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Servant Leadership: a Systematic Literature Review and Network Analysis

Servant leadership is a form of moral-based leadership where leaders tend to prioritize the fulfillment of the needs of followers, namely employees, customers and other stakeholders, rather than satisfying their personal needs. Although the concept is not new among both academics and practitioners, it has received growing consideration in the last decade, due to the fact that it can positively affect a series of individual and organizational outcomes, such as job satisfaction and organizational commitment. In particular, the latest trend in literature has focused on the identification of the antecedents, mediating and moderating mechanisms at the basis of this relationship, as well as on the development of a common scale to measure the construct across diverse economic and cultural contexts. The purpose of this paper is to depict the evolution of the scientific literature that has developed on the concept, to identify the main criticalities and provide avenues for future research. A dynamic methodology called “Systematic Literature Network Analysis” has been applied, combining the Systematic Literature Review approach with the analysis of bibliographic networks.


Introduction
With the beginning of the twenty-first century, the moral nature of leaders has started to be considered not only necessary for the good of society but also essential for sustainable organizational success (Freeman et al., 2004;Gulati et al., 2010;Padilla et al., 2007), thus marking a considerable shift in research. As a consequence, moral leadership theories, such as transformational, ethical, authentic and servant leadership, have recently received considerable attention from the scientific community.
Servant leadership seems to be the most promising and most investigated over the last few years, especially due to the holistic approach and broad focus adopted compared to the other philosophies, as well as to its important role in affecting individual and teamlevel outcomes, such as organizational commitment, organizational citizenship behaviour, job performance and job satisfaction. Like most other leadership constructs, the definition and measurement of servant leadership were primarily developed in the United States. In particular, the term servant leadership was coined by Greenleaf in 1970 in his essay "The Servant as Leader" to describe an emerging style of leadership where leaders focused on followers' personal growth and development, by treating them in an ethical way. The author asserted that the servant leader is "primus inter pares" or "first among equals", meaning that his/her highest priority is service to others in order to fulfill their needs, rather than fulfilling his or her personal needs. Greenleaf's conception was then refined by many other scholars, such as Ehrhart (2004), who claimed that servant leadership is one in which the leader goes beyond the financial success of the organization recognizing his or her moral responsibility towards subordinates, customers and the entire company's community. The emphasis of the servant leadership philosophy has been placed over time on serving and creating value for multiple stakeholders, both internal and external to the organization. Liden et al. (2008) further stressed the fundamental leadership behaviours of servant leadership, such as behaving ethically, helping followers grow and succeed, empowering, emotional healing, conceptual skills and creating value for the community.
Research on servant leadership can be categorized into three main phases: a first phase focusing on its conceptual development, a second phase investigating the measures and testing the relationships with some fundamental outcomes via cross-sectional research, and a third phase aimed at understanding the antecedents, mediating mechanisms and boundary conditions of servant leadership. The last "model development phase" is the most recent and has seen a proliferation of studies in the last twenty years. A significant contribution to provide an integrative theoretical framework has been recently made by Eva et al. (2019), who offered a clear conceptual distinction of servant leadership compared to other approaches, evaluated and assessed the most rigorous scales of the construct developed so far, and highlighted the most important antecedents, outcomes, moderating and mediating mechanisms identified in the literature.
The purpose of this research is to provide a further and complementary review of the literature on servant leadership through bibliometric methods, in order to assess the evolution of the field over time as well as the current state-of-art on the key trends and provide avenues for future research. In particular, the authors aim to identify: -The structure of the field, the most consolidated research and its temporal and geographical evolution -The most recurring theoretical underpinning and constructs -The most cited articles representing milestones of the literature -The most impactful authors and journals -The disciplines and subject areas involved by the topic -Research implications -Future research directions The structure of the paper is as follows. In the first section, the methodology adopted for the literature review and the steps taken in developing the research are presented. In the second section, the results of three different analyses are explained: namely, the paper citation network consisting in the connected components and the main path, the keywords analysis, and finally the global analysis with the basic statistics. In the third and final section, the main conclusions are drawn and questions to be addressed by future research are provided.

