Effect of Chief Executive Officer’s Sustainable Leadership Styles on Organization Members’ Psychological Well-Being and Organizational Citizenship Behavior

Situational leadership theory and the contingency approach of leadership were utilized and applied based on situational theory. Based on a total of four foundational theories, that is, bottom-up spillover theory, theories of prosocial behavior, and so on, this study empirically analyzed what influence a chief executive officer’s (CEO’s) sustainable leadership styles (servant, ethical, and authentic leadership) have on the psychological well-being and organizational citizenship behaviors of organization members. The study was conducted on adult employees of midsized or larger companies (including subsidiaries) across four countries: South Korea, the United Kingdom, the United States, and South Africa. Data were obtained from 649 adult employees. SmartPLS was used to conduct structural equation modeling analysis of the data. The results were as follows: (1) CEOs’ servant and authentic leadership styles had statistically significant positive (+) effects on employees’ psychological well-being; however, ethical leadership did not. (2) CEOs’ ethical leadership had a statistically significant (+) effect on employees’ organizational citizenship behavior; however, servant and authentic leadership did not. (3) Employees’ psychological well-being had a statistically significant (+) effect on organizational citizenship behavior. CEOs are attracting more attention than ever, leading companies in today’s rapidly changing times. This suggests that it is necessary to comprehend principles that show when, where, and how important leaders are and sustainable leadership styles that can increase their chances of success. Moreover, this study derived constructive implications that a leader can overcome today’s challenges through sustainable leadership styles.


Introduction
The concept of sustainability has been gradually expanding to encompass more than environmental protection [1]. Furthermore, the term has been applied to various aspects of commercial entities. Thus, research on sustainability based on leadership theories is actively underway, with case studies and empirical tests, specifically, on leadership styles that have sustainable effects on organizations' functioning (e.g., [2,3]). For example, studies have shown that servant and ethical leadership styles have sustainable effects [2].
Leadership is an important area of research in the field of social science, especially in business and corporate management [4,5]. An organization needs a leader who exerts sustainable influence and leads employees to efficiently and effectively achieve sustainable goals [2,3]. In this study, we state that leadership with sustainable effects encompasses maintaining positive leadership within the organization for the long term.
The chief executive officer (CEO) is the head of a company, and they have the authority as the company representative to look into its operations [6]. While CEOs of startup companies lead only a few employees, others of large corporations have tens of thousands of employees with a company history of over 100 years [7]. Every CEO strives to help their interests of others in business management activities and pay attention to solving ethical problems of the organization [46,47].
In fact, there is a growing trend of studies that have recognized the importance of ethical leadership, but these have focused on the leadership of managers or direct superiors rather than that of the CEO [48,49]. However, considering that the CEO leads the ethical activities of the company and establishes ethical norms, research on CEOs' ethical leadership is important. For example, Eisenbeiss et al. [50] found that CEOs' ethical leadership contributes positively to the formation of ethical cultures within their organizations, which in turn has positive effects on brand performance (outcome). Therefore, ethical leadership has statistically significant effects on company and organizational culture, beyond the dimension of simply regarding the CEO's remarks and behaviors as personal matters [50,51]. Furthermore, ethical leadership has been reported to have positive effects on the psychological well-being and organizational citizenship behavior of members [52].
Finally, authentic leadership involves clear awareness of goals, putting firm values into actions, and openly communicating with the organization members. In other words, authentic leaders are not leaders who are given the authority, but those who inspire people to work based on their sincerity [53,54]. They take care of organization members, nurture them, help them build confidence, teach them skills, and discipline them if necessary to lead them to achieve more than what they would otherwise be capable of. Therefore, authentic leadership insists on discarding the third-person model, in which the company creates stories and styles of the leader in a uniform fashion and forces members to adopt it [53,54]. Authentic leadership advocates democratization so that organization members can create leadership styles for individual leaders. In other words, authentic leaders insist on leading by example [55]. As such, they ultimately embrace people; they instill trust and happiness among organization members and foster them to become greater leaders [56]. Furthermore, it has been reported that authentic leadership has positive effects on the psychological well-being and organizational citizenship behavior of members [57,58].
From a corporate perspective, organization members with high psychological wellbeing have a positive perception about the organization, support their colleagues, put forth more effort than expected, and have more consideration for others [59]. If psychological well-being is high, there is a higher probability of organizational citizenship behavior appearing positive [60][61][62]. Psychological well-being is thus positively (+) related to organizational citizenship behavior and is closely related to the organization's performance creation [63,64]. Organizational citizenship behavior therefore induces desirable behavior among organization members and improves organizational effectiveness [65]. Moreover, it has an additional effect of improving organization members' sense of belonging and organizational satisfaction [66]. Therefore, it increases organizational effectiveness in addition to reducing the burden on the leader [67,68]. In other words, psychological well-being is a mediating factor in the relationship between servant, ethical, and authentic leadership and organizational citizenship behavior.
After previous studies were examined comprehensively and applied to this study, it was expected that CEOs' sustainable leadership styles (servant, ethical, and authentic) would have positive effects on their organization's members' psychological well-being and organizational citizenship behavior. Nevertheless, after deriving the CEO's sustainable leadership styles, previous studies have not specifically examined the influence of these styles on the psychological well-being and organizational citizenship behavior of organization members. Previous studies have rarely investigated the effects of organization members' psychological well-being on organizational citizenship behavior. This study, therefore, empirically analyzed how CEOs' sustainable leadership styles affected organization members' psychological well-being and organizational citizenship behavior. Furthermore, the study assessed the influence of organization members' psychological well-being on organizational citizenship behavior. The research questions are summarized as follows: This study focused on the sustainable leadership of CEOs and clarified the enhancement of organizational capabilities through the aspects of human resource and personnel management [69][70][71]. In particular, it derived the key factors of the sustainable leadership of CEOs that enhance organization members' organizational citizenship behavior such as diligence, openness to experience/challenges, responsibility, cooperativeness, discipline, proactiveness, altruistic behaviors, conscientious behaviors, righteous behaviors, courteous behaviors, and efforts.
In order to understand sustainable leadership, one needs to first examine the concept of corporate sustainability management. Corporate sustainability management means that companies can continue to survive and grow only when they incorporate a symbiotic relationship with various stakeholders, undertaking social and environmental responsibilities along with pursuing economic profits [1][2][3]. Therefore, sustainable leadership should function to create continuous value for the economy, society, the environment, and all stakeholders. This can never be achieved by a leader alone. Ultimately, people do all the work, and sustainable leadership is possible only when both the leaders and members of an organization work together. In other words, leaders should build trust based on the basic virtues of listening and communication, self-awareness, transparency, and ethical integrity, present a sustainable vision based on the accumulated trust, and work together with organization members [1][2][3].
It was inferred from the above that servant, ethical, authentic leadership could be included as specific traits of sustainable leadership. These three types belong to the appropriate range of sustainable leadership styles. In addition, they can meet (contribute to) the purpose of a sustainable leadership style [20,22,25,27,28].
It should be noted that transformational leadership and transactional leadership were excluded from the sustainable leadership styles examined in this study. Transformational leadership is a type of leadership that presents new goals to bring about organizational change. Transactional leadership is a leadership style conducted as a transaction between a leader and subordinates in terms of cost and effectiveness. Therefore, it is evident that these are somewhat removed from the concept and characteristics of sustainable leadership mentioned previously. This study examined the underlying theories in detail, as outlined below.

