights, and paying legitimate wages. A number of researchers reported on the plight of garment workers in Bangladesh and labour-related issues such as excessively long shifts, lack of job security, threat of lay-off, physical and verbal abuse, discrimination in the wages of male and female workers, and sexual harassment (Carlson and Bitsch, 2018;Kabeer and Mahmud, 2004;Lipschutz, 2004;Pashaei et al., 2018). Mani et al. (2016c) studied social issues and practices from three different perspectives, namely suppliers, manufacturers, and customers. Their findings suggest that the degree of adoption of social sustainability varies from industry to industry. Technically advanced sectors, such as energy and power, exhibit better social sustainability compared to labour-intensive industries such as textiles and mining. Corruption and bribery were also reported by many researchers as a challenging social issue, especially in emerging economies (Carlson and Bitsch, 2018;Maria-Ariana, 2017). A summary of social issues in supply chains has been provided in Table 6.

Barriers of social sustainability in multi-tier supply chains
Barriers, which can be internal or external, are the factors which thwart the implementation, realisation, and achievement of sustainability practices (K€ oksal et al., 2017). Cost reduction is an internal barrier whereas consumers' demand for low-priced products is an external barrier (Walker and Jones, 2012). Many researchers admitted that financial considerations such as pressure of cost reduction by the buyers (Shen et al., 2015); lack of financial assistance from the government and industrial associations (Chi, 2011) and lack of loan availability from banks for sustainability related activities (Panigrahi and Nune, 2018) are important barriers. Lack of pressure from the stakeholders, namely trade unions and shareholders (Mani et al., 2016a) and lack of top management commitment (Shen et al., 2015;Akbar and Ahsan, 2019;Walker and Jones, 2012) are additional barriers which prevent adoption of social sustainability practices. Lack of strict government laws and their lax enforcement make it easy for the suppliers to escape from the responsibility of adopting social sustainability practices (Panigrahi et al., 2019;Mani et al., 2016a;Majumdar and Sinha, 2019). Panigrahi and Nune (2018) pointed that lack of employment stability, lack of health and safety measures, poor community economic benefits, lack of scope of improvement in products' characteristics, inadequate infrastructure development, and lack of skilled human resources are important barriers to adoption of sustainability practices. Negligence of health and workplace safety are also considered as barriers (Akbar and Ahsan, 2019;Panigrahi and Nune, 2018). Other barriers are lack of awareness (Mani et al., 2016a;K€ oksal et al., 2017); lack of moral and values (Movahedipour et al., 2017); lack of willingness and initiatives of political parties (Carlson and Bitsch, 2018); cultural mismatch (Huq et al., 2014); bribery and corruption (Carlson and Bitsch, 2018;Huq et al., 2014); lack of training and education (Zorzini et al., 2015;Ali et al., 2018); and a lack of competitive pressure (Mani et al., 2016a). Multi-tier suppliers are typically small and medium scale enterprises and they lack in terms of resources. Lion et al. (2016) also observed that it is very difficult to implement sustainable practices at smaller companies because of insufficient competencies or resource constraints even when top management may have high interest in it. The barriers have been categorised for easy understanding and are listed in Table 7.  (2017); Joint effort and planning Joint effort and planning by supply chain partners for sustainability goals for the entire supply chain promotes social sustainability.
Kumar and Rahman (2017) Monitoring and auditing supply chain partners Auditing, monitoring, and sustainability reporting drive sustainability practices.
Kumar and Rahman (2017) Awareness Making suppliers aware of sustainability practices promotes sustainability in the supply chain.
Diabat and Govindan (2011); Meixell and Luoma (2015); Mani et al. (2015); Kumar and Rahman (2017); Training and education of auditors Training and education of local buying team and auditors about how to conduct auditing and training the suppliers about best practices help the suppliers in achieving sustainability targets. Huq et al. (2014) Social concern Concern for the society drives social sustainability. Mani et al. (2015) Employee involvement including middle management Active involvement of employees and middle management promotes social sustainability. Hanna et al. (2000); New et al. (2000) Availability of resources and firm size The availability of resources and firm size influences sustainability. Large companies have greater expertise, resources, and buying power. Gimenez and Tachizawa, 2012;Walker and John (2012) Community economic welfare Commitment towards the community's economic welfare drives social sustainability. Diabat et al. (2014) Country related National culture Positive culture in the country related to sustainability acts as a driver.
Ciliberti et al. (2008) Enforcement of the law Tight enforcement of laws may encourage implementation of social sustainability practices.
Yu (2008); Huq et al. (2014) 6.4. Sustainability tensions in multi-tier supply chain Tension is generally considered as a negative consequence which emerges from contradictory goals and interests among collaborating partners (Fang et al., 2011). For example, ethical sourcing and purchasing improves a focal firm's reputation, but may force their suppliers to change their operations or even break the relationship if they do not comply. Such a situation leads to mental and financial stress among the suppliers (Jackson and Young, 2016). Tensions are also known as side effects or "the darker side" of sustainability (Tura et al., 2019;Johnsen and Lacoste, 2016). In supply chain, tensions can arise from competing goals between a number of factors such as: short-term profitability and long-term environmental sustainability, cost efficiency and sustainability, and stakeholders' and shareholders' interest (Xiao et al., 2019). As sustainability in the supply chain is a multi-dimensional issue, its implementation often leads to the development of multiple tensions (Fayezi et al., 2018). The major sustainability tension is the competitive demand between cost optimisation and socioenvironmental sustainability (Brix-asala et al., 2018). It is generally believed that sustainable business practices lead to winwin situations but implementation leads to potential tensions and conflict. Tura et al. (2019) identified 20 different types of tensions categorised under four broad areas while implementing sustainable business practices. These broad categories are economic, structural, psychological, and behavioural in a business network comprising of suppliers, focal firms, customers, and other partners. Economic tensions include higher investment, cost of operational changes incurred by the suppliers in order to adapt new processes, and code of conducts or sustainability requirements (Tura et al., 2019). Similar findings were also reported by Huq et al. (2014) as suppliers incur additional cost to fulfill sustainability requirements of buyers. Structural tensions arise when buyers need to exercise, monitor, and control their suppliers, in order to ensure that suppliers fulfill the code of conduct. For distant suppliers, buyers generally rely on third party auditing, which sometimes leads to confrontational relationships with suppliers (Huq et al., 2014;Hannibal and Kauppi, 2018). When suppliers fail to fulfill buyer's code of conduct, the latter tends to terminate business relations. Mistrust among the buyers and their suppliers is considered as Table 6 Issues of social sustainability in multi-tier supply chain.