Methods
The paper is based on a two-step method, referred to as "Systematic Literature Network Analysis (SLNA)" (Colicchia & Strozzi, 2012): a Systematic Literature Review (SLR) and a further analysis of the subset of relevant articles obtained through a bibliographic Network Analysis (NA): namely, the citation network analysis, the co-occurrence networks analysis and the basic statistics. The first qualitative assessment is mainly based on the researchers' judgements as to the selection of keywords and leverages on an explanatory approach; while the bibliometric assessment provides more objective insights through quantitative and statistical evidence (Aliyev et al., 2018). In particular, bibliographic data analysed through bibliometric methods include the most impactful author names, journal titles, article titles, article keywords and article publication years (Block & Fisch, 2020). The aim is to "complement the traditional content-based literature reviews by extracting quantitative information from bibliographic networks and detect emerging topics, thus revealing the dynamic evolution of the scientific production of a discipline" (Strozzi et al., 2017). This dynamic analysis has proven to be effective in different research fields, as it highlights the literature development, identifies authors network and topic clusters, examines gaps and criticalities as well as presents further research directions. In contrast to narrative literature reviews, which aim to summarize the content of the studies of a particular research field, SLNA focuses on assessing the conceptual structure of the field and its development over time (e.g. how has the number of studies evolved, how have the topics evolved, how have the outlets evolved, etc.). It goes beyond a mere descriptive summary of prior literature, by leading a discussion of what we know and where we can go, and allows the measurement of the knowledge diffusion within and between disciplines, by identifying interdisciplinary links. Moreover, compared to traditional methods which lack a clear methodological approach, quantitative bibliographic studies make it possible to avoid the researchers' selection bias by selecting clear keywords and exclusion / inclusion criteria and by adopting clear boundaries at every stage to ensure a systematic search of papers (Fetscherin & Heinrich, 2015;Block & Fisch, 2020), to the point that the process can be replicated at any time. Lastly, SLNA is characterized by a more up-to-date and broader scope (with regards, for instance, to the journals and publication years considered), thus minimizing the risk of producing an over-reflective and biased argument by the authors but rather leading to evidence-based conclusions. Figure 1 clarifies all the steps of the methodology. The reference database chosen for the development of the research was Scopus, due to its coverage, convenience, and in alignment with the current literature. According to Falagas et al. (2017) as well as to Block and Fisch (2020), Scopus includes a more expanded spectrum of journals and a faster and broader citation analysis compared to other research databases, such as Web of Science (WoS). This result has been confirmed by Chadegani et al. (2013), who assessed that Scopus covers a superior number of journals compared to WoS, even though it is limited to more recent articles, and by Bergman (2012), who demonstrated that Scopus also provides higher citation counts than Google Scholar and WoS. Moreover, compared to these two other databases, Adriaanse and Rensleigh (2013) proved 1 3 that Scopus delivers the least inconsistencies regarding content verification and quality, such as author spelling and sequence, volume and issue number.
The key search criteria and final query were defined on the basis of the keywords used by scholars to address the concept of servant leadership, according to one reference paper among the main pillars of the literature: "Servant leadership: a systematic review and call for future research" (Eva et al., 2019) from which this paper mainly differs due to its quantitative citation-based methodology. The most common keywords in literature, also employed in this study, consist of: servant leadership, servant leader, service leadership, servant behaviour and servant organization. In order to develop a more comprehensive definition and consequently to obtain a more comprehensive sample on the topic, the search criteria were loosened to "servant leader*" OR "service leader*" to include both "servant leadership" and "servant leader(s)" OR "service leadership" and "service leader(s)". Also, considering the different spelling between British English and American English, both terms "behaviour" and "behaviour" were included. As the literature on leadership is very broad, the terms above were limited to three streams of search in the section "Article title" to include only articles that were strictly related and focused on the topic, and not dealing with it in a marginal way, but also to obtain a moderate number of papers to conduct the analysis. This systematic literature review is most suitable when the number of papers is not too limited nor too big. The authors tried to conduct a broader search stream also including keywords and abstracts, but it resulted not applicable: it provided several thousand results and the content of papers obtained was in most cases out of scope. Since the focus of the research was servant leadership from a human resource and organizational perspective, areas were investigated individually to assess whether they were pertinent or not with the topic. On the basis of this analysis, it was possible to include: Business, Social sciences, Economics, Econometrics and Finance, Psychology, Arts and Humanities, Decision sciences, Environmental science and Multidisciplinary. Papers written in languages other than English were excluded. With regards to the time span, the year in which the study was conducted (2020) was eliminated in order to consider only papers of concluded years. Finally, the search was limited to articles and conference papers, as they contain very clear citations and make it possible to achieve ideal results. See Table 1 for the final specification of the query.
This procedure allowed us to obtain a subset of 357 papers published between 1984 and 2019, which were then analysed using VOSViewer Waltman et al., 2010) and Pajek software (de Nooy et al., 2011) to identify the main citation path emerging from the citation network and also key concepts and trends emerging from the co-occurrence network. Afterwards, the basic statistics of the whole subset of papers were examined in order to provide some general insights: the temporal and geographical evolution of the literature, the subject areas involved, the ranking of the 10 most cited papers and the most influential authors and journals. The findings of these analyses are reported in the following sections.