Situational Theory Situational Leadership Theory
This theory posits that performance differs depending on the situational factors acting on the leadership process. It was argued that the determinants of leadership are not in the personal characteristics of the leader, but in the organizational situation the leader is in [72]. Early leadership theories viewed the situation as just a guiding concept and were biased toward leaders themselves. However, the idea of leadership has been adjusted and developed in terms of cooperation with members of the organization according to the trend of the times such as social change [73]. This theory contends that to exert the most effective leadership in any situation, leadership must be differently applied depending on the environment in which the leader is put [72]. For example, leadership (e.g., servant leadership) that elicits potential with an attitude of serving members of the organization may be needed immediately, or moral/legal/normative beliefs and practices (e.g., ethical leadership) may be further emphasized [74,75]. In addition, the structure and nature of the tasks to be performed, transparent human relationships between leaders and members of the organization (e.g., authentic leadership), and so on may be further highlighted [76]. Therefore, which guidance behavior is most effective in a certain organizational situation is determined by the exercise of sustainable leadership according to the environment with which the leader is faced. Furthermore, leadership styles arising from these situational changes can have a vast influence on organizational effectiveness (e.g., job satisfaction, organizational immersion, and organizational citizenship behavior) [40,77,78]. Therefore, sustainable leadership such as servant, ethical, and authentic leadership can have a significant influence on organizational citizenship behaviors. Taken together, situational leadership theory was utilized and applied as a foundational theory in the path of sustainable leadership → organizational citizenship behavior in this study.

Contingency Approach of Leadership
This theory contends that the effectiveness of leadership depends on how the leader's behavior is appropriately combined with various complex situational variables such as personality, task structure, formal authority system, role expectation, and maturity level of members of the organization [79][80][81]. In the contingency approach of leadership, there is no single best type of leadership that can be applicable to all situations. In other words, it is necessary to determine which type is most effective under which contingency condition. Characteristic theories and behavioral approaches on various leadership types have made significant theoretical contributions [79][80][81]. In addition, many studies have been conducted on various types of leadership linked to organizational effectiveness levels such as job satisfaction, organizational immersion, and organizational citizenship behaviors of organization members [40,77,78]. In particular, according to the contingency approach, the effectiveness of leadership in making high collective achievements depends on the styles (e.g., servant, ethical, authentic leadership) by which leaders control the situation of organization members and positively influence various factors (e.g., human respect, potential, ethics) [82][83][84]. All things considered, the contingency approach of leadership was utilized and applied as a foundational theory in the path of sustainable leadership → organizational citizenship behavior in this study.

Bottom-Up Spillover Theory
In the study of life quality and psychological well-being, theories related to spillover emerged from Roscoe and Roschelle [85], who argued that satisfaction in one area of life has a spillover effect on other aspects of satisfaction. Sirgy et al. [86] divided the directions of spillover into upward and downward, and the spillover theory began to be studied in both directions. According to the bottom-up spillover theory, the efficacy of each subarea constituting life has an influence on overall life satisfaction, thus enhancing life quality and psychological well-being. In short, this theory maintains that factors such as happiness, quality of life, and psychological well-being are determined by the sum of various pleasant moments experienced in everyday life [86]. Through the application of this concept, a leader's virtues, consideration, encouragement, clarified accountability, and leadership, as well as organization members' annual salary raises and stable work life, may bring about the enhancement of job satisfaction, thus boosting happiness, life quality, and psychological well-being [87][88][89]. Taken together, the bottom-up spillover theory was utilized and applied as a foundational theory in the path of sustainable leadership → psychological well-being → organizational citizenship behavior in this study. As actions defined on the basis of society, these theories concern actions that generally benefit other people or ongoing policy systems [90,91]. A prosocial behavior is an action that occurs in the interaction between people, and at least one helper and more than one beneficiary must exist. It is additionally noted that since a prosocial behavior is socially defined, a helping behavior is not limited to a specific form, and the form of help may vary depending on the situation [90,91].
In particular, prosocial organizational behavior was applicable in this study. Such behavior refers to an action performed by organization members with the intention of promoting the interests of individuals, groups, or organizations while members perform their organizational duties [92,93]. By this definition, it refers to all kinds of behaviors performed for others with the intention of benefiting them within an organization, such as helping behavior, sharing behavior, and creative and proactive behavior [92,93]. Williams and Anderson [94] raised the need for the concept of prosocial organizational behaviors by dividing organizational citizenship behaviors into those that benefit the organization in general (OCBOs), targeting organizations, and those that immediately benefit specific individuals (OCBIs), targeting people.
Prosocial organizational behavior is greatly influenced by leadership. In addition to instructions/reports, explanations/persuasion, and orders/questions, various factors and cases such as leaders' virtues, consideration, encouragement, human respect, potential, ethics, and so on mostly appear in relation to bosses [95]. In particular, in cases of a team leader or department head as a direct supervisor, its importance becomes even greater. A leader's fair attitude can reduce the negative effects of silence and increase organizational citizenship behavior [96]. Furthermore, members of organizations have different attitudes in terms of contributing to their organization according to their leader's style, including evaluation/compensation procedures, distributional fairness, and supervisors' attitudes and behavior toward subordinates [97,98]. Prosocial behavior was utilized and applied as the foundational theory of organizational citizenship behavior in this study.

