An alternative pathway for explaining the indirect effects of SL is the quality of the leader–follower relationship (van Dierendonck,) or LMX (Brower, Schoorman, & Tan,). Research has drawn on social exchange theory to posit LMX as an important mediator of the SL–outcome relationship, based on the assumption that a key aspect of enacting SL is the development of high‐quality relationships with followers (Ehrhart,; Liden & Maslyn,; van Dierendonck,). SL has also been shown to predict follower behaviour through LMX (Newman.,). Our analysis, therefore, tests the proposition that LMX is a key mediator through which SL influences outcomes.

The second mediator examined is trust in the leader, defined as willingness to be vulnerable based on a trustor's positive expectations about trustee actions and behaviours (Mayer, Davis, & Schoorman,). SLs build trust by prioritizing follower goals and desires over self‐interest, demonstrating their focus on the greater good. In risky situations, followers feel that they can rely on their leader to prioritize their welfare over organizational objectives (Dirks & Ferrin,). Leaders also generate a climate where followers feel valued, listened to, and invested in and reciprocate by showing trust in their leader. Trust in the leader has, therefore, been posited as a key mechanism for explaining the influence of SL on follower outcomes (Joseph & Winston,; Schaubroeck.,), and is argued to create a safe psychological climate that explains the positive consequences of SL (van Dierendonck,).

Fairness is particularly pertinent to SLs, given their ethical orientation and sensitivity towards follower needs (Mayer, Bardes, & Piccolo,). A SL's concern for follower's growth and prosperity is instrumental for ensuring that rewards are distributed fairly amongst subordinates. Procedural justice captures the perceived fairness of the procedures that are used to determine outcomes (Thibaut & Walker,), and procedural justice climate has been shown to mediate the relationship between SL and both individual‐ and team‐level outcomes (Ehrhart,; Mayer.,). Accordingly, van Dierendonck () proposes that fairness is a key dimension for creating a safe psychological climate through which SL engenders more favourable follower behaviour. Procedural justice is a critical factor in predicting employee cooperative behaviour (Konovsky,), proactive behaviour (Crawshaw, Van Dick, & Brodbeck,), and task performance (Aryee, Chen, & Budhwar,). Overall, when employees feel they are fairly treated, they are more willing to accept leaders’ decisions and facilitate organizational performance by ensuring that work‐related objectives are achieved, and demonstrating performance‐oriented behaviours such as OCB, voice, and creativity (Naumann & Bennett,; Organ,; Rupp, Shao, Jones, & Liao,).

Eva et al . ( 2018 ) highlighted that many mediators have been studied to explain the effects of SL. Their extensive list of 35 mediators demonstrates that significant attention has been paid to examining the indirect effects of SL. However, most studies assess single mediators only (Bouzari & Karatepe, 2017 ). Given the conceptual and empirical overlap between many of the mediators examined (e.g., trust and LMX), it is likely that there is a certain amount of redundancy between them. Further, single mediator designs make it impossible to assess which mediators are most important for particular outcomes (see Hughes, Lee, Tian, Newman, & Legood, 2018 ). Given the large number of mediators identified in Eva et al .'s ( 2018 ) review, we are not able to test a full model with all potential mediators tested together. Instead, we draw upon two alternative theoretical explanations posited by van Dierendonck ( 2011 ), high‐quality leader–follower relationships and psychological climate, and operationalize these pathways using three mediators that have been consistently examined in the literature (Avolio, Zhu, Koh, & Bhatia, 2004 ; Eva et al ., 2018 ; Schaubroeck et al ., 2011 ): procedural justice and trust in the leader (capturing the psychological climate pathway), and LMX (capturing the leader–follower relationships pathway; van Dierendonck, 2011 ). Our key aim here was to provide a rare test of the relative effects of these alternative mechanisms and provide insight into which pathway offers the most empirically powerful explanation for the indirect effects of SL.

Ethical leaders focus on the promotion of appropriate conduct through interpersonal relationships and personal actions in organizations (Brown.,). It is like SL in that both theories emphasize caring for people, integrity, trustworthiness, and serving the good of the whole (van Dierendonck & Nuijten,). However, EL focuses on directive and normative behaviour, while SL focuses on the developmental needs of followers. Unlike ethical leaders whose main concern is doing the right thing, SLs focus on doing what is in the best interests of their organizational community, which, in turn, should engender a deeper level of trust and commitment amongst followers, manifesting in heightened outcomes above and beyond those accounted for by EL.

Authentic leaders are self‐aware, showing openness and clarity regarding who they are, and consistently disclosing and acting in accordance with their personal values, beliefs, motives, and sentiments (Avolio & Gardner,). AL is operationalized as focusing on a leader's authenticity in interactions with others and being true to their inner thoughts, while showing an open mind and the willingness to change. In relation to SL, the most obvious overlap is with two characteristics: authenticity and humility. Both servant and authentic leaders are moral leaders who share characteristics such as integrity, honesty, and humility (van Dierendonck,), but AL has a more explicit focus on authenticity. Regarding humility, only willingness to learn overlaps with SL; willingness to stand back and give room to others is missing (van Dierendonck,). None of the other four SL characteristics are explicitly conceived as belonging to AL. In comparison with AL, SL emphasizes responsibility to society and stakeholders (Ehrhart,; Walumbwa, Hartnell, & Oke,). Research has shown that such self‐sacrificing behaviour engenders prosocial behaviour (De Cremer, Mayer, Van Dijke, Schouten, & Bardes,), and positive emotion and cooperation amongst followers (De Cremer,), which is likely to translate into the work‐related outcomes discussed earlier. AL, on the other hand, has a greater emphasis on ensuring that a leader's behaviour is consistent with one's ‘true self’, which we argue will have less of an impact on follower outcomes.

Servant leadership also has many conceptual overlaps with emerging forms of positive leadership that emphasize ethical and moral behaviour (authentic, spiritual, and ethical) and those that emphasize putting followers first (empowering and self‐sacrificing). However, far less is known about the distinctiveness of SL against these newer forms of leadership. Based on the availability of studies to date, the current research focuses specifically on the incremental predictive validity of SL over AL and EL. These leadership approaches are commonly aggregated to reflect a morally based perspective on leadership (Dinh et al ., 2014 ; Lemoine et al ., 2019 ) and have therefore been argued to generate homogenous and potentially redundant findings. However, Lemoine et al . ( 2019 ) note that meaningful differences can be derived based on each definition, with SL emphasizing the need to benefit multiple stakeholders, AL emphasizing self‐awareness and internal consistency, and EL emphasizing the importance of complying the normative standards.

