Supplier sustainability: A comprehensive review and future research directions

Sustainability is not a one-player task. Organizations have started to realize that their supply chains have a signiﬁcant social and environmental impact, usually greater than their own operations, and managing sustainability at suppliers is crucial for supply chain-wide sustainability. Supplier sustainability management (SSM) research is fast evolving across multiple disciplines but lacks an interdisciplinary review to guage the progress made, and to decide the path forward. Heightened global focus on sustainability compels us to explore research avenues in SSM for meaningful progress. In this paper, we provide a comprehensive review of SSM research including the most recent work. We propose the Motivation-Measure-Analyze-Improve-Govern (MMAIG) framework for supplier sustainability, identify the limitations of current SSM research in enabling this framework, and propose future research directions. Our key observations are that (a) current SSM research is heavily focused on measuring and monitoring supplier sustainability, and (b) supplier sustainability improvement/development research is limited and the majority of it is about sustainable supplier selection. The future research directions that we propose are centered around (a) optimizing the investments towards supplier sustainability through collaboration, proposing mechanisms that consider risks, liabilities, and gains of all parties, and (b) considering behavioral aspects to overcome SSM implementation issues. Organizations can achieve eﬃcient improvement in supplier sustainability by using a collaborative approach that is data-driven and trust-based. We discuss several mechanisms within our MMAIG framework that can help organizations in their collaborative approach.


Introduction
Large multinational firms have sought and availed the opportunities in globalization and outsourcing practices.By sourcing from emerging economies, multinational firms have saved on operating and labor costs, which has outweighed the downsides such as increased lead times, inventory, loss of control, and disruption risks.In last few decades, another important factor has become part of this outsourcing equation-the social and environmental impact of suppliers' activities and associated reputation and regulatory risks to focal firms [1] .Multinational firms are being held responsible by stakeholders such as consumers, NGOs, media, and governments for the social and environmental impact of their suppliers' activities e.g.sweatshops [2] , child labor [3] , deforestation [4] , water pollution [5] , etc.These firms are being compelled to play a larger role in ensuring sustainable practices at suppliers.
More recently, focal firms are beginning to see supplier sustainability as an opportunity rather than just risk management.As the interest in responsibly-made and responsibly-sourced products is growing, companies are able to leverage supplier sustainability for marketing and competitive advantage (e.g.fair-trade labels on products).Focal firms are able to realize the cost-saving opportunities in sustainable operations (e.g.reduced energy consumption, reduced waste) more efficiently when they are considering the supply chain as a whole, instead of restricting their thinking within organizational boundaries.Focal firms are also pursuing supplier sustainability in light of evolving corporate values that focus on serving all company stakeholders including suppliers and society [6] .
As a result, there is a growing interest in research investigating how to measure, analyze, improve, and govern supplier sustainability, and what the direct and indirect benefits are to the focal firm.This research area is referred to as Supplier Sustainability Management (SSM).
There are existing literature reviews on the specific topic of SSM [7][8][9] , as well as on the wider topic of SSCM -Sustainable Supply Chain Management [10][11][12][13] .We discuss these reviews in detail in Section 2 .https://doi.org/10.1016/j.smse.2022.100003In light of this existing literature review work, we aim to make the following contributions: • Bridge the gap in existing reviews by presenting a comprehensive and interdisciplinary picture of SSM.Our focus is to elaborate on the important themes in SSM research to advance the understanding.This dictates that we do not limit the review to a mere classification of existing research papers.• Distinguish between the research that specifically considers supplier sustainability and the research that considers sustainability in general.This is important because considerable interest in sustainability as a topic often means that SSM researchers risk getting buried into a myriad of research papers not specific enough for their purpose.• Introduce the MMAIG framework (Understand m otivations for supplier sustainability; m easure, a nalyze, i mprove, and g overn supplier sustainability).For researchers, this framework presents the scope of existing SSM research and also highlights the opportunities.For practitioners, this framework presents different facets of SSM implementation and various mechanisms available under each.In Section 2 , we discuss frameworks from literature and compare them with the MMAIG framework.
Another aim of this paper is to discuss future research directions for SSM, based on our literature review and industry experience.We make it clear that while there is an increasing interest in SSM research, it is mostly focused on measuring supplier sustainability through standards and audits.There is a dearth of research on analyzing supplier sustainability performance e.g.understanding why some suppliers perform better/worse than others.Insights from such analysis can and should be used for improving supplier sustainability e.g.understanding where to exert improvement efforts-on which specific suppliers or improvement areas.
When it comes to sustainability improvement, research is limited to sustainable supplier selection.Selecting new suppliers that are already at a high sustainability level may help the focal firm's supplier sustainability targets, but this does not improve the sustainability of the existing suppliers.Furthermore, as sustainability is just one of many criteria in new supplier selection, it is not guaranteed that the focal firm will see an increase in overall supplier sustainability.Focal firms need different mechanisms to continuously improve sustainability in their existing supplier base.The research in this area is quite limited.
Apart from measure, analyze, and improve aspects of supplier sustainability, existing research investigates how focal firms can govern supplier sustainability and decide appropriate governance mechanisms for different suppliers.Existing research also explores the motivations of focal firm for pursuing supplier sustainability.
Our review brings forward these key aspects of SSM and also gaps around 'Analyze' and 'Improve' parts of our MMAIG framework (Motivation-Measure-Analyze-Improve-Govern).We propose future research directions that can address these gaps.The key considerations in our proposed research directions are: • a more collaborative approach towards SSM by considering gains and risks for all involved parties, • efficient progress through optimal effort investment, and • consideration of behavioral aspects to overcome SSM implementation issues.
Our overall aim is to complement existing SSM research with proposed future research directions to enable a robust MMAIG framework for SSM.
The rest of the article is organized as follows.In Section 2 we discuss the existing reviews and frameworks, our review scope and the MMAIG framework.In Sections 3 through 7, we discuss the elements of the MMAIG framework.In Section 8 we provide extensive recommendations for future research in SSM, and provide concluding remarks in Section 9 .