Sustainable Leadership Styles
Case studies and empirical tests have been conducted based on various existing leadership theories and styles that have sustainable effects in companies and organizations (e.g., [2,3,69,99,100]). Some researchers have revealed that these leadership styles are key components of human resources and capital development [2,69]. Furthermore, studies have reported that sustainable leadership styles contribute productively and efficiently to human resources and capital development at the personal and organizational levels [2,69]. They also enhance workers' autonomy, self-realization, and positive interpersonal relationships and ultimately help maintain people's health and the organization's well-being [101]. Moreover, sustainable leadership styles function in a strategic dimension to build a more positive and sustainable work environment in the organizational context [101][102][103][104].
Therefore, this study used three main sustainable leadership styles (servant, ethical, and authentic leadership) based on previous studies and conducted empirical tests to investigate how they affected psychological well-being and organizational citizenship behavior. The next section reviews the literature on sustainable leadership styles that was used in this study.

Servant Leadership
Servant leadership focuses on serving others and puts the interests of organization members before those of the leader [31,32]. The leader, as a servant, puts the employees, customers, and communities first and is committed to satisfying their needs [33][34][35]. Since it is based on the premise of serving others, it contrasts with traditional leadership, which focuses on the desire to lead others. While the leader's goal based on traditional leadership is to be served, the leader's goal based on servant leadership is to serve others [105][106][107][108]. A servant leader in an organization helps the members grow and develop with a serving Sustainability 2021, 13, 13676 7 of 29 attitude, rather than a controlling approach using commands or instructions, so that they can contribute to achieving the organization's goal [36,37,109].
Robert K. Greenleaf, an American business management scholar, first proposed the concept of servant leadership in 1977 [110]. However, it started to attract attention as a new leadership model in business administration 20 years later when Jossey-Bass, a business administration-related publisher in the US, published a book titled "On Becoming a Servant Leader" in 1996 [111]. According to Greenleaf, servant leadership is "a leadership in which the leader focuses on serving others, puts the employees, customers, and communities first, and is committed to meeting their needs." Therefore, to practice proper servant leadership, one must become a servant before a leader and create an environment wherein the organization members can develop capabilities on their own [110,112].
According to Spears, the former president of the Robert K. Greenleaf Center for Servant Leadership, which specializes in servant leadership education, there are 10 characteristic elements of servant leaders: (1) listening attitude, (2) making efforts to become more empathetic, (3) showing interest in healing the pains of organization members, (4) providing alternative solutions through clear awareness, (5) working together through persuasion rather than relying on blind obedience, (6) presenting vision through broad thinking, (7) making future predictions with keen insights, (8) serving based on stewardship or lowering oneself, (9) commitment to the capability development of organization members, and (10) helping to build a community of organization members [110,113]. In sum, the qualifications or attitudes that servant leaders should possess include: (1) committed service, (2) coordination of opinions between organization members, (3) provision of direction or vision, and (4) assistance in support of work and life.
Servant leadership differs from the traditional leadership style, wherein the leader dominates the organization members and commands; instead, here, the leader utilizes the organization members as much as possible and inspires them to perform their jobs. Furthermore, this leadership encourages the organization members to set and perform their roles proactively [114][115][116]. Servant leadership places importance on teamwork and building relationships. That is, it promotes the development of value-based management, creating conditions to enable the company to move forward as much as possible [117,118]. Furthermore, if servant leadership is used efficiently, each organization member can use their own skills and experience as much as possible. This creates advantages in the organization in the consequential aspect and encourages the involvement of all organization members in making certain decisions to help each member establish their own core performance indicators [119][120][121]. Furthermore, if servant leadership is used, the mission and goals of the organization can be set at the organization members' eye level. They can thus use their own capabilities as efficiently as possible and design a work-life balance [122,123]. This study, therefore, defined servant leadership as "the style of leadership in which the leader serves the organization members based on respect for people and supports them with a serving attitude to unlock their potential".

Ethical Leadership
Ethical leadership is defined as "the ability to demonstrate normatively appropriate conduct through personal actions and interpersonal relations and promote such actions to the organization members through communications, support, and decision-making" [124,125]. Furthermore, it is a style in which the leader's actions and the influence of leadership are consistent with ethical/moral values [126][127][128]. In fact, since leaders enjoy social power, they have a responsibility to respect other people's rights and dignity and exercise power in a manner that contributes to the community [125,129,130]. Ethical leadership, thus, entails the social responsibility of the leader and focuses on exercising of leadership while making decisions or taking actions that may affect other people [131,132].
Leadership is traditionally defined as the position or ability to lead a group or an organization and the ability to elicit the cooperation of organization members or help them achieve common goals. However, in the 21st century, ethical leadership has emerged as a new leadership condition and qualification, as it has been found that unethical behaviors of leaders often cause bankruptcies and social problems in large corporations [133][134][135]. Nowadays, good leadership does not just dwell on the ability to maximize the gains of the organization or group by matching the organization members' attitudes, values, and beliefs with the organization's goals and vision. It refers to recognizing and practicing one's ethical responsibilities and exercising influence to allow organization members to behave ethically and perform work in a manner that contributes to society [133][134][135].
Several researchers have identified three main factors as critical components of ethical leadership [136][137][138]. These are: (1) ethical integrity-ethical character and personality of the leader. These qualities are needed for ethical decisions, values, and beliefs [136][137][138]; (2) awareness of the importance of ethical values and the ability to match them with actionsan ethical leader is a person who takes responsibility for their actions and shows what moral integrity entails. Therefore, ethical leaders know how to act fairly and exercise their social powers. They are aware that they should work to serve not only the interests of their organization or themselves, but the interests of the whole of society, and thus act accordingly [136][137][138]; (3) the ability to motivate ethically-an ethical leader leads their organization and its members to behave ethically in the process of achieving common goals [139,140]. Ethical leaders endeavor to reward organization members for their ethical actions and promote such actions through communication. They reveal their clear views on ethical values and ensure that organization members are well aware of them. Based on this, ethical leaders elevate organizational culture and organization members' ethical values to the next level [139,140].
There are five principles of ethical leadership [125,141]. Detailed descriptions of the five principles are as follows:

1.
Honesty: a leader's dishonest behavior in the form of lying or distorting reality creates an atmosphere of mistrust wherein organization members cannot trust the leader and thus lose faith.

