Humble Leadership Affects Organizational Citizenship Behavior: The Sequential Mediating Effect of Strengths Use and Job Crafting

The purpose of the current study is to investigate the impact of humble leadership on organizational citizenship behavior (OCB) and the sequential mediation effect of strengths use and job crafting on the relationship. Data were collected at two points in time, spaced by a 2-week interval, with a sample of 260 employees working in a hospital in Beijing, China. Structural equation modeling was applied to test our predictions. The results indicated that humble leadership is positively related to OCB; the relationship between humble leadership and OCB was mediated not only by job crafting but also by strengths use and job crafting (sequential mediation). However, the indirect effect of humble leadership on OCB through the mechanism of strengths use was not statistically significant. This study advances the theory and research on the leadership–OCB relationship.


INTRODUCTION
More than five decades ago, Katz (1964) suggested that a class of discretionary and spontaneous behaviors beyond explicit role requirements is of great importance for organizational effectiveness. Smith et al. (1983) named such behaviors as "organizational citizenship behavior" (OCB). Subsequently, OCB was defined as "individual behavior that is discretionary, not directly or explicitly recognized by the formal reward system, and that in the aggregate promotes the effective functioning of the organization" (Organ, 1988, p. 4). OCB has received a great deal of attention among practitioners and scholars due to its positive effect on employees' performance (e.g., Podsakoff and Mackenzie, 1997;Becton et al., 2017). Given the importance of OCB for organizational effectiveness (Podsakoff and Mackenzie, 1997), numerous researchers have directed their research interests toward the antecedents to OCB.
To date, extant research has identified many determinants of OCB such as perception of organizational politics (Khan et al., 2019), emotional intelligence (Lim et al., 2018), perceived organizational support (Dai et al., 2018), psychological contract fulfillment (Ahmad and Zafar, 2018), and ethical leadership (Yang and Wei, 2017;Mostafa, 2018). However, little is known about whether humble leadership can affect OCB, and the potential mechanisms accounting for this relationship is underdeveloped. We thus aim to redress these gaps by proposing a sequential mediation model.
Based on the existing literature on humble leadership and OCB, we first expect that humble leadership is related to higher levels of OCB, because subordinates who follow humble leadership tend to experience high levels of self-efficacy (Wang et al., 2018), with meaning in work and self-determination  as significant predictors of OCB (Schlechter and Maharaj, 2007;Yan and Chen, 2013;Kao, 2017). The second purpose of this study is to examine the mediating role of strengths use in the humble leadership-OCB linkage. Owens and Hekman (2012) have suggested that humble leadership is characterized by appreciating subordinates' strengths, which can motivate subordinates to leverage their strengths at work (Govindji and Linley, 2007), ultimately resulting in an enhanced OCB (Van Woerkom and Meyers, 2015). In addition, we also test whether job crafting acts as a crucial mediator between humble leadership and OCB. The reason for this aim is that job crafting, as a proactive behavior which employees take to improve person-job fit (Wrzesniewski and Dutton, 2001;Tims et al., 2012), might benefit more from humble leadership  and can lead to increased OCB (Bavik et al., 2017). Importantly, since strengths use is associated with job crafting (Plomp et al., 2016), we also examine whether humble leadership will affect subordinates' OCB by strengths use and job crafting (sequential mediation).
With this study, we attempt to make two aspects of theoretical contributions. First, although leadership as a critical contextual factor such as transformational leadership (Podsakoff et al., 1990) and servant leadership (Walumbwa et al., 2010) has been revealed to relate to high levels of OCB, no prior study has investigated the relationship between humble leadership and OCB. Thus, this paper enriches the existing research on the leadership-OCB relationship by testing the association of humble leadership with OCB. Second, previous research concerning the leadership-OCB linkage has identified the mediating mechanisms between humble leadership and OCB from the perspectives of attitude or motivation (e.g., Nasra and Heilbrunn, 2016;Newman et al., 2017); very little research focused on positive behaviors as mediating mechanisms linking humble leadership to OCB. By investigating the mediating (and sequentially mediating) roles of strengths use and job crafting in the humble leadership-OCB relationship, this study contributes to unlocking the "black box" of the relationship between humble leadership and OCB and extends our understanding of the underlying mechanisms between humble leadership and OCB.