Review of reviews
There are several literature reviews on the wider topic of Sustainable Supply Chain Management (SSCM) [10][11][12][13] .These reviews cover SSM research, but they also cover other aspects of SSCM such as green logistics, reverse logistics, and downstream operations/networks of focal firms.As their scope is broader, these literature reviews do not have comprehensive coverage of SSM themes.
Some review articles specifically focus on SSM.Zimmer et al. [7] provide a list of the SSM papers under different subtopics (supplier selection, monitoring, improvement) and mention which dimension of sustainability each paper covers (social, environmental, economical).Their review focus is on the types of models and methods used in SSM research.Chen et al. [8] provide a list of SSM papers classified under different subtopics of SSM and mention which dimension of sustainability they cover.Gimenez [9] divides papers under SSM subtopics and specifies for each paper which type of activities it covers (assessment or collaboration) and which dimension of sustainability it covers (social, environmental, economical).In a separate section, the author reviews the papers discussing enablers of SSCM (and not SSM specifically).Agrawal and Lee [14] cover some improvement mechanisms of SSM.
Collectively, these reviews provide a good list of SSM papers under SSM subtopics and discuss certain aspects of SSM well.But they do not discuss various mechanisms of supplier assessment, improvement, and collaboration.This missing comprehensiveness is what we aim to address, by synthesizing and structuring all relevant research under SSM.We present a state-of-the-art understanding of SSM research in this article.

Frameworks in the literature
Existing reviews use different frameworks to organize and understand the literature.The Antecedents-Decision-Outcomes (or motivesdeterminants-outcomes) framework (ADO) is used by Seuring and Müller [13] and Gimenez [9] .The Pressures-Practices-Performance is a similar framework.These frameworks help understand the causal links in SSM.
The Boundaries-and-flows framework used by Sarkis [15] draws attention to information, material, and financial flows across organizational, cultural, economical, and geographical boundaries when thinking about green supply chains.Framework based on underpinning research theory is used by Sarkis et al. [12] , Dubey et al. [11] .
All these frameworks bring unique perspectives, based on the type of research questions they address or practice areas they support.We propose the MMAIG framework (Motivation-Measure-Analyze-Improve-Govern) to be comprehensive in terms of the insights it can generate for researchers, and to serve as a guide to practitioners.
MMAIG framework is in line with the Plan-do-check-act (PDCA) cycle.'Plan-Do' from PDCA (setting goals, processes) coincide with 'Measure' part of MMAIG.'Check' from PDCA (analyze the results) coincides with 'Analyze' part of MMAIG.'Act' from PDCA (take actions based on analysis) coincides with 'Improve' part of MMAIG.The underlying idea of continuous improvement in PDCA cycle is also central to MMAIG.It must be a cycle of Measure-analyze-improve for continuous improvement.Measure-analyze-improve sections of MMAIG focus on the WHAT and HOW of SSM.We add 'Motivation' on top to show causal links (similar to the pressure-practices-performance framework) and thereby focus on WHY.Governance at the bottom intends to cover WHO, HOW, and WHEN which is useful for practitioners.

Review methodology
We followed a typical review methodology in this study.Our review question aims at (i) exploring the key themes under SSM, (ii) identify- ing the gaps, and (iii) generating future research directions.We started with this review question to provide a breadth of coverage but limited ourselves to the SSM domain.Our selection process was iterative.We refined our search strategy as we studied more and more articles related to SSM and came across new themes.
We covered SSM studies from English language journals, conference proceedings, and books.The keywords we searched for included "supplier sustainability ", "social responsibility ", "supplier audit ", "corporate social responsibility (CSR) ", "environmental performance ", "green supplier ", "responsible sourcing ", "green procurement ", "sustainability improvement ", and "sustainability development ", among others.Several articles referenced in those we studied, and publications by the same authors were also included in our research sample.
We removed studies that do not consider both supplier and sustainability aspects together.We also removed the studies that were focused on product sustainability (since our focus is sustainable practices of the suppliers, and not sustainable products).
We reviewed the abstracts of all studies to make sure they align with our research question.Next, we selected certain papers under each theme for full article review.We discuss all the themes in detail highlighting how well they are covered by existing research, and where the gaps exist.We use MMAIG framework to provide an overview of the breadth of the literature, to serve as a guide to practitioners, and also to motivate future research directions.

Review scope
The scope of this review is structured using the MMAIG framework.The elements of MMAIG -Motivation, measure, analyze, improve and govern -are outlined below.The review scope is summarized in Fig. 1 Motivation.What motivates focal firms to engage in SSM and why they should be concerned about the sustainability practices of their suppliers?Motivations can be intrinsic e.g.focal firm has an organizational culture that promotes sustainable practices in its sphere of influence.Motivation can also be extrinsic e.g.focal firm is concerned about the negative publicity due to a sustainability-related incident at one of its suppliers.
Measure.How do the focal firms measure and validate suppliers' sustainability performance?First, they have to define the scope of measurement -which suppliers to measure, and across which dimensions (social, environmental, and economical).Second, they have to agree on standards against which suppliers will be measured -these can be public standards (like ISO14001) or industry standards (like the Electronic Industry Citizenship Coalition's code of conduct), or private standards developed by the focal firm.And finally, they have to validate the actual performance against agreed standards -this can be done through selfassessment by the supplier (first-party) or audits by focal firms (secondparty) or by third-party auditing firms.
Analyze.How do the focal firms analyze the supplier sustainability performance data to get insights?Focal firms may have thousands of suppliers and hundreds of measurement parameters.It is important to have an analysis framework to get meaningful insights from these data.Understanding why some suppliers are performing better or worse than others helps identifying areas for improvement.When comparing supplier performance or scores against one another, it may be important to consider supplier segmentation to ensure apples-to-apples comparison e.g.paint suppliers and transportation suppliers may have different sustainability scores because of the nature of their business.
Improve.What type of improvement efforts can be made and how to measure the effectiveness of such improvement efforts?Once supplier sustainability performance is measured and analyzed, the next step is to take action on improvement.These actions can be to (i) include sustainability criteria in new supplier selection (this will not improve the sustainability of any supplier, but it may improve the overall sustainability of the supplier base of a focal firm), (ii) using penalty and reward mechanisms, (iii) disclosing audit findings on a public platform, (iv) Govern.What governance styles and management practices can be adopted for SSM implementation?Focal firms may want to take a phased approach towards implementing the MMAIG.In the initial phase, they can use MMAIG in its entirety for the key suppliers and only parts of MMAIG for non-key suppliers.They can transition their entire supplier base to the MMAIG in a time-phased manner.Focal firms can consider factors like their relative power or influence over the supplier base to decide on different governance styles for different suppliers (e.g.measureonly, measure and mandate compliance, etc.).These strategic directions generated by governance can be converted into actions by effective management systems at the focal firm.
Above five elements (Motivation, Measure, Analyze, Improve, Govern) outline our review scope.It is important to note that our review is focused on the sustainability-related practices of suppliers (i.e.process sustainability) and not on the sustainability of products supplied by suppliers (i.e.product sustainability).Hence, product sustainability and associated topics like the collaboration with suppliers for sustainable product innovation, product packaging design, labeling, traceability, etc. are not in the review scope.