Citation Network Analysis
The initial procedure of the network analysis was aimed at identifying the main article clusters emerging from the citation network by using the VOS Clustering analysis Waltman et al., 2010). "A citation network is a network where the nodes are papers and the links are citations. The arrows go from cited to citing papers representing the flow of knowledge." (Strozzi et al., 2017). For the identification of the connected component, the minimum threshold of 0 was maintained in order not to exclude recent papers and less relevant authors. The largest connected component (a set of nodes connected by links) consisted of 291 items connected to each other, with 85 different clusters. No other significant connected components emerged from the literature. Figure 2 presents the network obtained with VOSviewer, where nodes are weighted by the citations and coloured with both a cluster and year overlay.
The following procedure of the network analysis consisted in implementing the key route algorithm (main path) of the network, using Pajek: a program providing powerful visualization tools. The objective was to identify the nodes that cite or have been cited the most, thus representing the most consolidated research in the field. This was possible by conducting the betweenness centrality analysis of a vertex, which is "the proportion of all geodesics between pairs of other vertices that include this vertex" (de Nooy et al., 2011, p. 131). The betweenness centrality analysis allows to focus on the importance of a node in the communication between any node pair in the network, to identify those playing a central role in information flows and being responsible for the system vulnerability (i.e. vertexes lying on many of the shortest paths between other vertexes). Figure 3 shows the flow of knowledge over time, with the network of the 25 essential articles, intensively cited and referring to other papers, labelled by Pajek with the name of the first author and the year of publication. It is clear how the research structure has changed over time: from 1996 to 2012 it developed linearly, while from 2012 on it has started to articulate towards different directions often interconnected to each other. One possible interpretation of this pattern is the following: originally, the novelty of the subject led to a straight evolution of the field over time, afterwards, once the topic gained ground and different research trends emerged, referencing papers and literature reviews started to come out.
Based on the previous analysis, the most relevant papers were studied not only to identify the key concepts expressed by the single paper but most importantly to understand the evolution of the field over time. The analysis of the main path allowed for pinpointing trends and variations that would not be very visible in the general set of papers. The main findings, which are the result of a quantitative analysis and have not been selected by the authors according to a discretionary criterion, are reported in the following section with the aim of depicting a landscape of the scientific literature on the concept of servant leadership.