Humble Leadership and OCB
Humility leadership representing social interpersonal characteristics (Owens and Hekman, 2012) consists of three aspects: a willingness to view oneself accurately, an appreciation of others' strengths and contribution, and teachability (Owens et al., 2013). In recent years, the construct of humble leadership has received remarkable scholarly attention (Owens et al., 2013). Extant literature has suggested that humble leadership makes subordinate growth and development legitimization (Owens and Hekman, 2012); promotes subordinate learning orientation, job satisfaction, work engagement, and retention (Owens et al., 2013); develops loyalty and commitment (Basford et al., 2014); elevates top management team integration and empowering climate (Ou et al., 2017); mitigates the negative impact of leader narcissism on positive follower outcomes (Owens et al., 2015); positively influences team performance through social contagion process (Owens and Hekman, 2016); and enhances subordinate creativity via psychological capital (Wang et al., 2018). Furthermore, humble leadership has been found to be related to team performance, and the relationship is mediated by team psychological capital and team task allocation effectiveness (serial mediation) (Rego et al., 2017a). In a similar vein, Rego et al. (2017b) also pointed out that humble leadership can increase team performance via enhanced collective humility and team psychological capital (sequential mediation). Liu et al. (2017) demonstrated that humble leadership can facilitate team innovation by shaping the team voice climate. Chen et al. (2018) identified a moderated mediation connection between humble leadership, identification with the leader, psychological empowerment, and employee proactive behavior. Although much research has stressed the importance of humble leadership to individuals and organizations, there is still a need for investigating the effect of humble leadership on a wider range of positive outcomes of individuals or organizations. Given that OCB plays a crucial role in facilitating organizational productivity, efficiency, and employees' performance evaluations and promotions (Podsakoff et al., 2009), further research on the antecedents of OCB is of great significance. Thus, the present study aims to examine whether humble leadership can significantly improve employees' OCB.
We speculate that humble leadership has a positive correlation with OCB. At least three reasons offer support for the prediction. First, Wang et al. (2018) demonstrated that when humble leaders appreciate subordinates' strengths, even view themselves as students of their subordinates' strengths (Owens and Hekman, 2012), subordinates' perceptions of self-efficacy and self-worth will be enhanced. Given that self-efficacy has a close relation to OCB (Dussault, 2006;Kao, 2017), humble leadership might have a positive correlation with OCB. Second, the appreciation of subordinates' contributions that humble leaders express can help subordinates recognize the importance of their contributions to organizations and, in turn, facilitates subordinates' perception of meaning in their jobs . Considering the positive impact of the meaning of work on OCB (Schlechter and Maharaj, 2007), it is reasonable to anticipate that humble leadership is positively related to OCB. Third, humble leaders used to acknowledge their deficits and show themselves to be open to new ideas, which can provide subordinates with opportunities for selfdetermination . Such self-determination can lead to an increase in OCB (Yan and Chen, 2013). Accordingly, it is possible to postulate humble leadership to be linked with OCB. Based on the above arguments, the following hypothesis is derived: Hypothesis 1: Humble leadership is positively related to OCB.