Focal firm motivation
It is important to understand why focal firms should be concerned about their suppliers' sustainability practices and what are their intrinsic and extrinsic motivations.Motivators and drivers of sustainable practices of an organization is a widely researched topic (for reviews, see [16][17][18] ).The majority of this research is not specific to organizations' practices related to its suppliers' sustainability, but considers organizations' practices related to sustainability in general.However, some studies specifically investigate the focal firm's motivations for improving their suppliers' sustainability.The key findings from this research are captured below and the summary is presented in Table 1 .
Brand/Reputation Risk.Sustainability incidents at suppliers of large brands attract unfavorable publicity and cause reputation damage.This can be also seen as an opportunity since focal firms' efforts in improving supplier sustainability add to their brand image.Perceived risks to the focal firm due to its inaction compels the focal firm to undertake supplier sustainability initiatives [19] .
Supply Risk.Social/environmental issues at the supplier (e.g.poor soil quality due to fertilizer overuse) can result in a supplier's inability to meet the time, quality, and quantity requirements of the focal firm.In extreme cases, focal firms may have to incur supplier switching costs [19] .
Mimetic pressures/Competition.Focal firms can be under pressure to imitate other competitive firms that are supplier sustainability champi-ons.A competitive environment, where environmental and social practices are extensively adopted, is more conducive for firms to extend sustainable practices outside their own organization [20] .Better social responsibility performance of suppliers also results in an improved competitive advantage for the focal firm [21] .
Coercive pressures/Regulations.Typically regulatory pressures on firms are to improve their own sustainability practices and seem to have less influence on a firm's actions for supplier sustainability improvement.Sancha et al. [20] show that coercive/regulatory pressures do not constitute drivers for supplier sustainability improvement though they are drivers for an organization's internal sustainability (Zhu and Sarkis [22] ).This is also in line with Zhu et al. [23] who found that coercive pressures do not aid the external green supply chain management practices of an organization.
Normative pressures.Normative pressures are pressures from social groups (e.g.NGOs, trade unions) to confirm social legitimacy.These groups have the power to influence consumers and shape public opinions [ 24 , 20 ].However, Sancha et al. [20] conclude that the normative pressure by such groups does not significantly influence supplier sustainability practices of the focal firm.They find the reasoning to be the same as in the case of coercive pressures, there seems to be a normative pressure on focal firms about their own practices but not about their suppliers'.
Customer demand.Focal firms are observing increased customer demand for products that are responsibly made throughout the supply chain.Trudel and Cotte [25] synthesize 30 years of research on socially conscious consumerism and observe a definite increase in the group of consumers interested in responsibly made products (products made through sustainable practices across the supply chain).Fan et al. [26] show that sustainable supplier improvement improves focal firm's sales performance.
Values/Culture.Firms are interested in improving their suppliers' sustainability due to their organizational culture and belief system (see the Ikea case study [27] ).Several CEOs now recognize the purpose of their organization as stakeholder focus rather than just shareholder value maximization.They want to ensure sustainability across their sphere of influence including suppliers [6] .
Financial Benefits.Improved sustainability practices at suppliers may result in a financial benefit to the focal firm.For example, lower fuel/energy consumption resulting in lower costs for suppliers may result in lower purchase prices for the focal firm.Hollos et al. [28] observe that sustainable supplier co-operation related to the environmental dimension has helped reduce costs for the focal firm.Carter et al. [29] provide initial evidence of a positive link between environmental purchasing and financial performance.Yu and Huo [30] find that green supplier management leads to a better financial performance of focal firms.
Psychological Loss.Serious sustainability incidents that occur at a supplier may negatively impact the morale of employees at the focal firm, and damage the firm's self-perception [19] .

Measuring supplier sustainability
To improve supplier sustainability, focal firms need to measure it first.Measuring supplier sustainability involves defining the measurement scope, agreeing on measurement standards, and conducting supplier audits to validate actual performance against the standards.
In the case of product sustainability, one can inspect the product from a supplier and get perfect information about its sustainability.Measuring process sustainability is not that straightforward and is effortintensive.As we will see further in this paper, suppliers may have the incentive to hide information from auditors.From the focal firm's perspective, the efforts required for measurement and information accuracy are key considerations in the measurement process.

Measurement scope
Focal firm needs to define the scope when it sets to design a measure for its suppliers' sustainability performance.Scope should specify the sustainability dimensions to cover (Economical, Social, Environmental dimensions as defined by Elkington [31] ), and what measurement parameters should go under each of these three pillars.
Scope should also specify which suppliers to cover.Focal firms can concern themselves with sustainability at the first-tier suppliers or extend their scope to second or third-tier suppliers.Tachizawa and Yew Wong [32] present an excellent review of 39 research papers specifically discussing the sustainability of second and third-tier suppliers.They review the importance of going beyond first-tier suppliers, focal firm's practices related to sub-suppliers, and its engagement approaches.
Extending beyond first-tier suppliers is always desirable but comes with its own challenges.Large firms with thousands of suppliers may not have the resources to manage sustainability beyond first-tier suppliers and may need to prioritize even among the first-tier suppliers.Even if the focal firm has the resources, lack of contractual agreement and often transaction-based relationships with second and third-tier suppliers can restrict the focal firm's influence [33] .Also, first-tier suppliers may be reluctant to share information about their suppliers due to the risk of supplier bypassing.Another challenge is unauthorised subcontracting by first-tier suppliers where the focal firm loses visibility completely [34] .Kazançoglu et al. [35] discuss criteria for diffusing sustainability standards throughout multi-tier supply chains.

Measurement standards
Focal firms use standards as an instrument to govern their supplier's sustainability performance.Various standards are developed by international entities (public standards/certifications), by industry organizations (industry-specific code of conduct), and by focal firms themselves (private standards/supplier code of conduct).
Public standards / Certifications.These have the benefit of lower asset specificity which is preferred by suppliers (Asif et al. [36] ).For example, if a supplier has ISO certification, it can be presented to multiple buyers.This is not true if the supplier adheres to the firm-specific code of conduct.Standardization has the benefit of lower transactional costs for focal firms (not having to develop and manage their own standards) but there is also a downside to this.For example, Schwartz and Tilling [37] argue that in an attempt to standardize corporate social responsibility, ISO 26000 overlooks organizational and cultural contexts.The legitimacy of standards and the factors driving the legitimacy is another important consideration [38] .In case of auditable certifications like ISO 14000/14001, certification is meant to be evidence for the conformance of standards.However, sometimes companies stop continuously complying with standard requirements after completing the third-party audit and getting the certificate [39] .
Private codes of conduct.These are issued by individual firms.Due to their content specificity for a focal firm, they are the most prevalent measures to achieve sustainable improvement besides regulations [40] .Schleper et al. [40] cite reviews of 233 codes [41] and 157 codes [42] .As discussed earlier, from the supplier's point of view, lower asset specificity is preferable.
Industry standards.These are midway between public and private standards.They have the advantage of more legitimacy, reduced transactional cost, and better enforcement compared to private standards (Sullivan [43] cited by Schleper et al. [40] ).An interesting case study by Rosen et al. [44] investigate two industry standards for supplier environmental performance and why one was adopted more widely than the other.They list remediability and legitimacy as the key factors influencing the adoption of industry-standard among suppliers.