Category
Specific issues Description Sources

Labour related
Child labour Age of the employee less than legal age for employment in manufacturing company.

Mani et al. (2018)
Forced or bonded labour Workers are forced to work against their will, under threat of punishment.

Mani et al. (2018) Lack of inclusion of disabled and marginalised people
Inclusion of specially abled and marginalised community in the workforce. Yawar and Seuring (2017) Lack of minority development Development of minority population to bring them into mainstream. Yawar and Seuring (2017) Low wages Wages lower than prescribed legal wage. Lipschutz (2004); Kabeer and Mahmud (2004); Maria-Ariana (2017) Forced overtime Overtime is voluntary. Workers should not be forced to do overtime.

Lipschutz (2004) Excessively longer working shifts
Workers are asked to perform their job in longer working shifts against their will. International Labour Organization (2015); Maria-Ariana (2017) Female workers related Poor condition of women Sexual harassment at workplace. Women are obliged to sign an agreement not to get pregnant as long as they work at the factory. The working condition for pregnant female workers is also poor. (2004) Workers are temporary and their job can be terminated without assigning any reason.

Kabeer and Mahmud
International Labour Organization (2015) General issues Discrimination Discrimination with respect to race, gender, religion, disability, and age. Lower wages for female workers compared to their male counterparts is very common. (2004)

Lack of social ethics and values
There is an important role of social ethics and values in sustainable development of business. In developing countries, political parties lack commitment for the well-being of workers.
Carlson and Bitsch (2018) Lack of financial assistance from government and trade bodies Lack of financial assistance from government and trade bodies hinders social sustainability.
Chi (2011) Supplier related Lack for concern for reputation Small scale organisations generally do not realise value of reputation.

Valmohammadi (2011); Shen et al. (2015)
High implementation cost Adoption of social sustainability is cost intensive and small companies often do not have enough liquidity. Buyer related Organisation size Large companies have greater expertise, resources, and buying power; hence, they can show greater commitment towards social sustainability.
Walker and Jones (2012) Buyers ignore violation of social sustainability Buyers ignore violation of social sustainability as they are not genuinely interested in social sustainability implementation. They are more concerned about reputation and bad publicity.

Buyers accept mock compliance or overlook violations
Auditors from buyers' side know mock compliance practices and ignore the same.

Huq et al. (2014)
Lack of social concern Lack of social concern shown by firms hinders adoption of social sustainability.

Mani et al. (2016a)
Lack of social audit Lack of social audit by the buyer allows stakeholders to be exempted from CSR reporting.

Valmohammadi (2011); Shen (2015)
Poor community economic welfare Poor community economic welfare by firms impedes social sustainability. Competition among suppliers to reduce cost To get orders from the buyers, suppliers compete among themselves to reduce cost risking social sustainability. Zorzini et al. (2015) Confrontational relationship between Third party auditors often give adverse report regarding violations of code of conduct so they can revisit and receive another fee. Huq et al. (2014) psychological tension. Suppliers feel that they are forced to measure, monitor, and report sustainability indicators that are not relevant for their own business. For example, overtime is a critical issue in developing countries, where the minimum wages are very low. Workers often have to work overtime so that they can earn decent money (Xiao et al., 2019). Many suppliers do not buy employee insurance as their proft is very low. If they buy employee insurance, their profits will go down further and they will not be able to survive (Xiao et al., 2019). Resistance to adoption of new work practices, process changes, learning new skills, sharing of sustainability related operations and process data with the buyers are all considered behavioural tensions (Tura et al., 2019). Rezaee (2018) reported that tensions can occur in various dimensions of social and environmental sustainability as the corporates find that the non-financial social and environmental sustainability may take away funds. Often, managers give greater priority to economic goals than to sustainability goals. Sometimes suppliers who wish to comply with sustainability norms become phased out because of cost related reasons (Xiao et al., 2019). Sounndararajan et al. (2019) proposed the concept of collective stakeholder orientation in the context of global supply chain. They claim that this would enable participants to seek and share value, to share responsibilities, make voluntary and sustainable collaborations, seek collective ownership of the responsibilities, and provide mutual benefit to all participants overcoming their differences. A summary of sustainability tensions is given in Table 8.