Main Path Analysis
The most recent paper dealing with servant leadership is the one by Yang et al. (2019), which builds on self-determination theory to investigate, through an empirical study conducted in the Chinese banking sector, how servant leadership affects employee creativity. The authors used a survey based on five-point Likert scales to assess that there is a positive relationship between servant leadership and employee creativity, mediated by follower psychological empowerment and moderated by work-family conflict. This paper can be considered as a pillar of the literature as it gathers the contributions of several articles, including a paper by the same author written two years before. Yang et al. (2017) previously provided evidence on other mechanisms influencing the relationship between servant  leadership) with transformational leadership, to assess whether they were able to explain incremental variance with respect to a series of relevant organizational outcomes. Servant leadership emerged as the only positive leadership style adding incremental variance to that explained by transformational leadership, thus being of significant utility. Previously, Van Dierendonck et al. (2014) leveraged on two experimental studies and one field study to differentiate servant leadership from transformational leadership in the way they affect organizational commitment and work engagement, as the former is mediated by follower need satisfaction while the latter by perceived leadership effectiveness. Hsiao et al. (2015) systematically integrated the three levels of organization, employee and customer to demonstrate that leaders displaying servant behaviours stimulate customer value co-creation (CVC) with the key mediating roles of positive psychological capital (PPC) and serviceoriented organizational citizenship behaviours (OCBs). Newman et al. (2017) found that at the basis of the link between servant leadership and followers' OCBs, there are also the mediating mechanism of leader-member exchange (LMX) and the moderating mechanism of leader proactive personality. Chiniara and Bentein (2016) previously provided other mediating mechanisms between servant leadership and individual performance outcomes such as OCBs and task performance: namely the satisfaction of three basic psychological needs of employees (autonomy, competence and relatedness). Ozyilmaz and Cicek (2015) tested the positive effects of servant leadership on OCBs and on job satisfaction, assessing that this second relationship is partially mediated by psychological climate. Hunter et al. (2013) further investigated both the direct effects generated by servant leaders at the individual level, such as decreased turnover intentions and disengagement, and the indirect effects generated at the team level, such as decreased turnover intentions, helping and sales behaviour through the mediation of service climate. Moreover, they investigated the basis for individuals enacting this mode of leadership and found that leader agreeableness represents a positive antecedent of servant leadership, while extraversion a negative one. Executive characteristics of servant leaders were also tested by Peterson et al. (2012), who assessed that narcissism is negatively related to servant leadership while founder status (i.e. founder or non-founder) is positively related to servant leadership; both effects are partially mediated by the chief executive officer identification in the organization. Sun (2013) further concentrated on the identity of servant leaders, by explaining the psychological factors, both cognitive and behavioural, that constitute it. Neubert et al. (2016) tested servant leadership effects in hospitals, accumulating evidence that there is a positive relationship with both nurse and patient satisfaction, moderated by organizational structure. Similarly, Chen et al. (2015) explored how managers' servant leadership affect the performance of frontline service employees', such as hairdressers, through the partial mediation of selfefficacy and group identification. In relation to these performance behaviours, they also found that servant leadership explains additional variance above and beyond transformational leadership. Liden et al. (2014) developed a model to test servant leadership in restaurants and stores. Specifically, they demonstrated that servant leaders propagate servant leadership behaviours among employees, such as increased job performance, creativity and customer service behaviours as well as decreased turnover intentions, by establishing a serving culture at the unit level (e.g. store) and fostering employee identification with the unit. Liden et al. (2015) contributed to the literature by providing the shortest-to-date 7-item scale (SL-7) measure of global servant leadership, starting from a previous 28-item scale (SL-28) developed in 2008, and tested it across three empirical independent studies. Besides the topic of employee creativity already investigated in literature, Yoshida et al. (2014) ascertained the effects of servant leadership on individual relational identification and collective prototypicality, which, in turn, fosters team innovation. Antecedents of servant leadership discussed above have been examined by other scholars, such as Hu and Liden (2011), who identified team-level goal, process clarity and team servant leadership as three mechanisms affecting team potency, performance and organizational citizenship behaviour. The authors also emphasized the role of servant leaders in moderating the link between team-level goal and process clarity with team potency. Similar outcomes were found a year before by Walumbwa et al. (2010), whose analyses revealed that the relationship between servant leadership and OCBs is partially mediated by commitment to the supervisor, self-efficacy, procedural justice climate and service climate. Hale and Fields (2007) leveraged on three servant leadership dimensions introduced by Greenleaf (1977), namely service, humility and vision, to point out cultural differences affecting the way servant leadership is perceived in different countries. Specifically, they found that countries with a higher level of power distance and collectivism experience servant leadership behaviours less frequently. They also assessed that, when great value is placed on uncertainty avoidance, vision has a significant stronger relationship with leadership effectiveness. Previously, Dennis and Bocarnea (2005) developed and tested a scale aimed at measuring five out of the seven servant leadership constructs based on Patterson's theory: agapao love (which means to love in a social or moral way), humanity, vision, trust and empowerment. This theoretical development was drawn on a literature review by Russell (2001), who provided an overview of the current individual and organizational values associated with servant leadership, deepening their role in three main attributes: trust, appreciation of others and empowerment. A sequential, upward-spiralling model based on the variables of vision, influence, credibility, trust and service was formerly developed by Farling et al. (1999) to explain how these variables relate one to another in defining the concept at the basis of servant leadership. This paper represented an evolution of two former analyses. The first one consists in a servant leadership model developed by Buchen (1998) within the context of higher education and based on five main dimensions: identity (the direction of ego and image), empowering (the sharing of power with collaborators), reciprocity (a relationship of mutual dependency between leaders and followers), commitment (the absolute devotion to academic discipline) and finally future (the alignment between faculty and institution). The second is a reflection paper on Greenleaf's definition of servant leadership by Spears (1996), which, on the one hand, emphasizes the primary goal of serving the greater needs of others and, on the other hand, draws the evolution of the topic from its genesis (1970) to the current time (2019). At first, from the mid 1990's to the late 2000's, research was mainly qualitative and moved towards the development of a theoretical framework of servant leadership, as well as of various scales aimed at measuring the main dimensions of the construct. The last stream of research from 2010 to 2020, instead, suggests the authors' orientation for a quantitative approach based on surveys, experimental and field studies to investigate the antecedents, mediating mechanisms and boundary conditions of servant leadership. Recently, some qualitative studies have emerged again on the topic; however, very few scholars are taking advantage of mixed methods combining the quantitative and qualitative approach.
From a theoretical perspective, the attempt of the present paper was also that of identifying meaningful constructs, underpinnings and framework used in the most consolidated literature on servant leadership, even if not explicitly mentioned by single studies. All papers, except one, were built on the basis of the servant leadership theory, often in combination with theories on other leadership styles, such as transformational, or on antecedents, outcomes, mediators and moderators of servant leadership, such as LMX theory. Moreover, the majority of paper explicitly employed more than one theoretical basis. The most recurring theory (6 out of 25 papers) was the social exchange theory, which was defined by Blau (1964) as "voluntary actions of individuals that are motivated by the returns they are expected to bring and typically do in fact bring from others" (p. 91) and is based on the central premise that the exchange of social and material resources is a fundamental form of human interaction. Motivational theories were also found several times (6 papers), with different sub-theories, such as goal-setting theory, motivational language theory and intrinsic motivation theory, emphasizing various factors that can foster personal or followers' motivation. The most important among these motivational theories came out to be the self-determination theory's (SDT) basic psychological needs, which consists in an empirically-based theory of human behavior and personality development aimed at identifying the social-contextual aspects that promote or prevent motivation based on the satisfaction of basic psychological needs such as competence, relatedness and autonomy (Ryan & Deci, 2017, pag. 3). The social learning theory (SLT), then evolved in the social cognitive theory (SCT), also emerged to be fundamental (5 papers), positing that learning occurs within a social context through the combination of individual experience, social interaction and environmental factors. Finally, the least recurring theory was the social identity theory (2 papers), which is a social psychological theory examining the role of self and identity in group and intergroup dynamics (Hogg, 2016).