Servant leadership is argued to be conceptually distinct from other major leadership theories primarily due to the emphasis placed on ethics and behavioural integrity (Ehrhart,; Hu & Liden,). In attempting to establish SL as a unique and influential leadership style, most research has examined its relation to TL – both styles emphasize individual growth and development (Bass,). However, a key point of conceptual departure concerns the focus of the leader's goal (Parolini, Patterson, & Winston,). While TLs are primarily concerned with advancing organizational objectives (Bass,), SLs ultimately aim to serve their followers and the needs of the wider community above all else (Graham,), potentially at the expense of their own needs and organization's interests. This unconditional concern for follower growth conceptually distinguishes SL from TL. In turn, followers engage in appropriate performance‐related behaviours, not through coercion or obligation, but because they genuinely believe in the guiding principles of the SL and view them as a role model (Liden, Panaccio,.,). This may explain the incremental predictive validity that SL has demonstrated over TL in recent studies (Liden.,; Peterson, Galvin, & Lange,). Here, we further explore the incremental predictive validity of SL over TL, exploring a wider range of follower outcomes.

Servant leadership has been consistently subjected to critical comparisons with other, more established leadership theories (van Dierendonck, 2011 ). However, Hoch et al . ( 2018 ) provided meta‐analytic support for the incremental predictive validity of SL over TL: Analysis showed that SL explained important incremental variance in employee OCBs, but little in job performance. However, the meta‐analysis was narrowly focused, comparing the effects of SL with TL only, and focused on a limited set of performance outcomes – job performance and OCB. Thus, it is unclear whether SL predicts other forms of employee behaviour (i.e., CPB, creativity, and voice) over other leadership styles (i.e., EL and AL).

Finally, we focus on followers’ organizational tenure as a third moderator. Employee organizational tenure has been found to directly link to work‐related needs, attitudes, and behaviours (Huang, Shi, Zhang, & Cheung,) and reactions towards management practices (Ashforth & Saks,), as well as being a key moderator. Wright and Bonett () found in a meta‐analysis that the correlation between organizational commitment and job performance is greatest with new employees and decays exponentially over time. Huang. () also found that participative leadership increases short‐tenure employee job performance and organizational commitment. We, therefore, propose that the effect of SL on follower performance will be stronger for employees with lower tenure for two reasons. First, shorter‐tenured employees may have a higher need for SL; leaders may create better career development prospects for such employees by taking a more active role in encouraging and supporting their needs for personal development. Second, based upon human capital theory (Becker,), longer tenure is likely to be associated with greater job performance; conversely, longer‐tenured employees may be less receptive to, SL, because their tenure is likely to be accompanied by increases in declarative and procedural knowledge, thus weakening the positive effect of SL on performance.

A second macrolevel influence that Meyer. () highlight as having a moderating influence on leader–follower relationships pertains to occupational characteristics. Building on this, Eva. () contend that the positive effects of SL could be partially contingent on the type of organization and industry within which leaders are operating. To explore this, we examine industry capital intensity as an industrial‐level moderator, which is a measure of the relative investment in fixed assets in an industry (Guthrie & Datta,). SL researchers have noted the role of industrial context as an important future research direction (Anderson & Sun,; Hunter.,; Liden, Panaccio,.,). High capital intensity industries include manufacturing, shipbuilding, and air transport because of the greater constraints imposed on employee performance by the degree of production technology or task structure (Terpstra & Rozell,). Low capital intensity industries include service industries (labour‐intensive), where the human element is the key to organizational effectiveness (Terpstra & Rozell,). We argue that SL will be most effective in low‐capital‐intensive industries where human capital is more emphasized: It is likely that leaders need to interact more frequently with followers in order to understand their needs and provide support and empowerment (Donia.,; van Dierendonck,).

With regard to individual–collectivism, its moderating effect on SL is harder to predict. In cultures with higher levels of collectivism, individuals are integrated into strong cohesive groups in which group goals often supersede individual goals (House.,). In contrast, individualist cultures tend to prioritize self and focus on enhancing their self‐esteem. Thus, the emphasis of SL to build community amongst followers could have particular relevance in collectivist cultures. However, collectivistic individuals are also likely to have invested time and effort into developing relationships with other individuals in the organization, such as co‐workers and subordinates. Therefore, in collectivist cultures not only is the leader less likely to play a dominant role in meeting employees’ needs in the workplace (Panaccio.,), the distinction between in‐groups and outgroups that collectivism reinforces is also likely to be at odds with the requirement of servant leaders to build relationships with co‐workers and subordinates (Hale & Fields,). Conversely, an individualist culture is likely to facilitate leadership with stronger focus on followers’ individual growth and well‐being, thus reflecting the core nature of SL. Indeed, a cross‐cultural study examining SL in the United States and Ghana confirmed that respondents from the United States experienced SL behaviours significantly more than their Ghanaian counterparts, arguably due to the high levels of collectivism and high power‐distance that characterize Ghanaian cultural norms (Hale & Fields,). Similarly, in a comparative study of SL, Pekerti and Sendjaya () found that while SL was perceived to be culturally universal, specific attributes of the SL construct held differential importance across cultures. More specifically, the individualistic low power‐distance orientation of Australians was associated with a more consistent and direct display of SL behaviours related to the authentic self, as compared to the collectivistic high power‐distance orientation of Indonesians.

Individualism–collectivism relates to how a person views herself or himself in relation to the collective, whereas power distance describes the extent to which individuals accept social stratification and unequal distributions of power in society (Hofstede, 2001 ). In high power‐distance cultures, one is expected to hold greater respect for authority figures such as leaders and elders; clear hierarchal systems and large differences in power are expected and accepted, and employees are more accustomed to authoritative leadership styles. By contrast, in low power‐distance cultures, people are less obedient to authority and are more likely to regard themselves as independent of and equal in status to others (Rockstuhl, Dulebohn, Ang, & Shore, 2012 ). The behavioural norms of low power‐distance cultures should thus be more compatible with SL due to the value placed on equality between leaders and followers and the opportunity for leaders to demonstrate humility and development of followers, both essential elements of SL (Hale & Fields, 2007 ).