Multi-tier supply chain structure, social sustainability governance models and practices
The structures of multi-tier supply chain (open, closed, and transitional) was first proposed by Mani et., (2013). In open structure, information and products flow from tier-2 to tier-1 and finally to buyer in a linear fashion. Buyer and tier-2 suppliers are connected through tier-1 suppliers, but not directly as depicted in Fig. 4. In closed structure, buyer establishes formal links and remains directly connected to tier-2 suppliers through exchange of information and regular interaction in transitional structure, which is in-between open and closed structure, the buyer and tier-2 supplier starts to build links between them with an aim to become closely structured. Mena et al. (2013) found that tier-2 suppliers supply raw materials for the final product and, hence, they play decisive roles if the buyer thinks of implementing sustainability in the supply chain. Tachizawa and Wong (2014) identified four modes of governance mechanism for sustainability in multi-tier perspective: Haugh and Talwar (2010) Table 8 Sustainability tensions in multi-tier supply chain.

Sustainability tensions Description Sources
Paradoxical tensions Paradoxical tensions arise because of contradicting yet interrelated sustainability and business goals. Such tensions may arise when a person tries to balance both personal and group identities; when an organisation's operations need to satisfy multiple stakeholders that results in competing strategies and goals; when organisations create competing designs; and when old working systems are replaced with new ones. (2011) Implementation of sustainable business practices may give rise to multiple tensions which can be broadly classified under four categories: economic tensions arise when one actor demands to invest into new technology, practices, etc. and the other actor perceives these demands asymmetric or unfair; structural tensions arise due to conflicting priorities of vertical and horizontal entities in supply chain, and generally manifest as increased monitoring and controlling requirements; psychological tensions arise due to differences in attitude and preference of different actors; behavioural tensions arise due to differences in nature of individuals: cooperative and, competitive. Fang et al., 2011;Toth et al. (2018); Tura et al. (2019) Tension between core labour standards and cost efficiency