2.
Justice: ethical leaders believe in justice and fairness. They prioritize treating every employee equally and place justice and fairness at the center of their decision making. They follow rules and do not give special treatment to any individual, except in special circumstances.

3.
Respect: leaders should listen carefully to others and confirm their inherent values. They should mentor others to become aware of their own purpose, values, and needs so that ethical qualities prevail throughout the organization. Furthermore, they always respect employees ethically, legally, and normatively [142,143].

4.
Community: ethical leaders behave altruistically, placing the welfare of their organization members in high esteem, and engage in activities such as team building, mentoring, and empowerment behaviors. This implies that they help to build a community and consider the value of the employees' goals and the goals of the whole organization.

5.
Integrity: Ethical leaders demonstrate appropriate values to people around them through their own behavior. Leaders who behave with integrity can strengthen their organization by hiring talented and ethical employees. People generally want to work with leaders who have integrity and are therefore more likely to be attracted to organizations wherein leaders act with honesty and integrity [142,143].
Based on the above, this study defined ethical leadership as "the ability to demonstrate morally/legally/normatively appropriate conduct through personal actions and interpersonal relations and promote such actions to organization members through communication, support, and decision making".

Authentic Leadership
Authentic leadership is defined based on the concept of authenticity [53,54]. Authenticity refers to self-awareness and consistency of inner thoughts, emotions, and values. It consists of two elements: self-awareness and self-regulation [144]. Self-awareness refers to Sustainability 2021, 13, 13676 9 of 29 recognizing one's true self at present and is a process of continuously understanding one's talents, strengths, goals, core values, beliefs, and desires. Self-regulation is the process by which a person aligns their behavior with their values and goals [144].
Authentic leadership is focused on encouraging positive self-development of leaders and organization members. Thus, it is defined as a behavioral style in which leaders create and enhance positive psychological competence and a positive/moral atmosphere to further develop self-awareness, internalized moral perspectives, balanced processing of information, and relational transparency [53,54]. Authentic leaders "are well aware of how they act and think, and other people recognize their values/moral perspectives, knowledge, and strengths" [55,145].
The components of authentic leadership include self-awareness, morality, transparency, and balanced processing. These components of authentic leadership can help develop psychological capital and positive emotions among organization members [146][147][148][149][150][151][152]. The components of authentic leadership are described in detail as follows.

1.
Self-awareness refers to how organization members perceive leadership and understand their own strengths, weaknesses, and motivations. The leader's self-awareness is the most essential element in authentic leadership [147,152]. Self-awareness occurs when the leader is aware of their existence in the circumstances that they are put in. Furthermore, it is not an ultimate goal but a series of processes in which efforts are continuously made to understand the leader's inherent talents, strengths, objectives, core values, beliefs, and desires. Self-awareness includes a basic understanding of the leader's knowledge, experience, and capabilities [147,152].

2.
Ethics/morality refers to how the leader acts according to internalized moral standards and values, rather than based on external pressures such as colleagues, organizations, and society. In this regard, authentic leaders seriously consider moral issues and develop ethical capability, efficacy, courage, and resilience to perform ethical actions sincerely and continuously. In short, morality refers to the process of ethical and transparent decision-making [153][154][155].

3.
Transparency refers to making efforts to share information and minimize inappropriate emotions. It is related to increasing trust through the expression of actual thoughts and emotions. Transparency is opposed to simply pleasing organization members, obtaining rewards, or avoiding punishment. It refers to actions that follow the leader's values, preferences, and needs [149,150].

4.
Balanced processing implies that the leader analyzes all the appropriate information objectively before making decisions. The leader performs balanced processing by collecting opinions in opposition to their own and sufficiently reviewing information and various opinions related to major decision making. Such balanced processing has positive effects on employees' behaviors through positive modeling [146].
Considering the above, this study defined authentic leadership as "the style of establishing firm values and principles based on clear self-awareness and exerting positive effects on organization members by forming transparent relationships".

Psychological Well-Being
Psychological well-being refers to accepting oneself positively and forming trust and warm relationships with others. It is the degree to which a person is satisfied while feeling happy and can be seen to have a healthy emotional state [156,157]. More specifically, psychological well-being can be defined as a multidimensional concept that includes both the positive emotional dimension (including happiness and satisfaction) and the negative emotional dimension, rather than a single dimension that shows the emotional health state of a person. Furthermore, it is a productive emotional state appearing in healthy individuals as members of society and a concept that focuses on self-realization [158][159][160]. Therefore, psychological well-being refers to the way a person accepts themselves as they are and maintains positive relationships with other people. Furthermore, it implies living with the motivation to strive to realize one's talents, abilities, and potential while self-regulating one's actions, which are not based on the standards set by others [156,157,160].
Psychological well-being refers to the sum of psychological aspects that constitute a person's quality of life [160]. Furthermore, for happiness, subjective judgment related to how one feels about one's life is more important than objective standards; this is called "subjective well-being" [161,162]. Some scholars have also argued that it implies living a happy life in terms of quality of life and should be based on the positive function of a person as a member of society. In other words, psychological well-being is a concept that places importance on an individual's meaningful life and encompasses their positive functions and potential as members of society [161,162]. Considering the above, this study defined psychological well-being as "the degree to which one lives in the direction of positively embracing life, having clear goals in life, and realizing one's potential".