The first moderator focuses on the cultural context of the primary studies. Eva et al . ( 2018 ) proposed situational strength theory (Meyer et al ., 2009 ) as a fruitful perspective for addressing concerns regarding the lack of context captured in current SL research (Eva et al ., 2018 ; Mumford & Fried, 2014 ). From the perspective of situational strength theory (Meyer et al ., 2010 ), national culture is a key macrolevel influence that could amplify the effects of SL in lower power distance contexts, such as the United States, where the SL construct was developed (Eva et al ., 2018 ). Drawing on House, Javidan, Hanges, and Dorfman's ( 2002 ) notion of culturally endorsed implicit leadership theory, scholars argue that cultural aspects are associated with SL effects (Donia et al ., 2016 ; Liden, Panaccio, et al ., 2014 ; van Dierendonck, 2011 ). Here, we use the Hofstede cultural dimensions to examine national cultural dimensions based on the geographic locations where samples were drawn (Hofstede, 2001 ). We focus on two aspects of national culture that are incorporated in Hofstede's cultural model: power distance and individualism. Research has demonstrated that individualism–collectivism and power distance are the strongest predictors of a range of outcomes at the societal level (Taras, Kirkman, & Steel, 2010 ). Further, these two dimensions of national culture are also most strongly associated with examinations of cross‐cultural leadership (Ng, Koh, Ang, Kennedy, & Chan, 2011 ).

The varied settings of the primary studies on SL suggest that a more nuanced understanding of the differences in individual and contextual settings is warranted (Liden, Panaccio, et al ., 2014 ). As Donia et al . ( 2016 , p. 724) note, SL ‘may not be equally effective across contexts’, yet ‘few studies have considered boundary conditions which may enhance or buffer SL's impact on employee outcomes’. Similarly, Eva et al . ( 2018 , p. 14) note that only a limited number of studies have examined moderating factors that influence the effects of SL, and that future research needs to break the traditional theoretical lens that has confined understanding of the construct to date. We address this call by examining three moderators concerning contextual (national culture, industry capital intensity) and personal (follower organizational tenure) factors.

Finally, we expect SL to be associated with individual voice. Voice refers to behaviours through which employees proactively make recommendations and express concerns (Van Dyne & LePine,). Relatively few studies have explored the role of SL in predicting employee voice. Those that have theorized that leaders persuade followers to believe that they have skills, knowledge, and abilities to improve work situations and meet personal and others’ needs and growth (Duan, Kwan, & Ling,).

The altruistic tendencies of SLs have also been shown to have a positive impact on employee creativity (Neubert, Kacmar, Carlson, Chonko, & Roberts,; Yoshida.,). SL is argued to create a climate of trust and psychological safety (Schaubroeck.,) where followers are inspired to take risks and develop new, more innovative ways of working (Kark & Carmeli,). Again, studies investigating the effects of SL on creativity have largely focused on the individual level.

Third, we focus on CPB. CPB comprises a collection of voluntary behaviours that detract from organizational objectives (such as unruliness, theft, or aggression) and ultimately harm organizational well‐being (Rotundo & Sackett,). SLs are role models for behavioural integrity and authenticity (Panaccio.,), so CPB is likely to be mitigated in the presence of SL. Conversely, those low in SL are more likely to be perceived as selfish and ego‐centric, putting their own needs above others, meaning that CPB is likely to increase. To date, research has focused mainly on the effects of SL on individual‐level CPB.

Secondly, SL theory posits that, as leaders encourage higher levels of moral reasoning in followers (Graham,), and act in ways that prioritize follower needs over self‐interest, subordinates experience an increased sense of empowerment, growth, and well‐being (Panaccio, Henderson, Liden, Wayne, & Cao,) and reciprocate with heightened OCBs (Liden, Wayne, Liao, & Meuser,). At the team level, SLs develop collective trust in the leader, which can also be enhanced by the way that the servant leader empowers the group and provides support designed to assist the team meet its goals. When team members feel that the leader can be trusted, members respond not only by performing well as a collective, but also engage in discretionary behaviours (Liden.,).

Firstly, we expect SL to be positively associated with individual and team performance. Research has demonstrated that SLs display a strong developmental orientation, provide timely feedback, and support skill development (Chen, Zhu, & Zhou,). This improves followers’ task performance by enhancing their abilities to meet objectives and perform as required (Chen.,; Hu & Liden,). SLs have also been shown to influence team performance by affirming team strengths and potential and providing collective developmental support (Hu & Liden,). Further, SLs promote the welfare of others by conveying support to individual group members, minimizing relationship conflicts, and nurturing the broader potential of individual members and a sense of community within the work group (Schaubroeck, Lam, & Peng,).

Servant leadership has been shown to relate to various performance‐related outcomes. Thus, for the present study, we take a multidimensional view of workplace performance. At the individual level, we focus on five specific dimensions: task performance; organizational citizenship behaviours (OCBs); counterproductive behaviour (CPB); creativity; and employee voice. At the team level, we explore team performance and OCBs. Thus, building upon Hoch et al .'s ( 2018 ) and Banks et al .'s ( 2018 ) meta‐analyses, which focused on in‐role job performance and OCBs, we examine a wider variety of work‐related behaviours at different levels of analysis in order to better understand the practical reach of the SL construct.

Servant leadership theory is based on the fundamental premise that servant leaders are primarily driven by empathy, altruism, and a sense of community stewardship (Chiniara & Bentein, 2016 ; Greenleaf, 1977 ), with this ethical imperative driving their deep and unwavering commitment to follower growth, empowerment, and well‐being (Liden, Wayne, Zhao, & Henderson, 2008 ). The premise of SL theory is that by first enabling the fulfilment of followers’ personal ambitions, the achievement of long‐term organizational objectives will follow. While the literature has struggled to reach a consensus on the precise definition and measurement of SL (Newman et al ., 2017 ; VanMeter, Chonko, Grisaffe, & Goad, 2016 ), we adopt the definition recently advanced by Eva et al . ( 2018 ), who argue that ‘Servant leadership is an (1) other‐oriented approach to leadership (2) manifested through one‐on‐one prioritizing of follower individual needs and interests, (3) and outward reorienting of their concern for self towards concern for other within the organization and the larger community’ (p. 4). This definition captures the essence of SL through three key features: motive (other‐oriented), mode (focus on follower growth), and mindset (concern for the wider community). It is also argued to afford a degree of flexibility for reflecting the different and multiple understandings of SL that exist (Eva et al ., 2018 ). As SL theory emphasizes a leader's ‘moral responsibility is not only to the success of the organization, but also to his or her subordinates, the organization's customers, and other organizational stakeholders’ (Ehrhart, 2004 , p. 68), Lemoine et al . ( 2019 ) argue that SL's distinctive focus is on serving multiple stakeholders.