Humble Leadership and Strengths Use
Strengths use is defined as the proactive behaviors which employees take to deploy their strengths at work (Van Woerkom et al., 2016a). Researchers have done much valuable work to explore the driving forces of employee strengths use. For example, Govindji and Linley (2007) demonstrated that strengths knowledge is a significant predictor of strengths use. In other words, individuals who recognize their strengths are more likely to use their strengths. In a recent review of strengths use literature conducted by Bakker and van Woerkom (2018), it was indicated that stable personality traits might have a significant correlation with strengths use. For example, neuroticism and extraversion are significantly and negatively related to strengths use (Bakker et al., 2019); a structural equation modeling analysis has also found that independent self-construal could significantly and positively predict employees' strengths use (Kong and Ho, 2016). Besides, a piece of research based on South African employees revealed that employees who gained more job resources (e.g., autonomy, information, and support for strengths use) were more apt to deploy their strengths in the workplace (Botha and Mostert, 2014). Consistent with the aforementioned results, Kong and Ho (2016) found that when employees perceived higher levels of autonomy support from leaders, they would have a stronger motivation to utilize strengths; Van Woerkom et al. (2016b) suggested that organizational support for strengths use (for instance, organizations help employees to identify their strengths) had a positive effect on strengths use indeed.
According to the above discussion, we know that both individual characteristics and situational circumstances have vital roles in elevating employee strengths use. It is important to note that leaders' role may be considered as one of the more prominent influencing factors of strengths use (Kong and Ho, 2016). In the present research, we anticipate that humble leadership has a positive association with strengths use. First, since humble leadership legitimizes subordinates' growth and development (Owens and Hekman, 2012) and previous literature demonstrates that strengths are the greatest room for subordinates' growth and development (Buckingham and Clifton, 2001), subordinates who follow humble leadership will be more likely to take various behaviors to leverage their strengths at work to achieve a high level of growth and development. Second, humble leaders always show appreciation of subordinates' strengths (Owens and Hekman, 2012), which conveys a piece of important information to subordinates that leaders encourage and support subordinates to use their strengths. As mentioned previously, support for strengths use is an important driving force of strengths use (Van Woerkom et al., 2016b). Thus, it is reasonable to predict that humble leadership has a positive impact on subordinates' strengths use. In sum, we propose the following hypothesis: Hypothesis 2: Humble leadership is positively related to strengths use.

Humble Leadership and Job Crafting
Job crafting as a specific form of proactive work behavior has been defined as the self-initiated changes that employees make in the task or in the relational boundaries of their work that are aimed at improving person-job fit (Wrzesniewski and Dutton, 2001;Tims et al., 2012). Job crafting is not explicitly authorized by the employer but initiated by employees (i.e., bottomup) (Hornung et al., 2010), which is a critical potential path where organizations can gain competitive advantage (Esteves and Lopes, 2017). Wrzesniewski and Dutton (2001) put forward three forms of job crafting: task crafting, relational crafting, and cognitive crafting. Task crafting may be achieved by altering the type, number, content, or scope of tasks and work routines (Ghitulescu, 2006). Employees can craft relations with others by changing the range, nature, or the number of their interactions at work (Kooij et al., 2015). Cognitive crafting refers to the change employees make to views on work (Wrzesniewski and Dutton, 2001). Subsequently, Tims and Bakker (2010) provided another perspective of understanding job crafting using the job demands-resources theory as a framework and proposed four dimensions of job crafting: increasing structural job resources, social job resources, and challenge job demands as well as decreasing hindrance job demands (Hetland et al., 2018). Empirical studies have found that transformational leadership (Wang et al., 2017), paternalistic leadership (Tuan, 2018), willingness to change, impact of change (Petrou et al., 2015), and autonomy support (Slemp et al., 2015) could significantly predict job crafting of employees. From an integrative perspective, Niessen et al. (2016) investigated the needed abilities and reasons for job crafting and found that selfefficacy (can do), need for control, need for human connection, and need for positive self-image (reason to) are positively associated with job crafting.
Although researchers have shown a sophisticated understanding of the conception of job crafting and identified some important driving forces of job crafting, we still have not found explicit literature on the association of humble leadership with job crafting. In the current study, we thus attempt to address the gap and hypothesize that humble leadership has a positive correlation with job crafting. Humble leadership, conceptually, has significant interpersonal implications with a strong motive for learning from subordinates (Owens et al., 2013), which contributes to eliciting subordinates' self-efficacy (Wang et al., 2018). Subordinates high in self-efficacy are more likely to believe that they can craft their job to achieve job objectives (Niessen et al., 2016). Therefore, humble leadership may have a positive correlation with job crafting. Moreover, humble leaders have an orientation toward subordinates and make the courageous decision to give up a certain portion of power so that subordinates have discretion in dimensions of their jobs . Such autonomy support encourages subordinates to craft their job (Slemp et al., 2015). Besides, qualitative research conducted by Owens and Hekman (2012) illustrated that leader-expressed humility has a positive association with employee engagement because humble leadership can cultivate the preconditions for employee engagement (Owens et al., 2013). A two-wave longitudinal study also demonstrated the work engagement of employees to be linked with job crafting (Lu et al., 2014). Accordingly, it is possible to expect humble leadership to relate to job crafting. Hence, we hypothesize the following: Hypothesis 3: Human leadership is positively related to job crafting.