Audits
Supplier's compliance with standards/certifications/codes of conduct is ensured through audits.Audits can be conducted by suppliers themselves (first-party), by focal firms (second-party), or by auditing firms (third-party).Often focal firms limit their supplier sustainability management to code of conducts and supplier audits.However, auditing alone does not improve supplier sustainability [ 45 , 46 ].In our experience with our partner focal firm, the auditing process presents many problems like low gage repeatability and reproducibility, lack of internal ownership and engagement when relying on external audits, and most importantly, scalability issues in auditing tens of thousands of suppliers (effort and cost investments).
Auditing efforts sometimes can even be counter-productive and result in audit fraud.Increasing auditing and publicizing negative audit reports motivates a supplier to exert greater effort to pass the focal firm's audit by hiding information rather than taking care to prevent harm [47] .When using third-party auditing, conflict of interest worsens the fraud.Auditors have a low incentive to reject bribery from suppliers [48] .If focal firms offer a bonus structure to auditors in order to prevent bribery, this can incentivize auditors to create false negatives in their auditing reports [49] .Levying penalties on auditors and suppliers when such collusions are detected and incorporating these conditions into the contract can help, especially in high-risk countries [50] .
From the suppliers' point of view, they have to face multiple individual audits by multiple buyers who enforce different codes of conduct.This results in audit fatigue and diverts suppliers' attention from actual improvement.In context of COVID-19, frameworks for auditing are evolving and concepts like remote auditing are being studied [51] .

Measurement systems
Focal firms implementing performance measurement systems for supplier sustainability should consider three steps.First, they have to properly define the scope of what needs to be measured.Second, they have to identify the party responsible for this measurement.Finally, they should also properly lay out how the measurements process will run.This structured approach covers WHAT, WHO, and HOW of measurement and is discussed in Beske-Janssen et al. [52] .Focal firms should also assess the maturity of their measurement systems by examining their design, implementation, and use [53] .External organizations, such as the Dow Jones [54] , also assess the measurement systems of focal firms.

Analyzing supplier sustainability performance
Once focal firms have the supplier sustainability performance data, they need to analyze it.They need to understand why some suppliers perform better than others and what the influencing factors are behind the performance.This analysis enables focal firms to identify key areas of improvement to focus future efforts.Supplier segmentation often goes hand-in-hand with supplier sustainability performance analysis.When comparing sustainability scores, supplier segmentation can provide important context e.g.Transportation suppliers may perform poorly on en- [61] , W-Wittstruck and Teuteberg [62] , M-Mette [27] , AS-Ahmed and Shafiq [63] .
vironmental parameters like CO2 emissions by the nature of their business processes.
We observe that SSM literature has largely focused on analyzing the influencing factors mentioned above.It lacks in providing an analytical framework that consists of scoring mechanisms, maturity assessment frameworks, and score trend analysis.We propose a future research direction in this area in Section 8.4 .

Influencing factors
Majority of research for influencing factors of sustainability is generic for any organization.But some research papers specifically investigate the factors influencing a supplier's sustainability.As argued by Foerstl et al. [55] , suppliers typically face less regulatory pressure than the organizations selling to end customers.Suppliers that act in B2B settings generally do not have big brand names and face less pressure from the public or NGOs.It is therefore important to understand the factors influencing supplier sustainability in the focal firm-supplier dyad.
Table 2 shows a comprehensive list of influencing factors identified in our literature review [ 45 , 56-62 , 27 , 63 ].However, with the exception of Locke et al. [45] and Villena et al. [64] , this research does not link the influencing factors to the measured and verified supplier sustainability performance scores to understand the extent of influence each factor has on supplier sustainability.

Improving Supplier sustainability
After measuring and analyzing supplier sustainability performance, focal firms can improve supplier sustainability with mechanisms covered in the sub-sections below.Focal firms are moving away from only monitoring suppliers and the trend is to collaborate with suppliers to improve sustainability.Choosing suppliers and setting improvement goals is only the first step for focal firms.For meaningful progress, it is important to have effective collaboration by ensuring a win-win for all involved parties considering risks, liabilities, and gains for each.It is also important to invest optimally when multiple supplier sustainability improvement options are possible.We observe that research is lacking in this area and we propose a future research direction in Section 8.2 to address this gap.

Sustainable supplier selection
Focal firms can incorporate sustainability criteria during the new supplier selection process, thereby selecting suppliers that are already at a certain sustainability level.This helps focal firms meet their supplier sustainability targets.However, this action does not increase the sustainability performance of existing suppliers.Sustainability is a moving goal and focal firms need to pursue continuous improvement for the same.Adding sustainability criteria in the new supplier selection process alone is not sufficient.
Key literature on sustainable supplier selection [65][66][67][68][69] is covered in the literature review by Wetzstein et al. [70] .This is summarized in Table 3 .It is obvious from the number of research papers that this is a very popular research area.
But there is little evidence of the transfer of this research to the realworld [65] .Also, a majority of these papers only focus on environmental aspects in supplier selection [68] with only a few considering all three dimensions of sustainability [69] .Furthermore, the sustainability criteria used in supplier selection do not refer to the taxonomies of criteria [69] .Reviews indicate that the methodologies used in sustainable supplier selection research are predominantly multi-criteria decision making, mathematical programming, and artificial intelligence.