Thirdly, while many existing studies demonstrate the association between SL and follower outcomes (see Eva et al ., 2018 ), the strength of the effects reported varies greatly, with some reporting relatively strong correlations (Ling, Lin, & Wu, 2016 ), and others suggesting far weaker relationships (Neubert, Hunter, & Tolentino, 2016 ), limiting our understanding of SL. Relatedly, there have been recent calls for future research to more clearly establish the boundary conditions of SL (Donia, Raja, Panaccio, & Wang, 2016 ; Liden, Panaccio, Meuser, Hu, & Wayne, 2014 ), particularly through the lens of alternative theoretical perspectives that go beyond those typically applied, namely social‐based theories (Eva et al ., 2018 ). In direct response to Eva et al . ( 2018 ), we use insights from situational strength theory (Meyer, Dalal, & Bonaccio, 2009 ; Meyer, Dalal, & Hermida, 2010 ) to more clearly establish SL's criterion‐related validity and clarify the magnitude of effects across various boundary conditions. Indeed, the consistency and generalizability of findings on SL in various contexts, including different cultures (i.e., vertical‐collectivistic vs. horizontal‐individualistic), industries (i.e., high vs. low industry‐capital intensive), and research designs (e.g., self‐rated vs. externally rated performance), are yet to be comprehensively assessed (van Dierendonck, 2011 ). To address these research objectives, we present the largest meta‐analysis of SL conducted to date. Given the imperative for more sophisticated research designs on SL (Eva et al ., 2018 ), meta‐analysis enabled us to test complex models incorporating multiple moderators and mediators that would be difficult, if not impossible, to test in single studies (Schmidt, 2010 ).

Secondly, it remains unclear exactly how SL elicits positive outcomes. While research has posited various processes to explain these effects (Chan & Mak, 2014 ; Newman, Schwarz, Cooper, & Sendjaya, 2017 ; Yoshida, Sendjaya, Hirst, & Cooper, 2014 ), such explanations have typically been derived from differing, often competing, theoretical frameworks (e.g., social exchange theory, Blau, 1964 ; social learning theory; Bandura, 1986 ; social identity theory, Tajfel, 1978 ), leaving it unclear as to which offers the most consistent explanation. Indeed, in the nomological network presented by Eva et al . ( 2018 ), 35 different mediators in four different categories (employee‐ and job‐centred; leader‐centred; team‐centred; climate‐ and organization‐centred) were identified as demonstrating significant indirect effects in previous research, suggesting that despite the proliferation of studies on SL, we are still no clearer on establishing which theoretical pathway(s) explain the effects. A more parsimonious model, proposed by van Dierendonck ( 2011 ), concentrated on two major pathways through which SL is argued to influence follower, performance, and organizational outcomes, namely high‐quality leader–follower relationships (LMX) and psychological climate (i.e., trust and fairness). In this paper, we therefore test these alternate pathways to uncover which, if any, provides the strongest explanation for understanding the psychological processes that underlie the SL–performance relationship, thus beginning to clarify the nomological network of SL, rather than simply adding more mediators to an already over inclusive framework. Testing this model also directly addresses calls to incorporate competing mediators in the same model as a means of building a more accurate understanding of the influence of SL (Antonakis, Bendahan, Jacquart, & Lalive, 2010 ; Eva et al ., 2018 ). As Leavitt, Mitchell, and Peterson ( 2010 ) point out, despite the potential utility in doing so, meta‐analyses are rarely used to compare alternative theories.

Firstly, a long‐standing debate regarding the conceptual overlap and potential redundancy of the SL construct with related leadership constructs (e.g., transformational, ethical, authentic: Antonakis, Ashkanasy, & Dasborough, 2009 ; Banks, McCauley, Gardner, & Guler, 2016 ) continues to plague the literature (Lemoine et al ., 2019 ). Indeed, findings from a recent meta‐analysis on leadership behaviours (Banks, Gooty, Ross, Williams, & Harrington, 2018 ) suggest large correlations between SL and transformational leadership (TL) ( r = .52; k = 5), as well as other morally based leadership approaches, namely authentic leadership (AL; e.g., Avolio & Gardner, 2005 ; r = .60; k = 1) and ethical leadership (EL; e.g., Brown, Treviño, & Harrison, 2005 ; r = .81; k = 1), although it should be noted that these results were based on very few (often single) studies. Furthermore, while Hoch, Bommer, Dulebohn, and Wu ( 2018 ) did find meta‐analytic evidence of the incremental predictive validity of SL over TL, its incremental predictive validity over AL and EL is yet to be empirically established. Eva et al . ( 2018 ) have since called for ‘researchers to use more robust methods to test (SL's) incremental or predictive validity over existing leadership theories’ (p. 4). Using meta‐analysis, this paper aims to provide much‐needed evidence for the empirical distinctiveness of SL, assessing its relative value for predicting a range of work outcomes over the aforementioned leadership styles.

In an era when corporate scandals have become increasingly rife, organizational leaders are striving to act more responsibly to rebuild trust with followers (Pless, Maak, & Waldman, 2012 ). Consequently, there is a growing appetite for more moral and ethical styles of leadership (Lemoine, Hartnell, & Leroy, 2019 ), with an emerging and increasingly influential form being that of servant leadership (SL). SL explicitly captures the dimensions of personal integrity and social responsibility, rarely seen in more traditional and positive leadership constructs (Ehrhart, 2004 ), leading to an upsurge of research testing the criterion‐related and incremental predictive validity of SL (Chan & Mak, 2014 ; van Dierendonck, Stam, Boersma, De Windt, & Alkema, 2014 ). Recently, Eva, Robin, Sendjaya, van Dierendonck, and Liden ( 2018 ) conducted a systematic review of the literature, consolidating what we know about SL in a comprehensive nomological network and proposing a detailed future research agenda based on considerable gaps in the literature. We draw upon several of these gaps below and discuss how the current paper seeks to address them.