Strengths Use and Job Crafting
A substantial body of research has found that employees who utilize their strengths in the workplace are inclined to perform better and be more proactive (Cable et al., 2013;Dubreuil et al., 2014) and perform more helping behaviors and less counterproductive behaviors (Kong and Ho, 2016;Lavy and Littman-Ovadia, 2017;Littman-Ovadia et al., 2017). While strengths use can bring about numerous desirable outcomes, little is known about whether strengths use can positively affect job crafting. In the current study, we assume that strengths use has a positive association with job crafting.
When employees use their strengths at work, they can feel their most authentic self (Harzer and Ruch, 2013) and are more positive, energetic, and active (Bakker et al., 2019). Subsequently, they will engage more effort, time, and energy in work. That implicitly signifies that employees with higher levels of strengths use will experience higher work engagement. Indeed, a recent daily diary study also indicated that strengths use is positively related to work engagement (Bakker et al., 2019). As demonstrated earlier, work engagement is a significant and positive predictor of job crafting (Lu et al., 2014). Thus, we expect strengths used to positively relate to job crafting. In addition, in a weekly diary study, Van Woerkom et al. (2016b) pointed out that strengths use can predict a change in self-efficacy. Given that self-efficacy is a precursor of job crafting (Niessen et al., 2016), we also believe that the positive effects of strengths use can be conveyed to job crafting. Based on the above arguments, we postulate the following: Hypothesis 4: Strengths use is positively related to job crafting.

The Mediating Role of Strengths Use
Strengths use was found to be not only linked with well-being and task performance (Littman-Ovadia and Steger, 2010;Wood et al., 2011;Kong and Ho, 2016) but also related to helping behaviors (Kong and Ho, 2016). In terms of helping behaviors, employees who use strengths at work can experience higher feelings of positive affect (Wood et al., 2011;Forest et al., 2012), and in turn, this leads to enhanced intention to help others (Kong and Ho, 2016); moreover, employees can gain more energy from strengths use (Dubreuil et al., 2014), which offers more resources that employees can deploy to help others (Kong and Ho, 2016). Given that helping behaviors is one aspect of OCB (Bachrach et al., 2007), we thus speculate that strengths use acts as the same role in enhancing OCB. Empirical evidence provides support for the notion. For example, Lavy and Littman-Ovadia (2017) concluded that strengths use can influence OCB by the broaden-and-build effect of positive emotions. Similar research also revealed that using signature strengths has a unique contribution to OCB . In addition, Van Woerkom and Meyers (2015) found that strengths-based psychological climate can significantly predict employees' OCB, which also offers a piece of indirect evidence for the relationship between strengths use and OCB. Based on the previous prediction that humble leadership is positively related to strengths use and the above discussion, humble leadership may affect OCB by strengths use. Therefore, we posit the following: Hypothesis 5: Strengths use mediates the relationship between humble leadership and OCB.