Penalties and rewards
Supplier improvement research (not specific to sustainability) points out that penalties and rewards have a direct relation with supplier performance [71] .Focal firms may use penalties or rewards for suppliers based on their compliance with agreed sustainability standards.Porteous et al. [72] investigate which penalties (e.g.termination of contracts, reduced business, fines) and which incentives (e.g.increased business, preferred supplier status, price premiums, public recognition) are positively associated with reduced sustainability violations and operating costs.Discuss methodologies and environmental criteria used in various papers as well as if these papers were backed by a real-world application.Igarashi et al. [66] 1991-2011 60 Discuss research type (analytical vs empirical), supply chain context (first vs second-tier supplier), environmental criteria considered in various papers.Also, discuss which steps in the supplier selection process incorporate environmental criteria Lee et al. [67] 1997-2009 20 Discuss the environmental criteria used in various papers along with the methodology used.Govindan et al. [68] 1997-2011 32 Focus on green supplier selection papers using multi-criteria decision-making techniques.Discuss methodology (individual/Integrated approach) and criteria used.Tuczek and Wakolbinger [69] 2000-2015 62 Discuss sustainability dimension (Environmental, Social, and Economic; or a Combination), Methodology (single/hybrid approaches), Environmental criteria and industry considered in various research papers.Chen and Lee [75] Aral et al. [76] Price premium X X Termination/ Financial penalty X Delayed payment X Bid markup increase X The penalty and reward mechanism needs a strong measuring mechanism, resulting in high audit (monitoring) costs for the focal firm.Auditing costs drive the extent of auditing efforts that in turn drive information accuracy.Thus, focal firms need to manage the trade-off between auditing costs, reward costs, and non-compliance costs.Suppliers need to manage the trade-off between penalties, rewards, and sustainability compliance costs.
Many research papers factor in the trade-offs mentioned above and develop game theory-based models to propose strategies for focal firms and suppliers.They use different rewards and penalties and deal with different scenarios, but their basic premise is the same.Cho et al. [73] develop strategies for the focal firm by balancing auditing efforts and wholesale prices offered to suppliers to ensure a child-labor-free supply chain.Zarei et al. [74] use the same premise of balancing auditing efforts and wholesale price.Plambeck and Taylor [47] show that termination of sourcing can reliably improve supplier's sustainability efforts but the penalties may be counter-productive (supplier will try to hide information during an audit).Chen and Lee [75] develop a model in cases where the relationship with suppliers is short-term in a lowregulation environment.This model proposes a delayed payment contract to mitigate supplier sustainability risk to focal firm when sustainability information may be incomplete/imperfect.Aral et al. [76] analyze trade-offs between auditing costs and supplier un-sustainability and show how focal firms can incorporate supplier un-sustainability in the supplier auction process as a bid markup.These papers are summarized in Table 4 .

Mandatory disclosures
While large corporations in developed countries are mandated by regulations to disclose their own environmental and social performance, usually they are not mandated to disclose about their suppliers.For example, the EU regulations on CO2 emissions do not require firms to disclose their supplier performance.There are some exceptions to this e.g.California's Transparency in Supply Chains Law.Suppliers, either small or in developing countries, are usually not mandated by regulations to disclose this information.
Focal firms may voluntarily disclose their suppliers' environmental and social performance.For example, following the Rana Plaza disaster of 2013, several apparel retailers started to publicly disclose all audit reports of their Bangladeshi suppliers.Public disclosure is expected to induce suppliers to perform better, but there is also a possibility that it may deter suppliers from revealing their sustainability issues due to a fear of negative disclosure and business consequences.To avoid this, focal firms should put more emphasis on supplier's transparency in the initial phase, rather than focusing on supplier's disclosed issues [77] .
Public disclosure of supplier's sustainability performance can also help the focal firm in terms of increased valuation, market share, and trust in the brand [77] .However, there are risks like negative consumer/investor response or revealing too much about its supply chain to its competitors [78] .Kraft et al. [79] , Kalkanci and Plambeck [80] have proposed game-theoretic models that can help focal firms to decide whether or not to disclose under different scenarios.
As with all sustainability-related reporting, supplier sustainability disclosures need to be reported in a consistent and standardized way for effective verification and comparison [81] .

Supplier improvement through collaboration
Knowledge transfer.Going beyond mandating the compliance and inflicting penalties, focal firms can collaborate with suppliers through knowledge transfer and training.When Nike transferred its knowledge to suppliers for improved scheduling, efficiency, and quality, the number of supplier noncompliance incidents fell significantly [45] .Existing research has empirically proven the effectiveness of Knowledge transfer and training on supplier sustainability performance [82][83][84] .
Direct Investment.Focal firms can also directly invest in a supplier to improve its sustainability performance, e.g.Walmart has provided 50 million dollar loans to its Bangladeshi suppliers to improve their workplace safety.Often focal firms have multiple investment options available in terms of different sustainability projects with different suppliers but have a constrained investment budget.There is limited research to help focal firms on optimal investment.Some papers have proposed simple mathematical programming models for sustainability project portfolio optimization [85] .Others have proposed group decision-making methods [ 86 , 87 ].But these are missing some realistic considerations, like the risk of not investing in a specific improvement action.
Joint Investment.Focal firms and suppliers can make a joint investment for sustainability improvement.Recently, Apple and 10 of its suppliers in China have created a joint investment fund of $300 million for renewable energy projects and have invested in three wind farms [88] .In such cases, game theory can be used to decide the optimal investment by each player and to share benefits fairly among the players.
If high sustainability level of suppliers results in increased demand/revenue for end products (marketed as responsibility manufactured product), focal firms can agree to pass benefits to a supplier.This  [100] can be done through better wholesale price or revenue-sharing contract.Xiao et al. [89] , Ghosh and Shah [90] , Panda et al. [91] , Xu et al. [92] , Ni and Li [93] , Hsueh [94] develop various game theory-based models where focal firms and suppliers jointly, or suppliers separately, determine the optimal sustainability investments, wholesale prices or revenue sharing contracts.
Effectiveness of improvement Initiatives.When focal firms invest time and money for supplier sustainability programs, it is important to measure the effectiveness of such initiatives.Dou et al. [95] propose a multicriteria decision-making model to evaluate improvement programs considering environmental outcomes, operational performance, organizational alignment, supplier involvement propensity.Fu et al. [96] propose an evaluation model using DEMATEL (Decision-Making Trial and Evaluation Laboratory).

Network collaboration
Network collaboration means going beyond supplier-focal firm dyads and exploring synergies among different players to improve supplier sustainability.Several focal firms from the same industry may share suppliers and can collaborate to avoid duplication of efforts.Joint audits of shared suppliers by multiple buyer firms have shown benefits.Improvement in supplier compliance, reduced goal conflict, increased effectiveness and efficiency of assessment, reduced information asymmetry is shown in Lechler et al. [97] , Caro et al. [34] .A shared technology platform for sustainability data is shown to be beneficial for a network of focal firms and suppliers [98] .
Network collaboration can overcome inefficiencies in a competitive environment.Some of these are counterintuitive.For example, Chen et al. [99] show that if two competing focal firms have a shared supplier and if focal firms are not collaborating, they both end up focusing less on the shared supplier.Network collaboration helps overcome this.
NGOs have traditionally played a confrontational role with focal firms regarding the sustainable practices of their suppliers.But recently NGOs have started collaborating with focal firms.Strategic alliances comprising of multiple focal firms, suppliers, NGOs, multilateral organizations are being formed [97] , and their effectiveness, governance, and the inter-organizational fit is being researched [ 100 , 101 ].Network collaboration research is summarized in Table 5 .