To explore SL's relative predictive validity, we conducted relative weights analyses, using Tonidandel and LeBreton's ( 2011 ) guidelines for understanding the relative associations that each leadership style had with the correlates. Furthermore, we conducted a series of hierarchical regressions using structural equation modelling to determine the variance explained in the criterion by TL, EL, and AL and then to assess the incremental variance explained by SL. The incremental variance was then assessed by the change in variance explained between these two, sequential, steps. Finally, we conducted mediation analysis, using Viswesvaran and Ones’ ( 1995 ) meta‐analytic structural equation modelling (MASEM) procedure. All path models were estimated using robust maximum‐likelihood estimation within Mplus (version 7; Muthén & Muthén, 2012–2015 ). For our incremental and mediation analysis, we used the harmonic mean (Viswesvaran & Ones, 1995 ) for the sample size. This was required because sample sizes varied across the various cells of the correlation matrices.

To complete incremental and mediational analysis, we used correlations from our meta‐analysis, and correlations obtained from other recent meta‐analyses (Hoch et al ., 2018 ; Martin, Guillaume, Thomas, Lee, & Epitropaki, 2016 ). For some relationships (e.g., between procedural justice and employee voice), we did not have enough correlations within our sample and could not find previous meta‐analytic estimates. In such cases, we conducted a separate search of the literature to isolate relevant studies and coded the corrected correlations (see Table 5 and Appendix A for full details).

We examined moderators of SL using meta‐regression. Specifically, we ran random‐effects meta‐regression to determine whether effect sizes were affected by various moderators. Meta‐regression is able to determine whether there is a significant difference between studies according to different levels of either continuous or categorical moderators (Borenstein, Hedges, Higgins & Rothstein, 2011 ). These moderator analyses were conducted using the meta‐analytic software, Comprehensive Meta‐Analysis (version 2.2.064, 2011, Biostat, Englewood, NJ). The categorical moderators we included in meta‐regression were as follows: rater (whether the outcome is self‐ or other‐rated); time (whether the outcome is measured at the same time or later than SL); capital intensity (whether the study is conducted in a high vs. low capital intense industry); and scale (the scale used to measure SL). These were all dummy coded, except for one instance where there were four categories for scale, which were coded 1, 2, 3, and 4 (see Table 5 ). The continuous moderators we included were tenure (the length of time an individual was working for their current organization) and culture (power distance and individualism–collectivism). For culture, we assigned culture scores ranging from 1 to 100 based on the three culture taxonomies obtained from Hofstede ( 2001 ). Each study was assigned a score for power‐distance and individualism ranging from 1 (representing very high collectivism and very low power‐distance) to 100 (indicating very high individualism and very high power‐distance). For example, the United States is very individualistic and has a score of 91. Spain has a score of 51, about the middle of the scale. China, with a much more collectivist culture, has a score of 20. We coded industry capital intensity based on whether the industry's cost involves substantial investment in physical or financial assets such as equipment, machinery, or other expensive capital assets. For example, industries that are considered to be high industry capital intensity typically include energy, telecommunications, and transportation industry, while industries that are considered to be low‐medium industry capital intensity typically include service, education, and hospitality industry.

We employed the commonly used Hunter and Schmidt ( 2015 ) approach for our investigation. This involved generation of a random‐effects model accounting for sampling bias and measurement error. This was achieved through calculation of a sample‐weighted mean correlation ( r ), and a sample‐weighted mean correlation that was corrected for unreliability in independent and dependent variables. The latter correlation is referred to as the corrected population correlation (ρ). We also report the 95% confidence intervals (95% CI) of the sample‐weighted mean correlation and the 95% credibility intervals (95% CV) of the corrected population correlation. The 95% CIs estimate variability in the sample‐weighted mean correlation due to sampling error; credibility intervals estimate variability in the individual correlations across studies that can be explained by moderating variables (Whitener, 1990 ). If the 95% CIs do not include zero, we can say with 95% CI that the sample‐weighted mean correlation differs significantly from zero. Further, 95% CIs can be used to determine whether two sample‐weighted mean correlations differ from each other: The two estimates can be considered statistically different when their 95% CIs are non‐overlapping. When the 95% credibility intervals of the corrected population correlation are large, they indicate considerable variation across studies and suggest that moderators are likely to be in operation.

The initial coding scheme was developed by the lead author on the basis of the extant SL literature. Using this initial coding scheme, two authors independently coded 15 randomly selected studies. The coding was discussed between the authors, and ensuing discrepancies and problems were resolved, resulting in a refined coding scheme. Based on this refined coding scheme, one of the authors coded all studies. The lead author randomly examined approximately 20% of the primary samples. The average intercoder percentage of agreement across the study variables was 95%. Any discrepancies amongst the raters were discussed by two coders until consensus was reached for the final coding. All discrepancies were resolved through discussion.

A study had to meet several criteria to be included in our final analysis. First, it had to include a zero‐order correlation between follower‐rated SL and an outcome, other leadership style (i.e., TL, AL, and EL) or mediator at the individual level. Second, it had to include the sample size used to arrive at the correlation. Third, the sample had to be independent from other studies; if a sample overlapped with another study, it was only included once. In total, 124 publications and 130 independent samples (several publications reported multiple samples) met these criteria. Appendix B details the individual studies used for each of the meta‐analytic relationship examined.

A thorough search was conducted in order to identify published and unpublished samples that examined the antecedents, correlates, and consequences of SL. To identify studies for our meta‐analysis, we first searched for studies examining SL published from 1998 to the end of 2018. To ensure completeness, we used electronic databases, EBSCOHost, Emerald, ProQuest, PsycINFO, and ScienceDirect, which collectively include a wide range of management and applied psychology journals. We included the search terms: servant leadership , servant leader , service leadership , servant organization , and servant behavior . This process yielded a total of 2,391 results including journal articles, dissertations, books, conference papers and proceedings, and working papers. In addition, we examined the reference lists from any relevant review articles and most recent papers (Banks et al ., 2018 ; Eva et al ., 2018 ; Hoch et al ., 2018 ; Parris & Peachey, 2013 ; van Dierendonck, 2011 ). Finally, we searched for possible unpublished and in‐press studies by sending email solicitations to members of the Academy of Management OB listserv.