Organizational Citizenship Behavior
Organizational citizenship behavior encompasses acts that members perform voluntarily for an organization and refers to behavior that contributes to improvements in efficiency beyond the requirements of core tasks [163,164]. Organizational citizenship behavior is discretionary and not specified by an official system of the organization; rather, members are assigned additional work and voluntarily help others. They comply with the organization's policies and continually develop expertise to help the organization. Furthermore, organizational citizenship behavior moves an organization forward or defends it. It includes actions such as expressing positive attitudes toward jobs and enduring discomfort [163,164]. In other words, it encompasses the voluntary and altruistic behavior of organization members shown toward the organization [165]. Hence, organizational citizenship behavior has a significant impact on the productivity of an organization and solidarity among its members. Therefore, researchers have emphasized that it is the basis of the personnel selection process. This is because it has a large effect on the organization's atmosphere and production efficiency [166,167].
Organizational citizenship behavior is classified into the following five behaviors: 1. Altruism: this occurs when an organization member voluntarily helps others without expecting anything in return. Examples of altruistic behavior include helping an overburdened coworker to finish early or helping a new employee adapt to the organization quickly [168].

2.
Conscientiousness: each member complies with the implicit and explicit rules of the organization faithfully according to their conscience, for example, not taking more breaks than necessary and saving office supplies as if they are personal assets [169,170].

3.
Sportsmanship: this refers to behaving fairly and not gossiping or spreading rumors when one is dissatisfied with or has complaints about the organization or other members. It also refers to making efforts to understand the positive aspects of a situation. This includes engaging in proactive behavior for trying to solve a problem by directly talking to the involved person instead of simply putting up with the matter of complaint [171].

4.
Courtesy: this refers to taking steps in advance to prevent sudden frustration with other members regarding one's work or personal circumstances. This behavior involves contacting other members who may be affected by one's decisions or actions to ask for their understanding in advance and coordinate disagreements [171,172].

5.
Civic Virtue: this refers to showing interest in and actively participating in various official and unofficial activities of the organization. It includes social activities and interpersonal relationships with other organization members through clubs and social gatherings within the organization, proactive activities, and making changes by suggesting improvements that may help in the development of the organization [173].
Considering the above, this study defined organizational citizenship behavior as "the degree to which an organization member voluntarily supports organizational development without proper compensation despite its non-official work status". Rivkin et al. [174] revealed that servant leadership has a positive (+) effect on psychological well-being, such as by strengthening the life objectives and vocation of the organization members and motivating them positively. Specifically, the servant leader stresses personal glory less and encourages organization members to take moral, ethical, and socially responsible actions. The moral and ethical behaviors of organization members are thus strengthened. This characteristic has a positive effect on psychological well-being, including providing a meaning of life, happiness, and a sense of accomplishment. Furthermore, servant leadership helps employees develop a positive self-concept [34]. In other words, it acts positively on grasping self-awareness in relation to others and understanding one's physical characteristics, personalities, abilities, and so on. Coetzer et al. [38,39] found that servant leadership could improve work-related psychological well-being, promote job immersion, and reduce burnout.
An ethically oriented leadership approach provides many benefits to organizations. Ethical leadership is evident and distinctively visible in an organization because of the friendly and strong emotional atmosphere prevalent therein [124,125,127]. In fact, ethical leadership behavior is important and effective. When a company pursues ethical leadership, such as ethical compliance and models, the organization members are more committed to institutionalizing ethical principles and taking the lead [126][127][128]. Ethical leadership thus has positive effects on the psychological well-being of organization members, which is reflected, e.g., in their performing ethical and value-based behaviors and respecting others [52]. For example, when a leader shows appropriate ethical behavior, considers the dignity of others, and maintains a serving attitude, they can improve and strengthen their relationship with organization members. Ethical leadership can thus increase the psychological well-being of organization members [52].
Authentic leadership induces organization members' psychological well-being and a positive organizational atmosphere simultaneously [175]. Furthermore, greater selfawareness, internalized moral perspectives, transparency of relationships, and balanced processing of information are promoted by referencing the behavioral patterns of the authentic leader [176]. Authentic leadership is related to a deep desire of the leader to build transparent relationships and interact with organization members [177]. When dealing with members, the leader's transparency builds stronger relationships based on trust. Furthermore, an authentic leader provides an environment wherein organization members can unleash their creativity and potential as much as possible [178]. An authentic leader exerts a positive influence on the psychological well-being of organization members through numerous mechanisms [58]. As a result, the authentic leader supports the autonomous development of organization members, encourages and stimulates authentic relationships among organization members, and increases the self-esteem (confidence) of organization members. Additionally, authentic leaders create opportunities to allow members to develop their creativity and potential [175]. One study also found that authentic leadership had a positive relationship with psychological well-being [179]. Based on the results of these previous studies, this study established the following hypotheses:

Hypothesis 1 (H1).
A CEO's servant leadership has a positive (+) effect on the psychological well-being of organization members.

Hypothesis 2 (H2).
A CEO's ethical leadership has a positive (+) effect on the psychological well-being of organization members.

Hypothesis 3 (H3).
A CEO's authentic leadership has a positive (+) effect on the psychological well-being of organization members.

CEO's Leadership and Organizational Citizenship Behavior
Servant leaders lead to the creation of common visions, values, and goals and encourage cooperation [38,39,180,181]. They also let organization members feel responsible and encourage them to participate in the decision-making process. Furthermore, they allow organization members to have organizational companionship and psychological ownership [40,182]. Servant leadership leads to voluntary involvement of organization members in departmental or team work, which promotes psychological ownership, builds organizational companionships, and is effective in improving the organization and work environment [38][39][40][180][181][182]. Moreover, servant leaders transcend their own interests, express authentic concern for and care for their organization members, and demonstrate leadership in a way that works best for members' interests (e.g., organization members' skills, knowledge, and capability development) [31,32]. Servant leadership can create a social environment in which members perform social actions as reciprocating actions beyond their roles [183,184].
A study reported that ethical leaders affected organization members' willingness to change ethical awareness and attitudes and had positive impacts on various behaviors related to ethics [185]. Ethical leaders behave fairly and thoughtfully toward organization members. This behavior strengthens social exchange relationships with organization members. Moreover, it encourages organization members to take constructive actions voluntarily to benefit the organization [186,187]. The ethical leader's behavior leads to mutually beneficial relationships with organization members through social exchanges among them. A study also found that it had a significant effect on organization members' prosocial behavior and citizenship behavior [188]. Furthermore, leaders following ethical behavior tend to treat organization members fairly by considering their personal desires and rights. Therefore, ethical leadership increases organization members' intention to stay and organizational citizenship behavior [78].
Authenticity of a leader produces positive emotions within an organization. Authentic leadership, therefore, has positive effects on the organization [53][54][55]145,155,189]. The leader's authentic leadership elicits organization members' self-regulation, strengthening their positive behaviors and trust in the leader [190]. The authentic words and behaviors of a leader can serve as a motivator for members' positive behaviors [191]. In a previous study, Avolio et al. [57] reported that authentic leaders created open and fair work environments that promoted organizational citizenship behavior among members. Other studies [77,192,193] argued that authentic leaders provided opportunities for demonstrating organizational citizenship behaviors by creating appropriate environments. Furthermore, they said that authentic leadership was a predictor for organizational citizenship behavior and had a positive effect on it. Based on the above studies, this study established the following research hypotheses.