Governance and management of supplier sustainability
The effectiveness of a collaborative approach in SSM has been demonstrated by many studies [9] .The MMAIG proposed in this paper can help focal firms to move to a collaborative approach.Focal firms may feel daunted by the at-one-go implementation of the MMAIG for its entire supplier base.Some firms may not have the resources to roll out MAI (Measure-Analyze-Improve) to their entire or even the first-tier supplier base.In such cases, focal firms can use different governance styles for different suppliers and move towards a collaborative governance over a period of time.
Vurro et al. [102] propose four different governance styles: Transactional, Dictatorial, Acquiescent, and Participative/Collaborative.They propose using them based on the factors like power or influence of focal firm, and network density (degree of ties between actors in the network).
Application of this governance model is explored in a case study by Li et al. [103] for fast-fashion retailer H&M.
Hajmohammad and Vachon [104] propose four governance styles geared towards risk acceptance, risk avoidance, risk mitigation through monitoring, and risk mitigation through collaboration.They propose the use of these styles based on perceived risk levels of suppliers and buyersupplier dependence structure.
As we saw in Section 6.5 , collaborating parties in SSM are going beyond supplier-focal firm dyads, and these networks require different governance styles.Tachizawa and Wong [105] take a more networkfocused approach for governance.They propose formal and informal governance styles depending on supply network complexity, density, and centralization (degree of power in some nodes).Network governance is also assisted by the emergence of multi-stakeholder technologies, like blockchains, that allow governance to be more poly-centric.
These strategic directions generated by governance are converted into actions by effective management systems in the focal firm.The research investigating management systems for supplier sustainability is limited.Mamic [106] discusses centralized/decentralized management of SSM and the role of different departments/functional groups (Corporate Social Responsibility, Purchasing, Human Resources, etc.).Krause et al. [107] discuss the role of the purchasing department in supplier sustainability.They propose to amend Kraljic's matrix (strategic, bottleneck, leverage, and non-critical) to include sustainability criteria.

Trends and future research directions
In the above sections, we explored the five subtopics under SSM and were able to identify the limitations of the current state of art.In this section, we propose several future research directions based on these limitations.These are summarized under the MMAIG in Fig. 3 .
Trends and future research directions for the wider topic of Sustainable Supply Chain Management (SSCM) have been discussed in several papers [ 10 , 108 , 109 ].Some of these also provide directions for SSM.Network collaboration, shared audits, behavioral research, use of emerging technology are some of the SSM future research directions that appear in multiple papers.While others suggest broad areas for research, we aim to be more specific based on our industry experience.As an example, existing papers have mentioned the growing trend of interdisciplinary research in SSCM and have proposed studying behavioral aspects of supplier sustainability performance.Our proposal is more specific about investigating intrinsic motivations of the focal firm and its suppliers by employing behavioral theories like the Theory of Planned Behavior.We also avoid generic research proposals, such as "use emerging technologies like the blockchain ".Blockchains can be used in several settings that involve multiple stakeholders collaborating with each other, and it can prove useful in SSM.However, to propose a specific research direction, we would need to point out a problem that is unique to SSM and can be addressed by blockchain.
We propose research directions to address existing research gaps, but they are also motivated by the UN sustainability goals around decent work, economical growth, and climate action.Managing supplier sustainability would be critical in meeting these goals.

Going beyond auditing
Existing research brings out several limitations of the auditing approach in getting reliable information on supplier sustainability.We have empirical evidence that auditing is not an effective method to improve supplier sustainability (see Section 4.3 ).Although some suggestions have been made to improve audits (such as conducting shared audits to remove audit fatigue), these still use the auditing approach.The research lacks in exploring alternatives to auditing and in verifying the effectiveness of such alternatives.
One alternative to auditing can be asking the suppliers to provide a structured self-assessment of their sustainability, backed by documen-tary evidence.For such an approach to work, the suppliers also need to be assured that they will not be penalized for any sustainability issues they reveal to the buyer.The motivation of the buyer should be to work together with its suppliers to address those issues.This can be done via knowledge sharing, financial support, and operational support.Companies like Philips have been using this approach for several years.They find that it not only saves auditing efforts but also brings much better visibility of supplier sustainability issues through open and honest collaboration.The effectiveness of these alternatives to auditing must be studied, and the expected decrease in efforts must be substantiated.Self-assessment has been explored in other research streams [ 110 , 111 ], we can leverage this research and apply it to study supplier sustainability.The key concerns around self-assessment are validity and reliability, which need to be empirically investigated.In particular, we need to address four issues: 1. Does self-assessment provide valid evidence about supplier sustainability performance?2. Does the accuracy of self-assessment vary across different dimensions or topics of sustainability?3. Does accuracy improve over time if a supplier is part of many self-assessment cycles?4. Is there a variation in self-reporting accuracy among suppliers?
If so, can this variation be explained through buyer-supplier relationship, supplier context, and other characteristics?