Co-word Network Analysis (Keywords Analysis: VOS Clustering)
A second type of analysis, focused on the authors' keywords, was carried out in VOSviewer on the basis of the co-occurrence network. Co-occurrence analysis assumes that the article keywords chosen by various authors represent an adequate description of the content or of the relationship that the paper establishes between investigated problems (Strozzi et al., 2017). The aim of the analysis was to frame the development of the research trends over time: if many co-occurrences can be identified around a term, this is likely to represent a specific research pattern of the discipline. An occurrence threshold of 8 was used, with the goal of ensuring clusters' consistency in terms of content and dimension. A set of the 17 most relevant keywords divided into 3 different clusters was obtained, as shown in Fig. 4.
The network's nodes correspond to the keywords of the 357 papers' authors and their link weights to how many times the words appear in the papers. Three colors (red, blue and green) differentiate the keywords belonging to one cluster from other clusters' keywords, while the dimension of the node stands for the total link strength.
In the following section, the keywords clusters are examined in order to address the most relevant research patterns in the literature. Hence, the topics below have been discussed on the basis of the output of a quantitative analysis, aimed at addressing the most used keywords in the literature and identifying research trajectories within each cluster.

Cluster 1: Servant Leadership, Leadership, Transformational Leadership, Leadership Development, Scale Development, Trust
Servant leadership is one of the most recently investigated and adopted approaches belonging to the branch of moral leadership theories. As such, it has been studied in parallel with other similar leadership styles, such as transformational leadership. Transformational leadership is a positive form of leadership developed by Burns in 1978 as an ongoing process where "leaders and followers raise one another to higher levels of morality and motivation beyond self-interest to serve collective interests". This concept was then expanded by Bass (1985) and applied to organizations as a guideline for leaders to make followers perform beyond expectations. From a theoretical standpoint, a significant overlap between servant leadership and transformational leadership has been assessed by scholars, especially in terms of vision, influence, credibility, trust and service shown by leaders, to the point that servant leadership has often been considered as a form of transformational leadership (Farling et al., 1999). Trust, in particular, has been addressed in both leadership styles as central to relationship: an important factor in the interdependence existing between leaders and followers, consisting in four distinct dimensions: competence, openness, concern and reliability. Nevertheless, an attempt has been made to define the major variables involved in the servant-leader follower transformational model. On the other hand, research has tried to identify and address the main differences, or better nuances, between the two leadership approaches: while servant leadership focuses more on supporting and developing individuals within an institution, transformational leadership emphasizes the role of leaders in inspiring followers to work towards a common goal (Allen et al., 2016).
In 2010s, another stream of literature has focused on the development of a reliable and multidimensional scale to measure various aspects of servant leadership. Examples include the 6-item Servant Leadership Behavior scale  to measure servant leadership behaviors in a leader, or the Executive Servant Leadership Scale (Reed et al., 2011) to measure executive servant leadership across different organizational contexts.

Cluster 2: Job Satisfaction, Organizational Commitment, Public Sector, China
Several empirical studies have analyzed the relationship between servant leadership and different organizational outcomes, both at the individual and collective level, such as job satisfaction and organizational commitment. Moreover, different mediating and moderating Fig. 4 Co-occurrence author keywords network (size = total link strength, color = cluster) mechanisms as well as various antecedents of this type of relationship have been investigated. For instance, Kauppila et al. (2018) demonstrated that HR manager servant leadership positively influences organizational employees' overall justice perception, which in turn enhances organizational commitment and job satisfaction. Moreover, they found that high leadership self-efficacy fosters a line manager's effectiveness to emulate servant leadership behaviors from HR managers and use these behaviors to advance positive justice perceptions among their followers.
Since the general concept of leadership and the specific concept of servant leadership were developed in the US and in western societies, a consistent research stream has examined the generalizability of servant leadership constructs in completely different cultural contexts, such as China. US society is indeed highly individualistic, short-term oriented and characterized by low-power distance, meaning that relationships are expected to be participatory, democratic and consultative, while Chinese society is permeated by a collectivist culture, long-term goals and high-power distance, therefore based on the expectation that power is distributed unequally. Also, most of these studies were conducted in the public sector, where servant leadership has proven to be very effective in fostering many positive organizational outcomes. Liu et al. (2015) partially confirmed the generalizability of servant leadership constructs from Western society to China and also found a positive relationship between supervisors' servant leadership and the public service motivation.