The Mediating Role of Job Crafting
In the current study, we assume that job crafting may play an important mediating role in the relationship between humble leadership and OCB. Job crafting as a form of proactive behaviors has been demonstrated to be associated with many valuable outcomes, such as colleague ratings of in-role performance Tims et al., 2015), work enjoyment (Tims et al., 2014), well-being (Tims et al., 2013), employees' fit to the organization, job satisfaction (Kim et al., 2018), and intrinsic need satisfaction (Slemp and Vella-Brodrick, 2014). Bakker et al. (2012) illustrated that employees engaging in job crafting tend to mobilize their job resources and create a challenging work environment to foster enthusiasm and absorption. Also, when employees are dedicated to their work, they will be more likely to perform higher levels of OCB (Babcock-Roberson and Strickland, 2010). Accordingly, we believe that job crafting may have a positive association with OCB. A three-wave study indicated that when employees engage in job crafting, they can control their environment, which in turn results in higher self-efficacy and optimism; employees crafting their job can also experience a high level of hope by the process of goal setting and finding a way to achieve those objectives; more importantly, job crafting can help employees overcome some difficulties, which then leads to increased resiliency (Vogt et al., 2015). That implicitly means that job crafting has a significantly positive correlation with psychological capital. In line with the notion, Wingerden et al. (2016) also found a positive relationship between job crafting and psychological capital. Given the positive association of psychological capital with OCB (Norman et al., 2010), we believe that job crafting is positively related to OCB. Combining prior anticipation that humble leadership is positively related to job crafting with the above arguments, we can obtain the following hypothesis: Hypothesis 6: Job crafting mediates the relationship between humble leadership and OCB.

The Sequential Mediation
Integrating all hypotheses mentioned above, we anticipate that the relationship between humble leadership and OCB can be mediated by strengths use and job crafting (a serial mediation). Prior research has shown strengths use to be associated with increased job crafting (Plomp et al., 2016). The main reason for the positive relationship between strengths use and job crafting is that when employees deploy their strengths at work, they are more likely to experience more energy and higher levels of authenticity (Mahomed and Rothmann, 2019), and these positive feelings provide crucial psychological conditions for job crafting (Kira et al., 2012;Lu et al., 2014). According to social learning theory, employees might learn some important behaviors by imitating the behaviors of role models such as leaders (Bavik et al., 2017). Employees who follow humble leaders have the same propensity to use strengths at work as leaders (Wang et al., 2018); such strengths use behavior contributes to higher levels of job crafting. As demonstrated earlier, job crafting has predictive value for OCB through the mechanism of psychological capital (Norman et al., 2010;Wingerden et al., 2016). Therefore, strengths use triggered by humble leadership will affect job crafting, which in turn leads to an improvement in OCB. Taken together, we formulate the following hypothesis: Hypothesis 7: The relationship between humble leadership and OCB is mediated by strengths use and job crafting (sequential mediation).
The proposed model is presented in Figure 1.

Sample and Data Collection
A self-report research design was used in the current article. We adopted the convenience sampling to recruit participants. Participants were medical staff from a hospital in Beijing, China. An author of the present study contacted her friend working in the hospital and asked for her help to collect data. After obtaining consent from the manager and staff, we distributed electronic questionnaires to 316 medical staff in the hospital by WeChat (a type of communication tool) to gather data. Data were collected at two points in time spaced by a 2week interval to reduce the common method variance (CMV). We set the electronic questionnaires as follows: only when the electronic questionnaire was completely filled can it be submitted successfully. In the first stage, participants completed a questionnaire consisting of demographic variables and scales of humble leadership, strengths use, and job crafting. After 2 weeks, participants completed a measure of OCB. Finally, 260 valid paired data were deployed to test our predictions, the valid response rate was 82.3%. Demographic characteristics were described in Table 1.

Measures
A five-point Likert scale (1 = strongly disagree, 2 = disagree, 3 = neutral, 4 = agree, 5 = strongly agree) was applied to rate all items of main variables (i.e., humble leadership, strengths use, job crafting, and OCB). Since four scales used in the current research were initially developed in English, they were translated into Chinese depending on the process of translation and back translation to ensure the equivalence of items (Brislin, 1970).

Humble Leadership
Humble leadership was measured with the nine-item scale developed by Owens et al. (2013), which was composed of three dimensions: willingness to see the self accurately, appreciation of others' strengths and contributions, and teachability. One sample item was "My leader acknowledges when others have more knowledge and skills than him-or-herself." The Cronbach α for this scale in the current study was 0.96, indicating excellent reliability.