Optimizing direct/joint investments
Supplier improvement through direct/joint investment has been studied [112] where the focus is on financial or strategic outcomes from the improvement.When it comes to investment for sustainability, focal firm's non-financial goals are equally at play (see Section 3 ).Thus, investment decisions should be made considering trade-offs between financial and non-financial goals.These non-financial goals (improvement in sustainability levels of suppliers) are also harder to predict with certainty.We propose two research directions considering this.
Optimizing direct investment.When the focal firm has the option of investing in multiple sustainability improvement projects with multiple suppliers, it needs a methodology to select an optimal project portfolio for its direct investments.There are a handful of papers that address the improvement project selection and budget allocation with simple mathematical programming models (discussed in Section 6.4 ).But these miss out on important considerations and may not provide actionable insights in real-world scenarios.
We identify three key considerations.First, uncertainty in investments and returns is a typical feature of sustainability improvement projects.Deterministic models that ignore this uncertainty will not serve the purpose.Secondly, selecting sustainability projects involves chasing multiple and sometimes conflicting goals.Firms want to select projects to maximize financial returns and expected sustainability improvement, and minimize the risks from non-action.Models optimizing only one goal might prove too simplistic for practical use.And finally, selecting improvement projects goes hand-in-hand with selecting suppliers with whom these projects will be implemented.This makes supplier characteristics very important.Firms have to consider the size of their business relationship with the supplier, the supplier's current sustainability performance and their propensity to adopt sustainable practices.This exposes the firm to several trade-offs.Should they invest in a project with high financial returns with a supplier that is a late adopter of sustainable practices?Or should they invest in a project with lower financial returns with a supplier that is willing and an early adopter of sustainable practices?
These complex trade-offs are the main reason why focal firms need support from analytical models.Ignoring some or all of these trade-offs will defeat the purpose of building a model in the first place.We propose that future work should integrate sustainability research with project selection and portfolio optimization research.It should consider budget constraints (on money, time, and skills), and uncertainties in investment and returns.It should consider trade-offs between financial goals, sustainability goals, and risk minimization goals (risks of non-actions, risks from uncertainty).And it should also consider behavioral aspects like suppliers' inclination towards sustainable practices, and their willingness in adopting sustainable practices.These details are laid out in Fig. 2 for additional clarity.Several different methodologies such as multi-criteria decision making (MCDM), stochastic programming, and simulation-based optimization can be used to address these issues.
Optimizing joint investment.Another area for future research is optimizing investment in projects jointly funded by the focal firm and its suppliers.Both firms need to achieve a win-win solution, acceptably close to each party's individually determined optimal investment.We discussed a few analytical models in Section 6.4 that address this.
But when the focal firm is in negotiations with several suppliers on various joint investment projects, the problem becomes dynamic.What is an acceptably close solution for the focal firm is no longer a static value.It depends on and varies with how negotiations are going on with other suppliers.This is a practical scenario that requires more academic attention.We propose that new research should incorporate negotiation frameworks and disagreement dynamics to support the focal firm (and supplier) decisions.A game theoretic approach will be necessary to solve such a problem.

Enabling internal and external collaboration
External collaboration.Recurrent themes in SSM research are the lack of uniformity in standards and the challenges in establishing the legitimacy of these standards.Research also talks about duplication of efforts in governing these standards and exerting improvement efforts.Network collaboration of NGOs, regulatory bodies, industry associations, and focal firms with common suppliers can address all of these issues (as discussed in Section 6.5 ).But it is crucial to understand the conditions under which such collaboration can take place.Unless this is well researched, network collaboration will largely remain a theoretical concept with few real-life applications.Even though the parties involved are pursuing the same goals, there can be inherent conflicts of interests or hidden risks in collaboration.These conflicts and risks need to be properly addressed by sharing both efforts and benefits among the stakeholders.This needs further investigation.
Internal collaboration.So far, we have discussed the focal firmsupplier collaboration, network collaboration.Research often misses the aspect of internal collaboration within the various departments of a focal firm.Supplier sustainability is a cross-functional area that needs the involvement of purchasing, sustainability, corporate social responsibility (CSR), legal, marketing, and corporate strategy departments.While the role of internal collaboration is acknowledged by firms, effective cross-functional collaboration is often lacking in practice.Sustainability largely remains the responsibility of the sustainability department, if at all.
Internal collaboration faces challenges like goal conflict between different departments e.g.purchasing department's cost focus vs. sustainability department's sustainability focus.Existing research [106] has started to take notice of these problems, but more research is needed to examine the interplay between circumstances and behaviors that hinder or promote effective collaboration processes.This knowledge is necessary to be able to understand how focal firms can facilitate effective collaborative relationships.

Making data-driven plans for improvement
To pick effective improvement actions, focal firms should analyze the supplier sustainability measurement data.This analysis helps in deciding where to exert improvement efforts for maximum progress.As we have discussed in Section 5.1 , existing research in this area is scarce.We propose two core research directions.First, we need a framework that lays out different sustainability performance metrics and demonstrates the analytical methods that can generate them.To begin with, a scoring mechanism is needed to enable comparisons and benchmarking.An appropriate scoring mechanism should give good visibility over a large dataset but avoid oversimplification.It may be useful to look at green supplier selection research that uses multi-criteria decision-making extensively.These methodologies can be leveraged for final scoring.A review of green supplier selection by Tuczek and Wakolbinger [69] highlights several analytical methodologies used.These include multi-criteria methods (AHP-analytic hierarchy process, ANP-analytic network process, DEA-data envelopment analysis, TOPSIS-technique for order preference by similarity to ideal solution), artificial intelligence methods (ANN-artificial neural network, FST-fuzzy set theory), and a combination of these.
Apart from scoring, the analytical framework should also include a maturity assessment of suppliers, year over year score trend analysis, and prediction of future performance using statistical analyses or newer advanced machine learning methods.This type of analysis will help focal firms in deciding appropriate improvement actions for suppliers e.g. a supplier with a low sustainability score, but good year-on-year improvement can be chosen for improvement by the focal firm.
Second, research should focus on exploring the link between supplier characteristics and the measured (or validated) suppler sustainability score.This can be used to predict the performance of suppliers not being measured currently.This is a very practical problem-focal firms usually have many suppliers, but may only have data on a small subset of them.Supervised machine learning algorithms can be used for this analysis.

Using supply chain finance instruments
As we discuss in Section 6.2, some firms provide incentives to suppliers based on suppliers' sustainability performance (like premium pricing).Firms can also collaborate with financial institutions to provide incentives like vendor financing or supplier loans [113] e.g.PUMA and BNP Paribas Bank have launched a supply chain finance (SCF) program that provides lower-priced short-term financing for PUMA suppliers that achieve a high sustainability score.In recent years, many financial institutions are committing to sustainable investments [114] .This has led to a 20% year-on-year expansion in SCF instruments [115] , making it a very promising research area.Future research should explore and compare the different SCF instruments that can be used to promote supplier sustainability.Each instrument should also be investigated on its effectiveness, and understand how it can be implemented in practice.