Cluster 3: Organizational Culture, Humans, Human, Adult, Male, Female, Article
Organizations across different fields and geographical contexts need to understand the role of a leadership that is responsive to a "service mission" in driving the company's evolution and success. Effective servant leadership practices are "humane oriented"; they are implemented when managers or leaders invest in human resources to create a social exchange relationship with employees that makes them feel valued and repay the organization through positive outcomes (Karatepe et al., 2019). As a consequence, employees' commitment and creativity are stimulated and organizational citizenship and prosocial behaviors are fostered, leading to an increase in organizational performance. For instance, Zhou and Miao (2014) found that servant leadership positively influences employees' commitment through perceived organizational support as a mediator.
In this framework, culture, and particularly organizational culture, is strictly connected to the leadership style adopted within a company. On the one hand, servant leadership is more likely to apply in contexts characterized by specific cultural values such as paternalism, collectivism and low-power distance. On the other hand, servant leadership can be adopted to create a new organizational culture based on trust, fairness and high-quality leader-follower relationships (Lee et al., 2019).
Leadership attitudes also vary according to gender, as some studies reported that, relative to their counterparts, leading females are more likely to display behaviors of altruistic calling, emotional healing and organizational stewardship (Beck, 2014) and to hold service and altruistic value (De Rubio & Kiser, 2015); therefore, they more often behave as servant leaders.

Keywords Temporal Analysis
From a temporal standpoint, VOSviewer offers a graphic representation that allows us to identify the most recent keywords and therefore the core topics currently discussed in literature. Figure 5, overlay visualization, displays the ultimate research trends by coloring them in yellow, in contrast with the oldest, colored in blue. It is possible to infer that the concept of servant leadership has been recently discussed in relation to some important outcomes, such as organizational commitment and job satisfaction, to which it is linked by evidence. The effectiveness of this leadership style represented in several organizational contexts has encouraged the development of specific leadership practices, such as training or interventions, aimed at fostering the servant leadership behaviors. At present, most empirical studies have focused on the public sector, as it is often characterized by front-line employees who imitate servant leaders' behaviors displayed by their managers, thus promoting the quality of relationships with end-customers and providing significant benefits to the whole organization. In this framework, the influence of organizational culture is still relevant, as it determines the way servant leadership is built by leaders and perceived by followers, along with its effectiveness in achieving the desired outcomes. In fact, according to the cultural climate, servant leadership may relate to both individual and organizational outcomes through different mediating and moderating mechanisms.