Our findings provided some support for the role of each of the three mediators. However, the indirect effects found varied as a function of the outcome variable. For task performance and creativity, significant and positive indirect effects were found for both LMX and trust in the leader. For OCB and voice, LMX and procedural justice demonstrated significant and positive indirect effects, whereas the indirect effect through trust in the leader was negative. For CPB, trust in the leader and procedural justice mediated the effects of SL in the expected direction, with LMX having an indirect effect in a positive direction. Taken together, our mediation analysis highlights the important role played by each mediator explored: Trust in the leader, LMX, and procedural justice all played a role in explaining the effects of SL on follower behaviour. Thus, we largely found support for Hypotheses 12, 13, and 14 (but not H12a, H12d, H13b, H13e, or H14c). Interestingly, SL had a significant direct effect on OCB, creativity, and voice, indicating partial mediation through procedural justice, LMX, and trust in the leader.

Table 4 displays the results of our incremental predictive validity analysis and demonstrates support for most of our hypotheses. Specifically, in relation to individual‐level task performance and OCB, SL shows strong incremental predictive validity over all the leadership styles (supporting Hypotheses 9a,b, 10a,b, 11a,b). SL explained an additional 41.7% and 39% of variance in individual‐level task performance and OCB, respectively. SL also explained an additional 35% of variance in team‐level OCB compared to TL and EL supporting Hypotheses H9g and H11g. Furthermore, when included alongside the other leadership styles SL was the only significant predictor of employee creativity (supporting Hypotheses 9d, 10d, and 11d). Looking at Table 4 , the only outcome for which SL did not show incremental predictive validity was CPB. In fact, the results demonstrated positive main effects of SL on CPB when it was included in regression analysis with the other leadership styles. This result may indicate a suppression effect: Large correlations between the variables (particularly between leadership constructs) may artificially suppress the relationship observed in the path analysis.

Hypotheses 9, 10, and 11 predicted that SL would have incremental predictive validity over TL, AL, and EL, respectively, in predicting individual‐ and team‐level behaviour. To explore the effect of SL compared to the other leadership styles, we first conducted a series of relative weights analyses (see Table 3 ). These analyses assessed the relative contribution of each leadership style on the outcome of interest. SL played a dominant role when it came to explain individual‐level OCB (47.84%) and creativity (37.58), over TL, EL, and AL.

Table 2 displays the moderation results for cultural and industrial context and employee tenure. Overall, we found little support for our moderation hypotheses. With respect to national culture, the only significant moderation effect found was in the relationship between SL and team‐level performance; the relationship was larger in cultures that were higher in power distance and collectivism. The only other moderation effect found was for tenure. Specifically, contrary to Hypothesis 8a and 8c, the relationship between SL and individual performance and CPB was larger as employee tenure increased. Finally, we found no evidence that capital intensity moderated the relationship between SL and any employee outcome.

Finally, we explored differential effects of various scales used to measure SL. SL is most frequently measured using scales developed by Liden et al . ( 2008 ), Ehrhart ( 2004 ), Barbuto and Wheeler ( 2006 ), and van Dierendonck and Nuijten ( 2011 ). As seen in Table 2 , we found no significant difference in the relationships between SL and individual‐ or team‐level task performance or OCB as a function of the SL scale used. The only significant moderation effect we observed was that studies measuring SL using Ehrhart's ( 2004 ) measures reported significantly larger effect sizes than those measuring SL using Liden et al .'s ( 2015 ) scale.

As the studies included varied in terms of the source from which they obtained performance ratings, including self‐rated performance (common‐source) and other‐rated (leader‐, customer‐, or peer‐rated), we assessed whether rating source impacted on the effect of SL. As shown in Table 2 , the moderation analysis demonstrates that there were no significant differences in effect size according to rater source. Another design feature that was used by numerous studies in our analysis was time‐separation. This involved collecting measures of SL at a different time to the criterion variables. Specifically, many studies included a time lag after measuring SL and measured the criterion variable several weeks or months afterwards (Neubert et al ., 2016 ). As shown in Table 2 , the moderation analysis demonstrates that the relationship between SL and individual‐level performance is larger when both variables were measured at the same time. However, the same effect was not found for either individual‐level OCB or creativity which both had similar relationship with SL regardless of whether the study was cross‐sectional or time‐separated. We also tested for the possibility of publication bias, comparing whether the effects of SL vary for published versus unpublished studies. Due to limited numbers of unpublished primary studies, we were only able to compare published and unpublished studies examining task performance and OCB at the individual‐level and team‐level OCB. To explore this issue, we used Egger's regression and Rosenthal's N , which we computed using the meta‐analytic software program, Comprehensive Meta‐Analysis (Borenstein et al ., 2011 ). Egger's regression involves testing the asymmetry of a funnel plot to assess publication bias using a linear regression test (Egger, Smith, Schneider, & Minder, 1997 ). In this test, the intercept provides an assessment of asymmetry, with an intercept which is significantly different from 0 indicating bias. A negative intercept indicates that smaller studies are associated with bigger effects. One concern of publication bias is that some non‐significant studies are missing from our analysis and that these studies, if included, would nullify the observed effect. Rosenthal ( 1991 ) provided a computation to determine the number of studies that would be required to nullify the effect. If this number is large, we can be confident that the treatment effect, while possibly inflated by the exclusion of some studies, is nevertheless not nil. Details of these analyses can be found in Appendix A . To summarize these analyses, we found no evidence of publication bias. With the exception, of team‐level OCB, all of the Egger's regression were non‐significant and the relatively high Rosenthal's N for team‐level OCB suggests that publication bias is not a concern.