Job Crafting
We assessed job crafting with the 15-item Job Crafting Questionnaire developed by Slemp and Vella-Brodrick (2013), which rates the degree to which employees engage in various forms of task, relational, and cognitive crafting (Slemp et al., 2015). One sample item was "Introduce new tasks you think suit your skills or interests." The Cronbach α for this scale in the current study was 0.90, suggesting excellent reliability.

Strengths Use
We evaluated strengths use using five items of Strengths Use and Deficit Correction (SUDCO) questionnaire developed by Van Woerkom et al. (2016a). One sample item was "In my job, I make the most of my strong points." The Cronbach α for this scale in the current study was 0.83, indicating good reliability.

Organizational Citizenship Behavior
Organizational citizenship behavior was assessed with the 10item scale used by Bachrach et al. (2007). The scale consists of two dimensions: helping (seven items) and civic virtue (three items).
One sample item was "Help other employees out if someone falls behind in his/her work." The Cronbach α of the scale in the current study was 0.92, demonstrating excellent reliability.

Discriminant Validity
To check the measurement model fit, we conducted a confirmatory factor analysis (CFA) in AMOS 21.0. Prior to performing CFA, in order to control for inflated measurement errors originating from multiple items for the latent variable and enhance the reliability and normality of the resulting measure (Nasser-Abu and Wisenbaker, 2006;Shi et al., 2015), three item parcels for humble leadership, three item parcels for job crafting, and two item parcels for OCB were created according to their dimensions. These item parcels were considered as indicators of corresponding constructs. Besides, all items of strengths use scale were viewed as indicators of strengths use. We selected six indices, including χ 2 /df, the root mean square error of approximation (RMSEA), comparative fit index (CFI), Tucker-Lewis index (TLI), incremental fit index (IFI), and goodness of fit index (GFI), to assess the overall model fit.
A four-factor model including humble leadership, strengths use, job crafting, and OCB was regarded as the baseline model. In order to examine the distinctiveness of the key constructs in the proposed model, we compared the baseline model with three alternative models. Table 3 reported the results of CFA. As shown in Table 3, the four-factor model exhibited adequate fit to the data: χ 2 = 102.60, df = 59, χ 2 /df = 1.74, p < 0.001, RMSEA = 0.05, CFI = 0.98, TLI = 0.97, IFI = 0.98, GFI = 0.94. More importantly, there existed a significant difference in χ 2 of the baseline model and three alternative models, which demonstrated that respondents could differentiate the four constructs very well. Therefore, the four constructs in the present study had a good discriminant validity.

Common Method Variance
Although the present study gathered the data at two points in time to control CMV, the self-report questionnaire survey might give rise to the CMV. Thus, we adopted the "controlling for the effects of a single unmeasured latent method factor" method to examine the degree of the CMV (Podsakoff et al., 2003). In accordance with prior studies (e.g., Ng et al., 2014;Xu and Lv, 2018), we established a new measurement model comprising a common method factor and four focal variables. All items were loaded on their theoretical constructs and the method factor. The results demonstrated that the new measurement model exhibited a good fit to the data (χ 2 = 93.97, df = 58, χ 2 /df = 1.62, p < 0.01, RMSEA = 0.05, CFI = 0.98, TLI = 0.98, IFI = 0.98, GFI = 0.95). However, variance interpretation of the method factor was 12.75%, less than 25.00% (Williams et al., 1989). Hence, the CMV did not appear to be a serious threat to the interpretation of our results.