Hypothesis 5 (H5). A CEO's ethical leadership has a positive (+) effect on organization members' organizational citizenship behavior.
Hypothesis 6 (H6). A CEO's authentic leadership has a positive (+) effect on organization members' organizational citizenship behavior.

Psychological Well-Being and Organizational Citizenship Behavior
Organization members with high psychological well-being participate more in organizational citizenship behavior such as helping others and complying with regulations [60][61][62]194,195]. Furthermore, organization members with high psychological wellbeing have a more positive impact on the organization as a whole than those with low psychological well-being [63,64,196]. In a meta-analysis of 101 empirical research papers on psychological well-being, Kim and Park [197] showed that organization members with high psychological well-being had low turnover intention and high income and performance and were more involved in organizational citizenship behaviors. That is, organization members with high psychological well-being contributed more to organizational performance [197]. Based on the above studies, this study established the following hypothesis.

Hypothesis 7 (H7).
Organization members' psychological well-being has a positive (+) effect on their organizational citizenship behavior.

Research Model
Situational leadership theory and the contingency approach of leadership were utilized and applied based on situational theory. Based on a total of four foundational theories, that is, bottom-up spillover theory, theories of prosocial behavior, and so on, this study empirically analyzed what influence a CEO's sustainable leadership styles (servant, ethical, and authentic leadership) had on the psychological well-being and organizational citizenship behaviors of organization members. Figure 1 shows a schematic diagram of the research model.

Psychological Well-Being and Organizational Citizenship Behavior
Organization members with high psychological well-being participate more in organizational citizenship behavior such as helping others and complying with regulations [60][61][62]194,195]. Furthermore, organization members with high psychological well-being have a more positive impact on the organization as a whole than those with low psychological well-being [63,64,196]. In a meta-analysis of 101 empirical research papers on psychological well-being, Kim and Park [197] showed that organization members with high psychological well-being had low turnover intention and high income and performance and were more involved in organizational citizenship behaviors. That is, organization members with high psychological well-being contributed more to organizational performance [197]. Based on the above studies, this study established the following hypothesis.

Research Model
Situational leadership theory and the contingency approach of leadership were utilized and applied based on situational theory. Based on a total of four foundational theories, that is, bottom-up spillover theory, theories of prosocial behavior, and so on, this study empirically analyzed what influence a CEO's sustainable leadership styles (servant, ethical, and authentic leadership) had on the psychological well-being and organizational citizenship behaviors of organization members. Figure 1 shows a schematic diagram of the research model.  Table 1 details the survey items measured in this study. The measured survey items of all variables were based on previous studies and modified and supplemented in accordance with the aim of this study. There were 32 questions rated on a 5-point Likert scale (1= strongly disagree to 5 = strongly agree).   Table 1 details the survey items measured in this study. The measured survey items of all variables were based on previous studies and modified and supplemented in accordance with the aim of this study. There were 32 questions rated on a 5-point Likert scale (1 = strongly disagree to 5 = strongly agree).

Settings of Respondents
The subjects of this study included ordinary workers (male and female adults) employed by midsize or larger companies (including subsidiaries) in four countries: South Korea, the United Kingdom, the United States, and South Africa. This study defined midsize companies as having total assets of more than 500 billion KRW and less than 10 trillion KRW, or with an average revenue of between 40 billion and 150 billion KRW for three years, regardless of the industry. This study defined large enterprises as having more than 10 trillion KRW in assets. The survey was conducted through Netpoint Enterprise Inc. (http://www.netpoint.co.kr/ accessed on 1 October 2020), a global research company, over four months, from 1 October 2020 to 31 January 2021. The questionnaire was prepared in two languages: Korean and English. In particular, the researcher requested that survey respondents (general office workers) honestly rate their CEO's leadership disposition on a 5-point Likert scale. Data were obtained from 649 adult employees. Table 2 provides a detailed description of the collected samples. In addition, under demographic characteristics, this study excluded some items such as the gender of the CEO or the respondents' own service periods, as many respondents were either reluctant to or did not respond.

Data Analysis
IBM SPSS (version: 18.0) and SmartPLS (version: 3.3.3) were used to conduct the statistical analysis in this study, and the data analysis procedure was as follows: (1) a frequency analysis was conducted to examine the demographic characteristics; (2) a reliability analysis using Cronbach's alpha was conducted to test the reliability of the measured survey items for all variables; (3) a factor analysis was conducted to test the validity of the measured survey items for all variables; (4) a correlation analysis was conducted to examine the degree of closeness (i.e., correlation) between the variables; (5) structural equation modeling was conducted to test the causal relationships between the variables.

Reliability and Validity
The reliability and validity of the measured survey items for all variables (servant leadership, ethical variables, authentic leadership, psychological well-being, and organizational citizenship behavior) used in this study were analyzed, and Table 3 shows the specific results. According to the analysis, Cronbach's alpha of every variable was at least 0.823, indicating high reliability. Furthermore, the loading value of each factor was at least 0.728, indicating high validity. Because of some predictor variables' high correlations with other predictor variables, multicollinearity was also studied to examine the phenomenon of negative effects. According to the analysis results, the variance inflation factor value was less than or equal to 10 in every case, confirming the absence of multicollinearity.