Smith and Lewis
Tensions arise due to psychological conflict between fulfilment of core labour standards and cost efficiency requirements in socially sustainable supply chains.
Kuntner and Weber (2018) Procurement sustainability tensions Procurement sustainability tensions are tensions encountered by procurement professionals. Procurement professionals face tension internally from the company and externally from the suppliers to fulfill sustainability requirements. Fayezi et al. (2018) direct, indirect, work with third parties, and don't bother. In direct method, the focal firms directly collaborate with tier-2 and higher tier suppliers without involving tier-1 suppliers. In indirect method, the focal firms engage tier-1 suppliers to monitor sustainability practices of higher tier suppliers (Plambeck and Denend, 2011). In third party mode, lead firms delegate responsibility to implement, monitor and evaluate sustainability to NGOs, certification bodies, or competitors. The focal firms do not know much about higher tier suppliers and their activities; however, they use third party mode. Through their case studies, Wilhelm et al. (2016b) found that the institutional distance between focal firm and suppliers plays an important role in deciding mode of governance.
Focal firms adopt open methods when they are sure about the sustainability practices of their suppliers. They follow a closed method when the institutional distance is high and suppliers' sustainability practices are uncertain. When the complexity at subsuppliers level is high, the focal firms may rely on the assistance of third parties. The authors introduced a new approach called don't bother, which the focal firm uses when they deal with suppliers located at high institutional distance, transparency is low, the capability of tier-1 suppliers is limited in terms of handling sustainability issues of sub-suppliers, and when there is the presence of high vertical and horizontal complexity. Sauer and Seuring (2019) proposed a cascade design to manage multi-tier mineral supply chains. Gong et al. (2018) studied the structure of multi-tier supply chains and governance mechanisms to disseminate sustainability learning by MNCs to their suppliers. Their research showed how focal companies can devote resources in the early phases of sustainability learning of tier-2 and higher tier suppliers and then delegate responsibility to tier-1 suppliers. Finally, the supply chain will evolve from an operating to a sustaining stage to become a closed supply chain (Darbari et al., 2019). Meinlschmidt et al. (2018) applied transaction cost economics theory to study how buying firms use different approaches to manage their lower tier suppliers' sustainability. They identified eight approaches and grouped them into three categories, namely direct-in-house, indirect-hybrid, and neglect-market. Managers of focal firms can select any one of them depending on several factors. Dickson and Eckman (2006) proposed a socially responsible apparel and textile business model consisting of three levels: first consisting of stakeholders such as individuals, groups, and society; the second level consisting of workers, business, consumers, and national interests; and the third level consisting of production, merchandising/retailing, and consumption/disposal taking on environmental considerations.
It is often difficult for focal firms to control and monitor sustainability practices of tier-2 and higher tier suppliers. To do this, the focal firms assign this task to their tier-1 suppliers. This is known as a double agency role of tier-1 suppliers (Wilhelm et al., 2016a). Grimm et al. (2016) emphasised that assessment and collaborations of sub-suppliers can help to improve corporate social sustainability (CSS) standards. Strategic partnerships with suppliers, a firm's channel power, awareness of perceived risks of multi-tier suppliers' non-compliance with CSS, and public attention on tier-1 suppliers can help the adoption of sustainable practices by multi-tier suppliers.
To ensure social sustainability, focal firms mainly rely on compliance, audit, and certification of their suppliers. Under the pressure of human rights related protests, the GAP clothing initiated an awareness program for its vendors, developed a vendor "code of conduct," and established systems and process of measurement. Their endeavor resulted in a significant improvement in operational performances and human rights conditions. This in turn helped GAP's sourcing managers to identify right suppliers (Worley et al., 2010). Fayet and Vermeulen (2014) studied the cotton supply chain and their findings revealed that cooperation and collaboration among the stakeholders such as industries, certification organisations, NGOs, and farmers can bring environmental and social sustainability by working towards shared value creation for the entire supply chain. Signing contracts that include specific terms and conditions of sustainability forces suppliers to follow the buyer's sustainability code of conduct. Besides, they must also Fig. 4. Structures of multi-tier supply chain (adopted from Mena et al. (2013)). ensure that their sub-suppliers will respect the same (Lion et al., 2016). A recent study by Mejías et al. (2019) shows how fast fashion companies promote sustainability in multi-tier fashion supply chains.
The process of auditing sometimes fails the very purpose of ensuring adoption of social sustainability by the suppliers; some of them adopt questionable and unethical means to hide their unethical and problematic labour practices. To pass social audit, upon occasion suppliers will produce forged paystubs and encourage employees to lie about their wages and working hours (Egels-Zand en, 2007). They may also use model productions (Harney, 2008) and present fake certificates of raw materials (Roloff and Aßl€ ander, 2010). To avoid all such unscrupulous behaviours of suppliers, Ablander et al. (2016) advocated a stewardship theory as a powerful and effective means to foster better social and environmental sustainability performance, as it allows building longterm collaboration, the open exchange of ideas and mutual trust between buyers and suppliers which makes them willing to work on common objectives. Chen and Kitsis (2017) argued that the relational capability of the lead firm and the moral motives of top management facilitate sustainability and improve sustainability performance. Supplier development by the lead firm enables suppliers to solve their sustainability issues and helps them to improve sustainability performance (Yawar and Seuring, 2018). Hannibal and Kauppi (2018) suggested that the third-party social sustainability assessor may act as a bridge between upstream suppliers and consumers. However, they also observed that the social sustainability assessors do not cover all the tiers in the supply chain. They suggested three different approaches for social sustainability assessment. Koberg and Longoni (2019) reviewed the existing literature covering important aspects of sustainable supply chain management in a global context. They found that the closed configuration is used to manage sub-suppliers' environmental and social sustainability whereas the open configuration limits buyers' capacity to address social issues at the supplier's site. There is no ideal configuration to properly address sustainability in a global supply chain. Lechler et al. (2019) studied how companies collaborate within an assessment sharing strategic alliance (ASSA) to manage suppliers with respect to sustainability; they reported that information asymmetry and goal conflict can be mitigated by collaboration within ASSA and that such an approach helps to improve multi-tier suppliers' sustainability compliance. Although studying social sustainability is complex since it involves value statements, morals, and other intangible and non-measurable aspects, it can be overcome by systems analysis and systematic redesign approach (Missimer et al., 2010). Cultural intelligence is important for inter-firm collaboration as it helps suppliers to connect with customers and collaborate with those from different cultural backgrounds (Awan et al., 2018). The social sustainability governance model and practices are listed in Table 9.