Hypothesis Testing
According to Anderson and Gerbing's (1988) suggestion, a two-step procedure was applied to examine our hypotheses. As shown in discriminant validity, the measurement model involving four latent variables (humble leadership, strengths use, job crafting, and OCB) reported an excellent fit to data. Moreover, all the factor loadings for the indicators of latent variables were significant at the 0.001 level, demonstrating that all the latent constructs can be represented by their corresponding indicators.
With respect to the structural model, three control variables were included in all structural models. The direct effect of humble leadership on OCB without mediators was first tested. The results suggested that the model (χ 2 = 25.52, df = 18, χ 2 /df = 1.42, RMSEA = 0.04, CFI = 0.99, TLI = 0.99, IFI = 0.99, GFI = 0.98) showed an excellent fit to the data, and the directly standardized path (β = 0.33, p < 0.001) was significant, indicating that humble leadership has a positive effect on OCB. Hypothesis 1 was supported.
The standardized path coefficients for the modified model were reported in Figure 2. The modified model explained 35.1%  a Strengths use and job crafting combined into one factor. b Humble leadership, job crafting, and strengths use combined into one factor. c All combined into one factor. ***p < 0.001. of variance in OCB. As shown in Figure 2, the standardized path coefficient between humble leadership and strengths use was 0.42 (p < 0.001), supporting Hypothesis 2; the standardized path coefficient between humble leadership and job crafting was 0.35 (p < 0.001), which is supportive of Hypothesis 3; the standardized path coefficient between strengths use and job crafting was 0.57 (p < 0.001), providing support for Hypothesis 4. As suggested previously, the standardized path coefficient from strengths use to OCB was not significant; thus, Hypothesis 5 was not supported.
We performed bootstrapping procedures (2,000 draws) with 95% bias-corrected confidence intervals to estimate the indirect effect in the revised model and its significance (MacKinnon et al., 2004). As reported in Table 4, the indirect effect of humble leadership on OCB via job crafting was significant, offering support for Hypothesis 6; the indirect effect of humble leadership on OCB through strengths use and job crafting (sequential mediation) was also significant, which was supportive of Hypothesis 7.
Finally, a post hoc power analysis was carried out in GPower with a sample size of 260 and six predictor variables as baseline to examine the appropriateness and representativeness of our sample and findings. According to Cohen's (1977) suggestion, the following effect sizes were used for this evaluation: small (f 2 = 0.02), medium (f 2 = 0.15), and large (f 2 = 0.35). Post hoc power analysis showed that the power to detect obtained effects was 0.999 for the entire regression in the prediction of OCB at the 0.05 level beyond the value of 0.8 recommended by Cohen (1977) and Mustafa et al. (2016). As such, we can conclude that the power to detect small effects was enough with a sample of 260 and that our findings are appropriate and representative.

DISCUSSION
Researchers have identified many determinants of OCB (e.g., Lim et al., 2018;Khan et al., 2019). However, there is still a paucity of literature on the relationship between humble leadership and OCB. By a two-wave survey with a sample of 260 employees working in a hospital in Beijing, China, the present study examines the prediction that when subordinates experience higher levels of humble leadership, they will respond to this with higher OCB. Moreover, this paper tests the mediational roles of strengths use and job crafting, respectively, in the relationship between humble leadership and OCB. More importantly, this paper further examines the sequential mediation effect of strengths use and job crafting on the relationship between humble leadership and OCB. The findings of this paper showed that humble leadership is positively related to OCB, strengths use, and job crafting and that job crafting plays a significant and positive mediating role in the relationship between humble leadership and OCB. More importantly, strengths use and job crafting serve a sequential mediation role in the relationship between humble leadership and OCB. However, the mediating role of strengths use in the humble leadership-OCB relationship is not significant. These findings have several theoretical and practical implications, which are discussed below.