Table 1 displays uncorrected and corrected correlations for the relationships of SL with each outcome variable. We interpret the magnitude of corrected population correlation as reported by Paterson, Harms, Steel, and Credé ( 2016 ) based on their investigation of effect sizes from over 250 meta‐analyses in the field of organizational behaviour which found that the average corrected effect size was, ρ = .28. Thus, we interpret the magnitude of the true mean correlation estimates in reference to these estimates by Paterson et al . ( 2016 ) rather than the conventional benchmarks proposed by Cohen ( 1992 ), and accordingly, we characterize our estimates as ‘below‐average’, ‘average’, or ‘above‐average’ in relation to Paterson et al .'s benchmark.

Discussion

Servant leadership represents a moral form of leadership that, alongside EL and AL, has seen a surge of interest in recent years (Lemoine et al., 2019). Recently, meta‐analytic investigation has provided some initial evidence for the utility of SL as a newer form of leadership that can provide a contribution to the leadership literature (Hoch et al., 2018). In the current study, we address recent calls to further understand and develop the SL construct (Eva et al., 2018). Specifically, in conducting this meta‐analysis our major aims were as follows: (1) to establish the main effects of SL on a wide range of employee behaviours at the individual level and team level, thus extending previous meta‐analytic findings; (2) to examine the incremental effect of SL over other moral‐based leadership styles (EL and AL); (3) to test some key mediators of the effects of SL in parallel to examine differential effects and potential redundancy; and (4) to examine moderators of the main effects in order to establish key boundary conditions of SL. With these aims in mind, we now turn to the implications of our findings.

Firstly, we found strong evidence for the hypothesized main effects of SL across the outcome variables at both individual level and team level of analysis. These relationships were all significant, suggesting that SL has good criterion‐related validity in relation to an array of workplace behaviours, above and beyond job performance and OCB examined in Hoch et al.'s (2018) meta‐analysis. Our analysis is also based on substantially more studies: The relationships between SL and task performance and OCB were based on 26 and 39 primary studies, respectively, compared with 8 and 6 in Hoch et al. (2018). Finding meta‐analytic support for such effects corroborates the influence of SL and verifies that this style of leadership goes beyond increasing followers’ growth and well‐being (Babakus, Yavas, & Ashill, 2010) and elicits performance‐related behaviour. Eva et al. (2018) also point to an over‐reliance on self‐report, single‐time point studies within the SL field; suggesting this type of design limits the ability to interpret the findings from the literature. To test the robustness of the main effects found in our analysis, we explored the effect of the two most common practices employed to reduce common method bias (CMBs, see Podsakoff, MacKenzie, & Podsakoff, 2012). The first involves collecting SL and the criterion variable from different sources (i.e., the leader and follower). This multisource approach was used in many studies but did not significantly influence the main effects observed. The relationship between SL and follower outcomes was similar regardless of whether the outcome was self‐ or other‐rated. The second widespread method used to reduce CMBs is the use of a time‐separated design whereby the criterion variable is measured at a separate time (usually after) the independent variable (i.e., SL). We found evidence that measuring individual‐level performance after SL did significantly reduce the magnitude of the association between the variables. The same effect was not found for either creativity or OCB. It should be noted that the two methods discussed above are inadequate methods to fully deal with issues of endogeneity and CMBs (see Antonakis et al., 2010; Kammeyer‐Mueller, Steel, & Rubenstein, 2010). As Eva et al. (2018) acknowledge, the issue of endogeneity within study design is not unique to the SL field, but we echo their call for future research to strengthen the survey design in SL research. This includes the use of experimental studies, longitudinal designs, and instrumental variables (Antonakis et al., 2010; Hughes et al., 2018).

Our second aim concerned the pertinent issue of construct redundancy amongst leadership constructs. The large correlations between SL and TL (ρ = .52), AL (ρ = .84), and EL (ρ = .82), found in the current research, confirm the possibility of empirical redundancy. These large correlations are similar to those reported by Banks et al. (2018). However, in support of the conceptual distinction of SL, we found evidence for its relative predictive validity on all outcomes (with the exception of CPB), over and beyond TL, AL, and EL, thus providing evidence for its unique practical value. SL had particularly strong incremental effects on individual‐level performance and OCB as well as team‐level OCB. Overall, our findings point to the importance of considering a wide range of employee outcomes when drawing conclusions on SL's utility. These findings are important to consider in relation to a recent review by Lemoine et al. (2019) who highlighted conceptual similarity amongst EL, AL, and SL but also noted that ‘considering any of the three approaches to moral leadership as “generically moral” obscures potentially important variance in both the constructs themselves and their correlates and oversimplifies much more complex phenomena’ (Lemoine et al., 2019, p. 150). Our findings support this position by suggesting the incremental effects of SL in particular. However, the large correlations between leadership styles do point to a need for future research to disentangle commonalities amongst empirical measures of these moral forms of leadership (Lemoine et al., 2019). Relatedly, most studies included in our review treat SL as an aggregate variable rather than examining the effects of the component dimensions. As a result, we were unable to examine the relative effects of the various dimensions theorized to make up SL (i.e., behaving ethically, creating value for the community, putting others first, helping others grow and succeed, emotional healing, empowering others, and conceptual skills; see Ehrhart, 2004; Liden et al., 2008). Future research should continue to examine the effects of these individual dimensions – particularly those that focus on benefiting multiple stakeholders which Lemoine et al. (2019) highlight as particularly unique to SL.

The third aim of our research was to establish which theoretical pathway posited by van Dierendonck (2011), psychological climate (comprising trust and fairness), or follower relationships (LMX) exhibits consistently stronger effects for explaining the link between SL and outcomes. Such an examination represents an important theoretical contribution, as empirical studies typically do not test multiple mediators of the effects of SL concurrently, leaving it unclear as to which theoretical lens best explains how SL translates into beneficial follower behaviour. LMX was found to be the most consistent mediator for the effects of SL on all outcomes examined, showing significant indirect effects for all outcomes except CPB. This reinforces the notion that SL is crucial in the development of a high‐quality leadership follower relationship (Newman et al., 2017; van Dierendonck, 2011) and aligns with studies highlighting a positive relationship between leadership styles and LMX (Wang, Law, Hackett, Wang, & Chen, 2005). However, examining the indirect effect of SL through LMX is scant, and thus, an important contribution of our study was to ascertain the extent to which LMX can be considered a key mediator of SL. However, we did not find the other pathways to be redundant, with both fairness (i.e., procedural justice), and trust in the leader also being significant mediators for different outcomes. In fact, trust in the leader demonstrated the largest indirect effects on both CPB and creativity. Overall, our mediation analyses are a first step in trying to better understand the dominant mechanisms responsible for linking SL and follower outcomes. As Eva et al. (2018) highlight, individual studies have explored a multitude of mediators, yet these tend to be studied in isolation, with single mediator models limiting our understanding of the relative effects of these different pathways. In the current study, we empirically tested van Dierendonck (2011) theoretical model derived from social exchange and organizational justice theories. However, given that other theoretical accounts and related mediators have begun to emerge (Eva et al., 2018), we suggest that further tests of competing mediators drawn from different theoretical perspectives are conducted in order to more clearly establish the nomological network of SL.