Social sustainability performance and its measurement
Addressing social issues in manufacturing sector leads to better quality products, employee satisfaction, and knowledge enhancement (Pullman et al., 2009). Further, costs can be reduced (Carter, 2005), which leads to improved social performance, better brand image, and other strategic benefits (Mani et al., 2016c) for the focal firm.
Measuring social sustainability is not as easy as economic and environmental sustainability, nor can the analytical tools used for measuring economic and environmental sustainability be used for competently measuring social sustainability (Lehtonen, 2004). Some researchers have attempted to provide matrices to measure social sustainability (Sarkis et al., 2010). Rajak and Vinodh (2015) used fuzzy logic to evaluate social sustainability performance in Indian automotive components manufacturing organisations. Their approach is useful in terms of computing social sustainability indices and in identifying stronger and weaker attributes. Tamara et al. (2017) proposed three sets of quantitative indicators, namely generic, suppliers-specific, and industry-specific to assess social sustainability performance. Chen and Kitsis (2017) proposed a framework for performance indicators driving sustainability in supply chains based on relational capabilities among the supply chain partners. After extensive literature survey, they proposed performance indicators to measure social sustainability under two broad categories: human capital and societal capital. These two broad categories cover social indicators related to employees, the community, and customers. Popovic et al. (2017) and Tamara et al.
(2018) identified a number of quantitative indicators. They derived these indicators based on sustainability reports covering all echelons (upstream, midstream, downstream). These indicators are holistic and generic in nature and can be equally applicable to all echelons to measure performance, periodic monitoring, and policy making. Staniskiene and Stankeviciute (2018) proposed indicators considering the role of employees and their well-being in an organisation. Ajmal et al. (2018) identified an acceptable framework for social sustainability indicators from corporate and societal perspectives. Their research suggests that it is possible to manage economic and environmental sustainability together with social sustainability. Strategic supply chain management (Kot et al., 2019) and supply chain orientation (Jadhav et al., 2019) may positively influence social and environmental sustainability. Das and Shaw (2017) developed a model including both environmental and social concerns to minimise total supply chain costs. From the model, they concluded that stricter carbon caps lead to the opening of more plants, a decrease in a company's carbon footprint to a certain extent when negative social impacts from suppliers are reduced, and an increase in the certainty of materials supplied from selected suppliers. Ahmed et al. (2019) developed a model for solving the sustainable supplier selection and order allocation problems. Yang and Zhang (2017) empirically studied sustainable supplier management practices and their effects on suppliers' performance, buyer-supplier relationships, and the buyers' competitive advantage. They found that the practices positively influence all three constructs. Ahmadi et al. (2017) used best-worst method to assess social sustainability in the supply chain. The best-worst method, combined with interactive and MCDM methods, is used to assess and evaluate suppliers considering social sustainability (Bai et al., 2019). Zorzini et al. (2015) emphasised that there is a need for research on how to measure social sustainability and its impact on stakeholders beyond the immediate supply chain. Table 10 presents a summary of sustainability adoption and its effect on supply chain performance, while Table 11 outlines the social sustainability performance indicators.

Conceptual framework and discussion
A number of drivers, issues, barriers, and tensions have been identified through this comprehensive review of the literature. Drivers promote adoption and implementation of social sustainability practices. In contrast, numerous barriers hinder the adoption and implementation of social sustainability, especially in developing countries. Buyers and other stakeholders pressure suppliers to follow socially sustainability practices. On the other hand, suppliers, in response to a number of barriers, tend to not implement social sustainability and instead try to present faux compliance practices to hide violations. Suppliers, in other words, experience tension due to buyers' pressure to implement social sustainability practices. This tension needs to be overcome by practices which promote long-term trustworthy relationships among the actors and stakeholders of the supply chain. Such practices would be reflected in terms of improved social sustainability indicators and supply chain performance by the mitigation of social risks. In order to improve social sustainability in multi-tier supply chains, efforts to establish links among drivers, barriers, issues, tensions, practices, and performance have been proposed through the conceptual framework depicted in Fig. 5.
Recently, many researchers have contributed to multi-tier supply chain literature in terms of structure, governance mechanisms, social issues, thematic analysis, and conceptual frameworks (Mena et al., 2013;Tachizawa and Wong, 2014;Zorzini et al., 2015;K€ oksal et al., 2017). Most researchers point out that research in the area of multi-tier supply chains is in its nascent stage and it needs further explorations from the point of developing conceptual frameworks and theories to explain the dynamics of interactions of different actors. In this paper, the concept of sustainability tensions, which has not received much attention until now, has been incorporated along with drivers, barriers, issues, and practices. Table 9 Sustainability governance practices in multi-tier supply chain.

Governance practices Description Sources
Institutional pressures such as coercive, mimetic, and normative pressure on suppliers Collective coercive pressure from buyers' mimetic pressure created by increased competition for orders among suppliers and normative pressure via education and training push the suppliers to adopt social sustainability by complying with local social sustainability laws, compliance with buyer's code of conduct. Proactive initiatives by firms go beyond local laws and buyers' code of conduct.
Huq and Stevenson (2018) Multi-stakeholder collaboration Multi-stakeholder collaboration helps to improve compliance, capacity, and capability to address complex issues. Worley et al. (2010) Assessment and collaboration for management of sub-suppliers Assessment includes audits, site visits, and suppliers' questionnaires. Under collaboration, buyers in general provide training workshop and, corrective action plans to their suppliers and sub-suppliers, leading to improved compliance. Vachon and Klassen (2008); Grimm et al. (2016) Structural variants: open (indirect), closed (direct), third party, and don't bother In open structure, the flow of material and information happen from tier-2 suppliers (T2) to tier-1 suppliers (T1) and then from T1 to lead firm. The lead firm delegates the authority for managing T2 to T1. This mode of governance is also known as "indirect." In a closed structure, the buyer and T2 establish mutual formal or informal contacts and communications. This mode is also known as "direct". There is a transitional structure, in which buyer and T2 start to establish link. In third party mode, the buyer delegates responsibility of managing sustainability to a third party. In the last category, buyer focuses only T1 for sustainbility issues. Sustainable supplier development and monitoring Improvement in buyeresupplier relationships and their performances leading to stronger competitive advantage. Yang and Zhang (2017) Collaborating with multiple suppliers Cost saving and reduction in pollution. Shrivastava (1995)