Theoretical Implications
The present study makes two main theoretical contributions to OCB literature. First, by investigating the relationship between humble leadership and OCB, our study enriches the literature on antecedents of OCB. Crant (2000) pointed out that leaders serve as a crucial role in facilitating employee proactive behavior (e.g., OCB). Although scholars appreciated the importance of leadership to OCB and found that transformational leadership (Jiao et al., 2011), attributional charismatic leadership (Deluga, 1995), shared leadership (Khasawneh, 2011), and developmental leadership (Yan and Chen, 2013) are positively related to OCB, less is known about the association of humble leadership with OCB. In line with previous research that found humble leadership to positively relate to employee proactive behavior , our study indicated that humble leadership is positively related to OCB, which adds to the literature on antecedents of OCB. Second, the current study stresses the importance of the mediating role of job crafting, which contributes to unpacking the "black box" in the humble leadership-OCB relationship. Our study showed that job crafting acts as a mediator in the relationship between humble leadership and OCB. Concretely, when subordinates perceive a high level of humble leadership, they will have the feelings of being empowered by leaders and having more autonomy to make decisions at work ; they are more likely to engage in work (Wang et al., 2018) and experience higher self-efficacy (Owens et al., 2013). Such positive outcomes induced by humble leadership will lead to increased job crafting. However, our study did not provide evidence for the mediating effect of strengths use on the humble leadership-OCB relationship. According to the results of data analysis, a possible reason for this finding is that for employees working in a hospital, strengths use does not directly but indirectly affect employees' OCB; that is, the effect of strengths use on OCB should be understood through the mediating mechanism of job crafting. Previous research conducted by Van Woerkom and Meyers (2015) suggested that positive affect fully mediated the relationship between strengths-based psychological climate and OCB, which to a certain extent offers support for the argument. In a word, this study can help us to understand the underlying process mechanism behind the humble leadership-OCB relationship.
Further study found that strengths use and job crafting served as a tandem mediating effect on the relationship between humble leadership and OCB, which contributes to our understanding of deeply potential mechanisms behind the humble leadership-OCB relationship. Prior research revealed that psychological capital (Wang et al., 2018) and psychological empowerment  respectively played important mediational roles in the effects of humble leadership. However, to the best of our knowledge, no prior research explored the underlying process mechanisms of effects of humble leadership from the perspective of strengths use and job crafting. Our study addressed this concern and found that humble leadership could affect subordinates' strengths use and then lead to increased job crafting, ultimately promoting OCB of subordinates. The result unravels a more complex process mechanism underlying the relationship between humble leadership and OCB, which contributes to deepening our understanding of the humble leadership-OCB relationship.

Practical Implications
The current study has some important practical implications for organizations. First, this study found that humble leadership is effective in promoting employees' OCB. Thus, to enhance employees' OCB, organizations should promote employees with a high level of humble characteristics to be a leader or enable present leaders to learn more knowledge on humble leadership by training projects. Second, as the current study indicated that humble leadership has an indirect impact on OCB through the mediator, namely, job crafting, organizations should create conditions under which employees' job crafting can be elevated. For example, organizations can provide employees with more autonomy to create the condition eliciting job crafting (Slemp et al., 2015). Third, the current study demonstrated that strengths use and job crafting play a significantly sequential mediating role in the relationship between humble leadership and OCB, which signifies that promoting employees' strengths use is quite important for enhancing employees' OCB. Thus, we propose that organizations should help employees to identify and develop their strengths to encourage them to leverage their strengths at work. In addition, providing employees with higher autonomy support is also an important way of improving employees' strengths use (Kong and Ho, 2016).

Limitations and Directions for Future Research
The current study is not without limitations. First, although this study adopted a two-wave survey separated by a 2week interval to collect data, we still cannot make robust causal inferences about the relationship between humble leadership and strengths use, job crafting, and OCB due to the shorter time interval in the process of data collecting. Hence, future research should conduct quasi-experiment or longitudinal research to further examine the causal relationship among humble leadership, strengths use, job crafting, and OCB. Second, in the current study, almost all of participants are female from a hospital, which may raise the concern about the generalizability of our findings in that males and females might have a significant difference in cognition in leadership (Hyde, 1981). So it is quite necessary for researchers to conduct research on the relationships between humble leadership, strengths use, job crafting, and OCB using a more gender-balanced sample to expand the generalizability of our findings. Finally, the current study merely investigated the complex process mechanisms underlying the relationship between humble leadership and OCB. However, we still have little knowledge on the boundary conditions of the humble leadership-OCB relationship. Therefore, future research should try to identify the boundary conditions of the relationship between humble leadership and OCB.

DATA AVAILABILITY STATEMENT
The datasets analyzed in this article are not publicly available.
Requests to access the datasets should be directed to believedh@126.com.

ETHICS STATEMENT
The present study was carried out in accordance with the recommendations of the Ethics Committee of the North China Electric Power University with written informed consent from all subjects. All the participants were asked to read and approve this ethical consent before taking part in the present study and followed it in the process of research. The protocol was approved by the Ethics Committee of the North China Electric Power University.