Our fourth and final aim was to examine boundary conditions of SL. Eva et al. (2018) proposed situational strength theory (Meyer et al., 2009) could be applied in order better understand when SL is more likely to influence followers. As SL was developed in the United States, it can be argued that it is a style of leadership that is best suited to cultures where power distance between leaders and employees is low. However, contrary to this prediction, we found that for the most part SL was just as effective in high power‐distance cultures as it was in low power‐distance cultures. In other words, SL does not appear to be a more beneficial style in countries where power distance is low. In fact, our findings suggest that, if anything, the opposite is more likely as the relationship between SL and team‐level performance was stronger in higher power distance cultures compared to lower power distance cultures. Interestingly, this moderation effect echoes findings from a recent meta‐analysis examining empowering leadership which found stronger effects of empowering leadership on follower performance in vertical‐collectivist cultures (Lee, Willis, & Tian, 2018). Overall, this finding suggests that power distance is not an obstacle to the use of SL. While we directly address calls to ‘understand the role national culture plays for servant leadership’ (Eva et al., 2018, p. 15), there is a need to understand the role of national culture not merely from a national level, but also from an intranational level and/or individual level (Gamble & Tian, 2015; Tsui, Nifadkar, & Ou, 2007). While cross‐cultural differences are of great interest, it is also important to recognize the presence of heterogeneity within countries. Future research should therefore further explore the role of national culture by measuring individual differences in various cultural aspects (i.e., power distance) as a moderator of the effects of SL.

Another suggestion made by Eva et al. (2018) was for future research to explore industrial characteristics as a moderator of the effects of SL. Indeed, we posited that SL would be more powerful in industrial settings where the human element is the key competitive advantage, and there are fewer constraints on employee job performance from automation and technology (i.e., low capital intensity). However, we did not find evidence for moderating effect of industrial setting – SL was as effective in high capital‐intensive as low capital intensive industries. Thus, the industrial setting should not be considered a constraining factor for SL. Finally, we explored employee tenure as a moderator of the effects of SL. Contrary to our prediction, we found that the relationship between SL and individual‐level task performance and CPB was larger for employees who had a longer rather than shorter organizational tenure. This effect may be explained by the fact that employees who possess an array of attributes that enable them to be successful in an empowered environment will benefit most from this aspect of SL (Rapp, Ahearne, Mathieu, & Schillewaert, 2006). In other words, it may be the case that employees who have more work experience within an organization may be best able to harness the benefits of a leader who allows them to utilize this experience.

Additionally, the large number of studies included in our analysis also allowed for the testing of some important methodological moderating variables (time‐lagged design, multisource designs, and publication bias). Of particular interest is the significant finding regarding the impact of time‐lagged design on the effect of SL on individual performance. Despite the limitation of time‐lagged design in establishing causality between leadership behaviours and outcomes, the significant finding provides much‐needed meta‐analytic evidence to support the role of time in leadership studies (Castillo & Trinh, 2018). Taken together, our moderation analyses provide some evidence of differences across national culture and time‐lagged design. We urge future researchers to continue to explore the boundary conditions of SL.

Strengths, limitations, and future research directions This meta‐analysis has several strengths. In relation to incremental predictive validity, we included not only TL, but also AL and EL, thus extending previous meta‐analyses (Banks et al., 2018; Hoch et al., 2018). Further, our mediation analysis tested multiple mediators concurrently, allowing us to explore the relative effects of theoretically distinct pathways. Again, this is not typical of similar meta‐analyses, which have explored mediators of leadership constructs separately (Martin et al., 2016; Ng & Feldman, 2015). Our analysis was constrained by the availability of primary studies. We were only able to test the relative predictive validity of SL in relation to TL, AL, and EL. Future research is needed to determine whether SL also has incremental predictive over other leadership constructs such as empowering and self‐sacrificing leadership, for example (van Dierendonck, 2011), as well as exploring alternative possible mediators such as self‐regulatory focus (Neubert et al., 2008). A further limitation was that our analysis relied heavily on cross‐sectional research designs. This is a limitation, particularly in relation to any proposed causal effects (Maxwell & Cole, 2007). It is imperative that future SL research focuses on the longitudinal effects: Cross‐lagged designs are needed to better understand the causal effects of SL – for example, it may be that LMX is an antecedent rather than a consequence of SL. Finally, while we conducted a comprehensive search for relevant articles, any conclusions drawn about relationships for which we only had a few studies should be treated with a certain caution. For instance, we found only seven studies that examined the SL–voice relationship. When relatively few studies are available, it is harder to draw firm conclusions from the results, because such results may be heavily influenced by particularly strong or weak correlations. This limitation also indicates clear areas for future research by demonstrating which outcomes particularly require further investigation.

Practical implications This research has several practical implications for organizations. First, our results indicate that SL has predictive validity over other leadership approaches, and therefore, organizations would benefit by developing their current leaders into SLs. Given the incremental validity evident in the current study, organizations should aim to select SLs into influential positions: Training programmes and selection profiles and processes would need to be aligned and developed to capture attitudes and behaviours associated with SL inside and outside the organization. The findings show that, as expected, SL drives positive behaviours. Training targeted at increasing leader perspective taking, moral standards, and emotional intelligence, should develop SL and, subsequently, follower positive behaviours. Second, consistent with SL theory, our results also suggest that it would be advantageous to create or reinforce a culture that positively promotes the development of trust, fairness, and high‐quality leader–follower relationships: designing, structuring and organizing work and the work environment to enhance interdependency, facilitating relationship building, and promoting skills development can all be beneficial for building trust and leader–follower relationships.