Management of sustainability practices
Better financial performance. Sroufe and Remani (2018) Incorporation of social sustainability criteria along with environmental criteria for supplier selection Sustainable performance of supply chain in terms of minimisation of environmental and economic risks to society. Awasthi et al. (2018) Social sustainability orientation, adoption of basic and advanced social sustainability practices and long-term orientation Advanced social sustainability practices improve operational performance which is significantly moderated by long-term orientation. Basic social sustainability practices do not improve operational performance. Croom et al. (2018) 7.1. RQ1: What are the drivers and barriers of social sustainability in multi-tier supply chains?
To answer our first research question, drivers and barriers were listed in Tables 5 and 7, respectively, after our extensive literature survey. Positive motivations of stakeholders are drivers and their reluctance is a barrier of implementation of social sustainability. Cost reduction and customer demand for low-priced products are barriers. Stakeholders' pressure plays an important role in terms of spreading awareness, adoption, and implementation of sustainability practices (Meixell and Luoma, 2015). Sancha et al. (2015), and Huq and Stevenson (2018) used institutional theory to study different types of coercive, normative, and mimetic pressures in the context of developed and developing countries, respectively. Sancha et al. (2015) suggested that only mimetic pressures positively drive sustainability practices whereas the role of coercive and normative pressures are not significant. Sancha et al. (2015) also used a resource-based view of firms' capabilities to integrate suppliers into their supply chain and extend sustainability practices to them. Huq and Stevenson (2018) used institutional theory to study the role of government and collective pressure from buyers. According to them, collective buyers' pressure was the main coercive pressure and the effect of this pressure varied from suppliers to suppliers. The coercive pressure from the government was not effective because of alleged corruption in the system. The professional education of young business entrepreneurs was reported as a normative pressure and the competition to retain skilled workers was identified as mimetic pressure. The theory of transaction cost was used to study drivers and barriers in the context of developing countries; a high transaction cost was found to act as a barrier of social sustainability (Huq et al., 2014). Training and education by the buyers is one of the most important enablers to implement Table 11 Social sustainability performance indicators.

Indicators
Description Sources

Employee participation
Participation in terms of intensity, form and issues, are important in strategic and daily management are linked with human resource management and sustainability of organisation.