Correlation Analysis
A discriminant validity analysis (correlation analysis) was conducted, and the results are shown in Table 4. The square root of the average variance extracted (AVE) was examined. According to the analysis, the square root of the AVE exceeded the correlation coefficient value between each variable in every case. Therefore, the discriminant validity of each variable used in this study was verified.
The effects of a CEOs' sustainable leadership styles (servant, ethical, and authentic leadership) on organization members' psychological well-being was examined. According to the analysis, CEOs' servant (β = 0.173, t = 2.046, p < 0.05) and authentic leadership (β = 0.292, t = 3.399, p < 0.01) showed statistically significant positive (+) effects on the psychological well-being of organization members. However, ethical leadership did not. Therefore, Hypothesis 1 and Hypothesis 3 were supported, while Hypothesis 2 was not supported.

2.
The effects of CEOs' sustainable leadership styles (servant, ethical, and authentic leadership) on organizational citizenship behavior among organization members was examined. According to the analysis, CEOs' ethical leadership (β = 0.184, t = 2.039, p < 0.05) had statistically significant positive (+) effects on the organizational citizenship behavior among organization members. However, servant and authentic leadership did not. Therefore, Hypothesis 5 was supported, while Hypotheses 4 and 6 were not supported.

3.
The effects of organization members' psychological well-being on organizational citizenship behavior was examined. According to the analysis, organization members' psychological well-being (β = 0.493, t = 10.403, p < 0.01) had statistically significant positive (+) effects on organizational citizenship behavior. Therefore, Hypothesis 7 was supported.

Mediated Effect Test
An additional analysis was conducted to examine whether psychological well-being mediated the relationships between CEOs' sustainable leadership styles (servant, ethical, and authentic leadership) and organizational citizenship behavior. The results of the analysis are presented in Table 6.

1.
Psychological well-being showed a mediating effect in the path of servant leadership → psychological well-being → organizational citizenship behavior.

2.
Psychological well-being did not show a mediated effect in the path of ethical leadership → psychological well-being → organizational citizenship behavior.

3.
Psychological well-being showed a mediating effect in the path of authentic leadership → psychological well-being → organizational citizenship behavior. In particular, psychological well-being showed a fully mediated effect.

1.
CEOs' servant and authentic leadership had statistically significant positive (+) effects on organization members' psychological well-being. These results support the results of previous studies conducted by Coetzer et al. [38,39], Ilies et al. [58], Jensen and Luthans [179], and Rivkin et al. [174] and can be summarized as follows. First, in terms of servant leadership, the results imply that CEOs should endeavor to build empathy with organization members based on listening, communicating, and coordinating attitudes. Moreover, they mean that CEOs should show interest in solving the problems and discomforts faced by organization members and try to develop their capabilities. CEOs should always have a humble attitude and put forth efforts to work together based on persuasion rather than obedience. Furthermore, they should provide alternatives through clear awareness and present visions through broad thinking. Therefore, CEOs should strive to cultivate proper servant leadership, rather than making efforts focusing on temporary or superficial behavior-oriented servant leadership, to increase the psychological well-being of organization members. When CEOs demonstrate servant leadership to heed the troubles of organization members and motivate them ceaselessly while leading them by example, the psychological well-being of organization members increases. Second, in terms of authentic leadership, the results imply that CEOs should reflect on past behaviors for their growth and maintain their initial commitment through selfreflection. They should try to be consistent with organization members by controlling their emotions and implementing the company's business plans and goals while considering the capabilities of organization members. The results also imply that CEOs should be undeterred by external pressures, make decisions transparently, put employees at ease, and embrace their flaws. Thereby, CEOs' authentic leadership increases organization members' psychological well-being, which leads to progressive thinking and behavior and ultimately serves as a major factor in improving job performance. When the leadership lacks authenticity, CEOs use their relationship with organization members to pursue their own interests, and such leadership obviously has negative effects on the organization members' psychological well-being. However, CEOs' ethical leadership did not have a statistically significant effect on organization members' psychological well-being. This result contradicts the findings of a previous study by Teimouri et al. [52]. If a CEO's ethical leadership emphasizes honesty and trust morally/legally/normatively, they may be considered inflexible. This also means that if the CEO persuades organization members ethically based on the rules and principles to cooperate, it may disturb the members' psychological well-being. If the function of goal setting is emphasized only in terms of ethics in an organization, communication may not be clear. Instead, it may disturb the organization members' psychological well-being, ultimately reducing the function of motivation. Nevertheless, as a means of achieving organization members' performance goals, CEOs' ethical leadership is important. In this study, however, CEOs' ethical leadership did not enhance the function of giving meaning to tasks through the organization members' psychological well-being.

2.
Ethical leadership had a statistically positive (+) effect on organizational citizenship behavior. This supports the results of previous studies by Brown and Treviño [188] and Zhu et al. [78] and can be summarized as follows. CEOs should not lie or distort reality morally/legally/normatively and should act based on the principles of fairness and honesty. They should always respect employees as human beings morally/legally/normatively and pursue common interests with the company and employees. Only when the CEO behaves with honesty and trustworthiness morally/legally/normatively can an organization's members trust and follow them. We found that CEOs' ethical leadership could increase organizational citizenship behavior among organization members. However, CEOs' servant and authentic leadership did not have statistically significant effects on organizational citizenship behavior. These results contradict the results of previous studies by Avolio et al. [57], Krog and Govender [182], and Shah et al. [40].
In fact, researchers have emphasized that servant and authentic leadership are very important in business administration and management. However, this study focused on the servant and authentic leadership of CEOs rather than that of immediate superiors and managers. As organizational citizenship behavior of members is voluntary, there is no official reward system internally in the organization. Therefore, various unofficial rewards should be offered to promote it. Particularly, interest and recognition by the immediate superior or manager are more important than the CEO's leadership in terms of exerting influence on the attitudes and behaviors of on-site organization members. Therefore, on-site leaders have the power to encourage organization members to demonstrate organizational citizenship behavior more actively by closely observing organization members and immediately praising and encouraging them. Herein, however, CEOs' servant and authentic leadership did not increase organizational citizenship behavior.