Global Analysis: Basic Statistics
In the "analyze search results" section of Scopus, it is possible to gain some general insights into the whole subset of 357 papers used for the purpose of this literature review. Figure 6 shows how the topic of servant leadership dates back to the mid 1980s but started receiving significant attention only with the beginning of the new millennium. Particularly, it experienced exponential growth from 2007 to 2019. This corresponds to the period when the first scales for the measurement of servant leadership were developed allowing the conduction of several empirical studies across various organizational contexts.
From a geographical standpoint, it is possible to observe in Fig. 7 how the trend has interested primarily the United States (40.8%), where theories of servant leadership first Fig. 5 Overlay visualization originated, and China in the second place (13.8%), where the western construct of servant leadership was tested to assess its reliability and validity across cultures. Several cultural differences have indeed been assessed between the United States and the Chinese culture; for instance, China is a long-term and collectivist country committed to work loyalty and respect, while the United States are more individualistic and oriented to short-term business relationships. The topic has then spread in most Anglo-Saxon countries, such as Australia (7.7%), United Kingdom (7.4%), South Africa (5.8%) and Canada (4.5%), probably fostered by their use of a common language and their cultural and historical ties. The countries of Netherlands, Hong Kong, Spain and Turkey represent altogether the remaining 19.9%.
Servant leadership is a versatile and multidisciplinary topic, as it can be applied to a variety of contexts that also fall outside that of the typical corporation. Figure 8 shows how, besides Business, Management and Accounting (38.7%), the subject areas of Social Sciences (25.3%) Psychology (11.1%) and Arts and Humanities (7.9%) are also significantly involved in the literature. This can be explained by the fact that, rather than organizations, at the core of servant leadership are humans: specifically, leaders and their followers. Figure 9 shows the contribution of the most impactful journals over time. The Leadership and Organization Development Journal with 22 articles out of 63, is the leading in the field and has grown exponentially from 2015. The journal of Business Ethics, with its 15 articles, has also been very influential for contributions to servant leadership research in the last decade. Other articles focusing on servant leadership have been published in Leadership Quarterly (10), International Journal of Contemporary Hospitality Management (8), and Leadership Organization Development Journal (8), which are also the longest-running in time.
With regards to the scholars who significantly contributed to the academic research on servant leadership (Fig. 10), some are based in the United States, where leadership theories have originated and are still widely investigated; the remaining part are based in Europe and Australia but have worked for important American journals and communities, such as Greenleaf center for servant leadership. The most important contribution comes from van Dierendonck (2014Dierendonck ( , 2019, who has an expertise in leadership and leadership development  Liden et al. (2008Liden et al. ( , 2014Liden et al. ( , 2015 and Sendjaya et al. (2008Sendjaya et al. ( , 2019, who have been writing for some of the most important journals in the field, such as Leadership quarterly, Journal of business ethics and Leadership and organizational development journal, which were also highlighted by the previous analysis (Fig. 9). The remaining scholars, Winston and Fields (2015), Eva (2019), Ruiz-Palomino (2018), Bande (2015), Barbuto and Wheeler (2006), Cooper (2014) and Jaramillo (2009, 2015, have to be mentioned as they also provided considerable contribution to research, as proved by the number of citations of their works.   Figure 11 shows the output of an analysis performed in order to identify recent breakthrough papers that have provided a significant contribution to the topic and have received considerable attention from the scientific community. The ten most cited papers have been identified by extracting all 357 papers from the Scopus database and ranking them according to the overall number of citations of the last 5 years, divided by 5 (average value of citation per year). This reduced time span has been chosen instead of the time-length of the whole dataset in order to avoid the biased result of obtaining the oldest papers as the most cited, due to the fact that they would have received a greater number of citations over time. The article with the highest number of citations is the one by Liden et al. (2008) who developed and validated a multidimensional measure of 28 items measuring 9 essential dimensions of servant leadership. This scale has been widely applied to test the construct validity in various organizational contexts in recent time, together with its shortened version of 7 item developed by Wayne et al. in 2015. The remaining most cited articles can be divided in three groups, according to the research streams. One stream has focused on the clarification of the construct and the servant leadership theories in organizations, including  scale development and validation. A second stream has been investigating the mediating and moderating mechanisms through which servant leadership leads to a series of behavioral, attitudinal and performance outcomes, both at the individual and collective level. Finally, a third stream has compared servant leadership to other moral-based leadership styles, such as transformational, ethical and authentic leaderships in terms of focus and their associations with a wide range of organizationally relevant measures. All the most cited papers are part of the biggest component shown in Fig. 2; moreover, four out of the ten papers are included in the main path, while the remaining six are not. Being written by more than one authors, these impactful papers are the result of a significant investigation conducted on the topic by more scholars. Altogether, the articles suggest that the most consistent trend in literature is moving towards the measurement of servant leadership across various cultural and organizational contexts, at both the individual and collective levels (organization, employee, customer, etc.). This has been possible through the clarification of the common constructs composing servant leadership and the development of a scale able to test them across different organizations and organizational levels.