Considering behavioral aspects in SSM
We have discussed various enablers and barriers of SSM in Section 5.1 .Eventually, it is the decision makers at focal firms and supplier firms who face these enablers and barriers.Their behavior may not always be rational, since it is influenced by social preferences, personal factors, and context, among others [116] .Therefore understanding the influence of these behavioral aspects is key to successful SSM implementation.While there is a growing trend of interdisciplinary research in sustainable operations [10] , considering behavioral aspects is relatively new.We propose the following three research directions in this area.
First, behavioral research is needed to investigate the intrinsic motivation of focal firm employees and managers.So far, in discussing focal firm's motivation for supplier sustainability (see Section 3 ), we have argued that the majority of these drivers are directly linked to focal firm's self-interests (extrinsic motivations).However, intrinsic motivation is often more powerful than extrinsic motivation and can overcome imperfect implementation issues.This is a budding research area, and Swaim et al. [117] take the initial steps toward the understanding role of motivation in sustainability behavior.We propose a research direction to explore the factors that drive intrinsic motivation for supplier sustainability using established theories, such as the theory of planned behavior.We need to understand how focal firm policies and other mechanisms influence employee motivation for supplier sustainability.This is important because suppliers and focal firms can be geographically and culturally distinct, meaning the intrinsic motivation of focal firm employees for supplier sustainability is likely to be less.
Second, building on the first proposal, behavioral research is needed to explore the intrinsic motivation of the suppliers to engage in sustainable practices.Unlike focal firms, small suppliers do not have many extrinsic motivations for sustainability.Suppliers do not face significant regulatory, normative, and mimetic pressures to be sustainable, nor do they fear brand and reputation risks [55] .Thus, investigating mechanisms that increase intrinsic motivation is more important for the supplier than for the focal firm.
And third, research should also focus on the role of moral norms.Current research looks at auditing and penalties through utilitarian functions, and how these force suppliers to be more sustainable.As we have seen in Section 4.3 , these strategies can be counter-productive, and suppliers may engage in hiding information.Research is needed to check the impact of audits and penalties on moral norms.When suppliers adhere to sustainability standards due to moral norms rather than punishments, it may make the improvement process impervious to sub-optimal monitoring and enforcement processes [118] .

Integrating supplier sustainability in organization strategy
Large firms have started to look at sustainability as an opportunity for competitive advantage, cost reduction, and marketing, instead of treating it as a compliance issue.As a result, many of these firms have started to integrate their sustainability targets with their strategic goals.This enables them to leverage their existing operational excellence efforts like lean manufacturing, TQM (Total Quality Management), and six sigma to optimally achieve these goals [119] .
Empirical studies have shown an association between a firm's TQM capabilities and its sustainability performance [ 120 , 121 ].Furthermore, studies have shown that combined implementation of operational excellence practices and sustainability initiatives results in a greater impact on the firm's environmental and social performance than treating sustainability initiatives as stand-alone target [122] .However, these studies have been limited to an organization's own sustainability performance and not to the organization's supplier sustainability performance.
We propose that future research should explore commonalities between operational approach for supplier sustainability and existing operational excellence efforts, such as lean and six sigma.Operational excellence efforts involve supplier partnership and improvement, therefore the possibility of extending these frameworks to include supplier sustainability goals should be explored.Some of the operational excel-lence goals like waste reduction and efficiency gains may already be aligned with sustainability goals.Future research can look if there are any conflicting goals, and methods to resolve these conflicts aligning with an overall strategy.Concepts like TQEM (Total Quality Environment Management) leverage the TQM principles to meet environmental goals.Our proposal is to explore whether environmental and operational goals can be managed together, combining processes, tools, and methodologies to achieve both.

Knowledge management in SSM
We have discussed knowledge sharing and training of suppliers as a supplier sustainability improvement mechanism in Section 6.4 .Apart from acknowledging the usefulness of knowledge sharing in SSM, existing literature does not explore this further.On the other hand, knowledge management in supply chains has received wider attention (see literature review by Marra et al. [123] ).Knowledge accumulation, diffusion and obsolescence in global supply chain networks and influencing factors like geographic, cultural, relational closeness, fear of competition, role of transparency and reciprocity have been explored theoretically and statistically [124] .We propose that future research should establish a link between effective knowledge management and supplier sustainability performance, and also explore the drivers and barriers to effective knowledge management in the specific context of SSM.

Conclusion
In this paper, we have reviewed the Supplier Sustainability Management (SSM) literature structuring it under the MMAIG framework (Motivation-Measure-Analyze-Improve-Govern).There are several research papers focusing on supplier sustainability measurement, standards, and audits.However, we observed a limited research on what focal firms can do after they have measured the supplier sustainability performance.Multinationals with thousands of suppliers are collecting large amounts of data on their suppliers' sustainability performance through audits.They need methods and models to use this data effectively in driving real change.We propose several future research directions to calculate overall scores, assess maturity, detect trends in performance, and analyze the underlying factors driving sustainability performance.
This analysis will naturally lead to specific actions for improvement.Although the literature about supplier sustainability improvement is scarce, it is not so in practice.Several large organizations have supplier sustainability departments that have allocated budgets for improvement programs.We propose research directions that can help organizations in optimizing their investment towards supplier sustainability improvement considering financial/non-financial goals, risks, uncertainties, and constraints involved.Focal firms are moving towards a more collaborative approach for supplier sustainability improvement, but there is a lack of a collaboration framework in research and in practice.We propose research direction to enable collaborative framework.This includes methods for choosing the best-collaborating partners, making optimal joint investments, agreeing on benefit-sharing, and using negotiation strategies.
Our proposed future research directions are summarized in Fig. 3 .Together with existing research, we believe this provides a robust framework for SSM.
Our focus has been on process sustainability and we have included literature covering all dimensions of sustainability (social, environmental, economic).Our observation is consistent with other reviews that the integration of social aspects of sustainability in SSM is less prevalent than environmental aspects.
Managerial Implications of our review are as follows.
• Measurement of supplier sustainability performance alone is not sufficient in bringing improvement in supplier sustainability.Focal firms need to use different improvement mechanisms based on collaboration e.g.joint improvement projects, knowledge sharing, and direct investment.• Though not sufficient in itself, measurement of supplier sustainability performance is an important first step towards improvement.Traditional approaches of supplier sustainability measurement (audits) are effort-intensive, and yet ineffective (and sometimes even counter-productive) in getting accurate information.To address this, focal firms need to change their overall approach to SSM-from playing a big brother (dictating compliance, forcing corrective actions, using penalties), to a collaborative approach based on reciprocity and trust that puts a premium on suppliers' transparency.• Before jumping to improvement actions, focal firms need to analyze the measurement data to understand the underlying factors driving this data.This data-driven approach will help them in exerting improvement efforts in the right area.• Focal firms need to have a clear methodology for selecting suppliers and projects for sustainability improvement and allocating investment in an optimal way.They should also have a clear objective in terms of what they want to achieve through improvement actions.• Focal firms need to explore network collaboration (collaboration with other focal firms, NGOs, and governments) to avoid duplication of efforts in terms of establishing standards, collecting data & information, conducting audits, and exerting improvement efforts.

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.

Fig. 1 .
Fig. 1.Review scope and classification structure for Supplier Sustainability Management.

Table 1
Focal firm's drivers for supplier sustainability.

Table 2
Influencing factors for supplier sustainability.

Table 3
Sustainable supplier selection literature.

Table 4
Penalty/Reward mechanisms for improved supplier sustainability performance.

Table 5
Overview of research in network collaboration for supplier sustainability.