Staniskiene and Stankeviciute (2018) Equal opportunities
Equal opportunity encompasses fair employment practices such as fair selection, performance appraisal, and compensation system. social sustainability. In short-term, its initial costs are high, but these costs may decrease over the long-term. Due to high costs of implementation, sometimes buyers mainly rely on audit and compliance. On suppliers' side, some of them hide facts and attempt to demonstrate fake compliance to pass the audit process; such non-compliance is very costly for both suppliers and buyers in terms of loss of orders and the reputational damage to the brand. Huq and Stevenson (2018) studied only tier-1 suppliers and they emphasised that there could be similar research on higher tier suppliers. Behavioural components such as leadership, professionalism, ethics, change management, company culture, and commitment need to be included at the level of multi-tier suppliers to drive socially sustainable practices (Winter and Knemeyer, 2013). Barriers listed in Table 7 can also be grouped into three categories, namely economic (cost of implementation, price war, lack of financial capability, etc.); skill and knowledge; and behavioural (reluctance of focal firm, law enforcement bodies, political parties and other stakeholders). Many believe that implementation of sustainability requires investment from the owners of suppliers. Costs and fears of monetary loss among SMEs are strong barriers to adopt sustainability practices. It has been reported that long-term partnerships, financial assistance and knowledge sharing help suppliers gain confidence and adopt sustainability practices in their operations. Joint planning and efforts to address issues and problems of suppliers will help suppliers to cut down cost and motivate them to adopt sustainable practices. For example, better cotton initiatives aim at improving the productivity of cotton by training farmers. Besides this, they also help to bring social and environmental sustainability to the cotton supply chain. PUMA took a pilot project to train and encourage SMEs to write their own CSR report and adopt social sustainability for themselves. The result of such project was encouraging as the suppliers were able to implement sustainability practices, to become more transparent, and to measure their environmental and social sustainability performances. They were also able to improve their competitive advantage and reputation (Vurro et al., 2009). 7.2. RQ 2: What are the social issues in the multi-tier supply chains?
A number of social issues pertaining to multi-tier supply chain have been reported in literature. Social issues generally fare worse in developing countries and labour-intensive industries. In developing countries, the law and regulation enforcement agencies are relatively slack, officials are often corrupt, and there are large populations of poor and uneducated workers. Here either the owners of firms are not aware of importance of social sustainability or they have little concern about such topics. Child and forced labour, low wages, excessively long working time, discrimination between and within genders, health and safety, poor working conditions, violations of leave facility, delayed payment, noncompliant overtime wage payment, intimidation, threats of layoff, lack of the right to freely associate, the safety and dignity of female workers, verbal and physical abuse, and bad behaviour of supervisors and managers are some of the frequently occurring, serious social issues. Such issues are to be addressed and eliminated to the extent possible to make socially sustainable supply chains. Organisation offers relevant training, workshops and assistance by focal firms, government agencies, and NGOs and active participation by the owners, managers and workers will bring awareness about how to address social issues and eliminate their occurrences from the multi-tier suppliers. On the other hand, violators should be booked and penalised. Note that addressing core social issues within an organisation paves the way to improve other two dimensions of sustainability .
Values added to the main product by multi-tier suppliers may not always be significant but they cannot be ignored. A faulty part or component, or poor workmanship in the final product could be disastrous for the end consumer. Workers who are poorly motivated due to unfair wages or who are fatigued due to excessive working hours are not able to perform their task with precision, resulting in production of defective parts or components. Consequently, the end consumer will suffer to some extent due to defective products, and that dissatisfaction will cause loss of business and reputational damage to focal firms. Suppliers should also realise that their unethical and irresponsible behaviours will eventually bring a bad image to their supplier base and to their reputation, which will result in loss of business and adversely impact the economy of the country.
7.3. RQ3: What are sustainability tensions and social sustainability practices in multi-tier supply chains?
Sustainability tensions exist in the multi-tier supply chain, and often, focal firm managers follow instrumental logic to cope with them (Deegan and Shelly, 2014; Xiao et al., 2019). The reasons behind sustainability tensions are conflicts in the goals and requirements of buyers and suppliers, major changes in operations and processes at the supplier's end which requires capital investment for installing new machines and equipment, resistance to adapting to new systems and processes, or cultural differences between developing and developed countries. Organisations adopt certain strategies to cope with such tensions. These strategies can be clubbed under four categories: win-win, trade-off, integrative, and paradox (Van der Byle and Slawinki, 2015). In the win-win approach, the strategy is chosen in such a way that improving one dimension of the sustainability leads to the improvement in at least one of the other two dimensions. In the trade-off approach, the company chooses one goal over other in terms of priorities. The integrative view is a balanced approach and gives equal importance to all three sustainability goals (Brix-asala et al., 2018). Finally, paradoxes are defined as "contradictory yet interrelated [objectives] which coexist and persist over time" (Smith and Lewis, 2011). The collective goals of sustainability gives rise to paradoxical tensions as each of the three dimensions of sustainability in isolation seems to be logical; however, when they are juxtaposed, they seem to be somewhat contradictory (Brix-asala et al., 2018). Under the paradox approach, buyers embrace the tensions and put efforts and resources to train suppliers during early phases of supplier development process; they subsequently delegate these responsibilities to competent tier-1 suppliers (Brix-asala et al., 2018;Wilhelm et al., 2016a). Contextualising is another approach to deal with paradoxical tensions; it occurs when managers relax the norms and accept something which is more workable (Xiao et al., 2019). The extent of contextualising depends on the judgment of managers; too lenient contextualising can magnify supply chain risk (Xiao et al., 2019). Assessment and collaboration are common modes of ensuring sustainability in upstream suppliers. Considering the paradoxical approach, norms may be contextualised and suppliers may be given free space to come up with creative solutions to the problems. Bommel (2018) suggested that firms adopting paradoxical or integrative thinking can handle complexity by reducing tensions, ambiguity, and uncertainty and by finding it easier to make sense of sustainable practices than the firms which believe in instrumental logic. It is also reported that the ambidextrous capability of focal firms helps them to resolve paradoxical tensions . 7.4. RQ 4: How do the social sustainability practices impact performance of multi-tier supply chains?
Social sustainability performance measurement is difficult and complex due to global spread of supply chain with varying environmental, economic, social, and legal standards from country to country (Taticchi et al., 2013). The measures of performance of social sustainability are still emerging (Beske-Janssen et al., 2015). It is reported by many researchers that the implementation of socially responsible practices improves the condition of vulnerable workers, helps to develop long-term relationships with buyers, contributes to growth in a country's economy, improves productivity, and enhances corporate social performance (Mani et al., 2016c;Huq et al., 2014;Chi, 2011). Further, such practices contribute to competitiveness of the whole supply chain by reducing cost and increasing market share (Klassen and Vereecke, 2012;Rao and Holt, 2005). The adoption of social sustainability in the emerging economy by suppliers results in better social performance in terms of compliance of human rights, child labour and safety, a decrease in lead time, better product quality, increased reliability of products resulting in improvement in buyers' and suppliers' performance, and positive relations between buyers and suppliers . It is observed that better workplace conditions, fair pay, on-jobtraining, employee satisfaction, and knowledge enhancement, all help to improve product quality (Pullman et al., 2009;Rothenberg et al., 2001), address health and safety issues, reduce the number of accidents at the workplace, lower the employee turnover, and lower recruitment and associated liability costs (Torugsa et al., 2013). Loyal and happier employees work harder for the company and they may work at relatively lower salary when the company takes care of their well-being at the workplace (Rupp et al., 2013).
It has been found that supplier's sustainability assessment, monitoring, buyer-supplier collaboration and sustainability practices have positive impact on sustainability performance (Freise and Seuring, 2015;Tate et al., 2010;Shafiq et al., 2017;Alshehhi et al., 2018). Collaboration and trust building with suppliers and customers can help to mitigate sustainability issues, maintain quality, and reduce cost (Shrivastava, 1995;Paulraj et al., 2008). Sroufe and Gopalakrishna-Remani, 2019 found that implementation of green practices as part of sustainability improved social sustainability in the organisation. The operational and financial performance can improve in the long run as a result of adopting social sustainability practices through collaborative planning and coordination among the supply chain partners (Brammer and Millington, 2008;Croom et al., 2018). Focal firms must take leadership and initiative, and leave behind their bargaining powers (Jia et al., 2019).