Discussion
This paper represents an attempt to rationalize the content of research developed in the context of servant leadership. The limitations of the study are mainly related to the adopted methodology. First of all, it consists in a literature review based on a citation network analysis, which may not be fully representative of a paper's qualitative contribution to the body of knowledge, especially because VOSviewer shows only a part of the whole subset. Moreover, citations could be biased because scholars often tend to cite the most relevant articles in the literature, driven by their reputation and popularity. However, these limitations can be overcome due to the fact that the purpose of the current study is to depict an evolutionary path of the topic, rather than investigate in-depth the contribution of single papers.
The growing body of empirical studies on servant leadership, analyzed for the purpose of the analysis, has allowed to identify some consolidated streams of research and some areas of the literature deserving further investigation. First of all, there is evidence that servant leadership can foster employees' positive outcomes, with different antecedents and through various mediating and moderating mechanisms. These outcomes have been found at the individual, team and organizational level and are of various types: behavioral, such as organizational citizenship behavior (OCB) and proactive behavior (Chiniara & Bentein, 2016;Newman et al., 2017;Ozyilmaz & Cicek, 2015;Walumbwa et al., 2010) attitudinal, Fig. 11 Ranking of the ten most cited papers in the last 5 years (mean value of citation per year) such as engagement and job satisfaction (Chiniara & Bentein, 2016;Hunter et al., 2013;Kauppila et al., 2018;Van Dierendonck et al., 2014) and performance, such as customer value co-creation and team performance (Hsiao et al., 2015;Hu & Liden, 2011). Moreover, servant leadership has been found to be of significant utility in explaining incremental variance above and beyond transformational, ethical and authentic leadership, with regards to these organizationally relevant criterion measures (Hoch et al., 2018). For this reason, scholars have addressed their attention to this leadership approach and several companies are moving towards the adoption of an organizational climate based on service, ethics and healthy work relationships that could significantly contribute to the organizational success (Eva et al., 2019). This aspect makes servant leadership attractive for both future research and usage. Also, in the analyses of the present study (main path, keywords clusters, global statistics), no significant criticism of servant leadership, from both an empirical and theoretical level, has emerged; the development of minor critical examinations has always started from a prior enhancement of the construct and its positive outcomes. It has to be considered, however, that the unquestioned positive praise of servant leadership may be due to the so called "Matthew effect": the fact that those papers offering a promising perspective of servant leadership, which have previously been successful, are more likely to be cited again and again (Bol et al., 2018).
Although a positive interpretation of servant leadership prevails in the literature, the authors of the present study went beyond the outputs of the main analyses to identify in the literature some potential drawbacks associated with the adoption of this practice within organizations (Palumbo, 2015). First of all, it has to be considered that the servant leadership approach takes time to build, as it requires strong interpersonal relationships engaging the emotional, relational and ethical dimension of followers, in which both the leader and the followers play a vital role in maintaining them over time. Plus, the servant leadership style may not be suitable for all organizations, especially those characterized by a fast-changing environment where decisions have to be made quickly, due to the fact that they would require a fast top-down approach, rather than bottomup. Another risk is losing sight of the purpose of the organization and ultimate goals in favour of people development: the servant leader is in fact devoted to the individual employee and their growth rather than to the organization. This could have negative effects on the organizational effectiveness. On the other hand, a successful company performance is not always due to a visionary leader who establishes a climate of service, as this represents a common misperception of the business world: the halo effect (i.e. the tendency to make specific inferences on the basis of a general impression).
Lastly, too much healing and empathy shown by the leader may turn into merely protective behaviour towards followers, which would discourage them from adopting a proactive role and promptly dealing with critical issues within the organization. This would challenge the prevailing arguments of the literature of servant leadership by producing a disabling environment that disempowers employees and leads to a situation of dependency on the leader (Palumbo, 2015). To prevent this possible counterintuitive consequence, servant leaders should act as role models and lead by example, ensuring at the same time that followers have the right degree of autonomy and responsibility. In light of these considerations, the conceptualization of servant leadership should be revisited to contemplate its side-effects, in terms of followers' behaviour, leader-follower relationships and organizational outcomes, to prevent the impoverishment of the overall organizational effectiveness predicted by some studies (Andersen, 2009;Palumbo, 2015;Liu, 2019;Chenwei et al., 2021, Wu et al., 2021.

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In particular, the authors of the present study have leveraged on a critical assessment of the outputs of the main analyses of the literature on servant leadership to identify some research areas that have not been examined in detail and deserve further investigation: -servant leaders' system of beliefs and values (i.e. ethics) as well as other antecedents, that may significantly affect followers' and organizational performance; -other mediating or moderating mechanisms (i.e. contextual discriminants) influencing the relationship between servant leadership and positive outcomes, both at the individual and organizational level; -servant leadership behaviours displayed by followers, that are useful to promote customers' satisfaction, especially in the service sector; -the utility of servant leadership in contexts where it has not yet been evaluated, such as technology, to test its validity across industries; -longitudinal, multi-level studies confirming the effectiveness and generalizability of the most recent scale of global servant leadership assessment (SL-7) across culturally diverse countries (other than the US and China, as suggested by this literature review), according to well-known frameworks such as Hofstede's -critical theoretical and empirical investigation of the potential shortcomings of servant leadership often neglected by scholars, to challenge the current positive interpretation of the topic and advance the scientific knowledge Additionally, on the basis of the authors' considerations, the role of servant leadership, compared to other types of leadership, may be investigated within the institutional framework (e.g. public services and administration, where it has shown to be very effective) and companies' organizational change management.

Conclusions and Managerial Implications
Due to its holistic approach, broad focus and important role in affecting both individual and team-level outcomes, servant leadership has seen a proliferation of studies in the last approach may be particularly effective in the post covid-19 scenario and / or in contexts characterized by a high degree of volatility, uncertainty, complexity and ambiguity (VUCA). In these environments, leaders struggle to make all decisions, thus requiring the proactiveness and motivational orientation of all employees, which have been identified as important mediators to positive followers' outcomes in the servant leadership research (Eva et al., 2019). Specifically, further considerations are needed in relation to the potential role of this leadership practice in empowering and supporting followers, as well as in giving them the right degree of autonomy and responsibility to take on new challenges and act on behalf of the company when pressured by the external environment.

Data Availability
We ensure data transparency.
Code Availability Not applicable.

Conflicts of Interest/Competing Interests
We have no conflict of interest or competing interest to disclose.