3.
Organization members' psychological well-being had a statistically significant positive (+) effect on organizational citizenship behavior. This supports the results of previous studies by Huang et al. [60], Kang et al. [90], Kim and Park [197], and Xu et al. [62] and can be summarized as follows. Organizational citizenship behavior increases as psychological well-being increases, that is, organization members embrace life positively, have higher self-esteem (confidence) and clear life goals, take care of work that falls under their responsibilities, live so as to realize their creativity and potential, and are content with the results of life. Organization members with high psychological well-being are more willing to spare time to help busy colleagues at work and try to keep up with organizational changes and innovations. Furthermore, they do not infringe or interfere with the rights of colleagues, voluntarily comply with corporate rules and regulations, and refrain from complaints and unprofessional behavior at work, thereby showing strong organizational citizenship behavior.

1.
This study is significant in that it studied CEOs' sustainable leadership styles in the organizational behavior theory aspect of business administration and psychology and empirically analyzed how these leadership styles affected organization members' psychological well-being and organizational citizenship behavior. Specially, situational leadership theory and the contingency approach of leadership were utilized and applied based on situational theory. Based on a total of four foundational theories, that is, bottom-up spillover theory, theories of prosocial behavior, and so on, this study empirically analyzed what influence CEOs' sustainable leadership styles (servant, ethical, and authentic leadership) had on the psychological well-being and organizational citizenship behaviors of organization members.

2.
CEOs are faced with a number of responsibilities, ranging from being in charge of their company's performance, to serving as a major spokesperson, to setting strategic directions, maximizing organizational potential, to securing internal and external stakeholders' participation. Notably, such responsibilities become much more complicated in crisis situations because employees and stakeholders ask CEOs for direction, information, and motivation. Therefore, the results of this study suggest that CEOs' sustainable leadership styles and actions to improve the possibility of success are extremely important. In other words, they imply that a swift leadership pattern is needed for CEOs to meet the challenges of the times. More specifically: (1) through sustainable leadership, efforts should be made to surpass the general level; (2) important measures should be identified, and preemptive moves should be made; (3) a discriminatory and dynamic approach to strategies should be taken; and (4) positive social objectives should be clearly expressed and practiced. CEOs are attracting more attention than ever, leading companies in today's rapidly changing times. This suggests that it is necessary to comprehend principles that show when, where, and how important leaders are and the sustainable leadership styles that can increase their chances of success. This study derived constructive implications that CEOs can overcome today's challenges through sustainable leadership styles.

3.
Currently, among the various leadership theories, servant leadership, in which the leader serves the organization members based on respect for people and supports them with a serving attitude to unlock their potential [19,24,25,27], is a typical example. In fact, the perspective presented in leadership theories shows only one aspect of this kind of leadership. For example, CEOs' servant leadership was found to increase organization members' psychological well-being. However, not everything is solved by servant leadership alone. Depending on the situation, the leader sometimes needs to become an authentic leader as well. In other words, the leader should exhibit multifaceted behavior according to the situation. Therefore, as mentioned earlier, this study derived CEOs' sustainable leadership styles. In particular, it found that servant and authentic leadership increased organization members' psychological well-being when applied appropriately.

4.
An authentic leader sets inner moral standards clearly and then uses those standards to control themselves [20,21,26]. Some scholars have argued that authentic leaders should be sincere and show their inner morals transparently while building relationships with others [53,54,155]. As such, there is an underlying assumption in authentic leadership that if the leader shows authentic behavior rather than instructing organization members, those members watch, learn, and follow [53,54,155]. In other words, the leader should exhibit model behaviors [55,145,189]. Based on fundamental theory, this study showed that CEOs' authentic leadership could increase organization members' psychological well-being and had a sustainable effect on it.

5.
Many CEOs now lead multinational labor groups and face the challenge of working with several stakeholders in different organizational sectors. In an increasingly globalized and flexible organization, it is necessary to educate CEOs on the importance of ethical leadership. The results of this study indicate that organizational citizenship behavior increased when organization members recognized that the CEO had moral values of honesty, fairness, integrity, and transparency. Therefore, this study suggests that CEOs should provide an environment that promotes moral values through official systems (e.g., hiring process and incentive/job promotion system) and the organizational culture's unofficial elements (e.g., meetings and discussions on ethics, unofficial job promotion procedures perceived within the organization) to exhibit and maintain ethical leadership. CEOs should recognize the importance of moral values in the corporate vision and firmly establish them. Furthermore, CEOs should always engage in work as ethical leaders and inspiring role models for the establishment of these ethical values. 6.
Ultimately, organizations should recognize the importance of responsibility and sustainability more strongly in the context of business ethics and integrate these elements into their strategic agendas and value norms. The results of this study show CEOs' ethical responsibility and sustainability. If a CEO makes a decision that lacks ethical values and is not recognized socially, ethical issues from the past to the present accumulate, organization members do not trust the CEO, and organizational citizenship behavior does not grow. Future studies in social science research can focus on CEOs' ethical leadership and its impacts.

Limitations and Future Studies
First, when the sample composition of the survey was designed conceptually, the researchers limited the subjects to ordinary employees (male and female adults) of midsize or larger companies (including subsidiaries) across various industries in four countries. Since the survey targeted several workers across industrial sectors, diversity was achieved. However, there were limitations in the samples of this study. In particular, although samples were collected from one country per continent, it is somewhat unreasonable to generalize the results. Furthermore, while many employees noticed the importance of CEOs' leadership, they may also feel a perceived difference between male and female CEOs. Therefore, we need to conduct additional studies on CEOs' gender and leadership styles.
Second, it is worthwhile to conduct an empirical study on issues of leadership that are socially desirable and have sustainable effects, such as servant, ethical, and authentic leadership. However, the survey respondents may have responded to or highlighted the good sides of their organizations, CEOs, and themselves in exaggeration when answering the questionnaire. Therefore, these limitations need to be resolved in future studies. Data Availability Statement: Data sharing is not applicable. The data are not publicly available due to participant privacy.