Implications and future research directions
Literature on social sustainability in multi-tier supply chain perspective is rather scant, yet the research area is emerging. Most research works on sustainable supply chain focus on the brand (focal firm) and tier-1 suppliers, while only a few address the issues related to the sub-suppliers. Even after the conceptualisation of multi-tier supply chains, sustainability research is predominantly focused on economic and environmental issues, and unfortunately, the social dimension has not been adequately addressed. A conceptual framework to address sustainability tensions along with drivers, issues, barriers, and practices has been proposed. This framework will be useful in addressing social issues in upstream suppliers by adopting more practical approaches such as contextualising the paradoxes rather than adopting instrumental logics. This study will help managers and practitioners to understand interactions among drivers, barriers, tensions, practices, and performances in the context of social sustainability in multi-tier supply chains.
This research has determined that a number of gaps still exist, as listed below. These research areas are ripe for further investigations.
Development of generic and industry-specific conceptual frameworks and models for the adoption of social sustainability in multi-tier supply chains. Study on social sustainability practices and performance (wellbeing of employees, society, community, etc.) and their correlation at various stages of the multi-tier supply chain. As sustainability in multi-tier supply chains is an emerging area of research, exploratory research, that focuses on higher tiers of suppliers may be carried out in cross-cultural context, in different supply chains. From this review, it becomes evident that only a limited number of papers used theories to explain their research findings; this observation establishes the clear need to explore more research in this direction. Furthermore, the drivers and barriers have been classified for easy understanding; hence, there is a call for research to develop new theories or to implement changes to the existing ones. Measuring social sustainability is a challenging task and is still evolving. Industry-specific frameworks for accurately measuring social sustainability may be developed and deployed for further assessment. Quantitative models and indices may be developed for holistic and objective assessment of social sustainability performance of a multi-tier supply chain network. Ethical and financial frameworks to address social sustainability in multi-tier supply chains should be explored. The role of various stakeholders to mitigate sustainability tensions in multi-tier supply chains has research potential. The role of Industry 4.0 as a driver of social sustainability in multi-tier supply chains also promises to be a fruitful topic.

Conclusions
In this paper, social sustainability drivers, issues, barriers, tensions, practices, and performance of multi-tier supply chains have been explored. A conceptual framework integrating the aforesaid elements has been proposed. Social issues need to be addressed through adoption of social sustainability practices spanning various tiers of the supply chain. Barriers impede the adoption of sustainability initiatives and give rise to multiple tensions among the stakeholders. Tensions can be mitigated through practices that are enabled by a number of drivers. The performance matrices to measure social sustainability and how they affect the performance of socially sustainable supply chains have also been discussed. Different theories such as stakeholder's theory, institutional theory, a resources-based view, and paradox theory have also been reviewed.
From this review, it is evident that the management of social sustainability in upstream suppliers is a challenging task, especially in developing economies. As most of the upstream suppliers are SMEs, hence lack of CSR knowledge, insufficient financial resources and weak managerial skills impede the adoption and implementation of social sustainability practices. Therefore, they need assistance from stakeholders, focal firms, and governmental, and industrial associations. It is a fact that a large number of workers earn their livelihood from higher-tiersuppliers; hence, the adoption of social sustainability is necessary to promote well-being of employees, to ensure justice, fair wages, safer working condition, equity and equal opportunity, and to provide education and healthcare. It is a little unrealistic to expect people to care about global warming or extinction of species when they are hungry, seeking work, or feel unsafe in their own home. So, addressing sustainability through paradoxical lens and contextualising have been reported as practical approaches to overcome tensions and allow free space to the suppliers to come up with innovative solutions while fulfilling their business goals. There is a need to develop an appropriate social sustainability adoption model and framework integrating stakeholders, business processes, technology, government regulatory bodies, political parties, universities, NGOs, financial institutions, and trade associations. Inappropriate framing and unsupportive procedures can hinder effective implementation of social sustainability.
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Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.