A Literature Review into Leadership Styles Discussed in the Past Five Years

Leadership is one of the most discussed topics in literature across all different industries and sciences. There is a need for clarification and overview on the different types of leadership discussed in literature. This explorative research describes a systematic literature review to give a general overview of the types of leadership discussed in the literature in the time period between March 2013 and March 2018. In total 658 different types of leadership were mentioned in 380 analyzed articles. Of that transformational leadership was mentioned 200 times, effective leadership was mentioned 118 times, transactional leadership 110 times, participative leadership was mentioned 89 times and ethical leadership was mentioned 72 times. Often these types of leadership were also mentioned together and a great overlap on the different leadership styles was mentioned. Further research on leadership should therefore focus less on explorative research and more on developmental research of a new full range scientific leadership model including the different leadership styles discussed most often in the past five year.


Introduction
Suddaby [1] wrote about the need for more construct clarity in management research and the need to "create precise and parsimonious categorical distinctions between concepts" and to "show their semantic relationship to other related constructs" ( [1], p. 347). Nowhere is this need more apparent than in the enormous amount of literature on leadership styles [2]. Leadership is one of the most discussed topics in literature across all different industries and sciences.
Three theories of leadership are discussed most in literature. The most discussed model of leadership is the theory of Bass [3] who describes transformational, transactional and laissez-faire leadership in a model. Transforming and transactional leadership were first discussed by Burns [4] and a few years later Bass [3] extended the theory of Burns [4]. Bass [3] introduced the term transformational leadership and combined this leadership style with transactional leadership and laissez-faire leadership.
Transformational leadership consists of four elements according to Bass [3]: individualized consideration, intellectual stimulation, inspirational motivation and idealized influence. Yukl [5] defines transformational leaders as leaders with an appealing vision for their team and they intellectually stimulate others in a way that is demanding and appreciative of the individual needs of the team members, incorporating the four elements of Bass [3] in the definition. Transactional leaders exert influence on followers based on exchanging benefits, this can be internal or external motivators, and respond to their self-interests when they achieve defined goals [3]. Transactional leadership included two components: contingent reward and management-by-extinction [3]. Often the transactional leadership style is contrasted to the transformational leadership style [2]. Transactional leaders are task-oriented. In contrary to transformational and transactional leadership, Bass [3] defined leaders who do not take charge of their leadership as passive or laissez-faire leaders. As the laissez-faire leader believes people are unpredictable and uncontrollable and understanding people is a waste of time, this leader maintains a low profile and works with whatever structure is available without any suggestions or criticism [6] [7] [8].
The leadership model of Bass [3] shows a lot of similarities with two other well-known models of leadership. First of all, the model of Ha-Vikström [9] based on the theory of Blake, Shepard, and Mouton [10] shows three basic paths to think about leadership; relation-oriented or people-oriented leadership, task-oriented or goal-oriented leadership and laissez-faire leadership. And second, the leadership model of Lewin, Lippit and White [11], describes autocratic, democratic and laissez-faire leadership.
Finally, shared leadership can be seen as a meta-theory of leadership, as all leadership is shared leadership, it is simply a matter of degree according to Pearce, Wassenaar and Manz [12]. Shared leadership builds on the philosophical assumption that every person is capable of sharing the burden and responsibility of leading to some extent [13]. Shared leadership encompasses other leadership styles and provides a way of organizing them [12]. An overview of these leadership theories is mentioned in Table 1.
A lot of research is based on the research done before and many different types of leadership are discussed in literature, as Suddaby [1] stresses. This research aims to bring clarification to the most discussed types of leadership in the past five years. There for the research question is: What types of leadership have been described in scientific literature between March 2013 and March 2018?

Methodology
This study is an explorative study into the different types of leadership described in literature in the past five years. As the research question is broad, this is an explorative study, which is a starting point from which other research can be build [16]. The research method is a systematic literature review. Systematic literature reviews started in the medical sciences and were developed because of a need to describe objective, generalizable and reliable data from literature on the application of medicine and treatment methods [17]. However, systematic literature research is also widely used in social sciences [18].
"A systematic literature review is the process of systematically locating, valuing and synthesizing research results, obtained with scientific research, to obtain a reliable analysis and overview" ( [17], p. 178). The systematic review process generally comprises five steps: the development of a protocol to guide the review, screening or inclusion criteria, quality appraisal, data extraction, and synthesis [19]. In this research it is to get an overview of types of leadership described in literature in the last five years. The studies selected meet the inclusion criteria, but the quality of the articles is not further appraised.
A type of leadership is defined in this research when a definition could be given to clarify what is meant and the noun leadership is combined with an adjective which says something about the noun leadership.
A range of electronic databases was accessed across December 2017 to March 2018 to find the articles as can be seen in Table 2. The only selection criteria for the articles is that leadership is mentioned in the title. The articles are selected from the first options the catalogue gave and so on. The selection criteria in the search engines is: a time period between March 2013 and March 2018, full text articles and scientific journals. The only search term in de search engines is leadership; no synonyms and the word leadership should be present in the title.
The broad and explorative nature of this study makes the precision of the found articles great. Of the 380 articles analyzed, 370 add adjectives to leadership and there for talk about different types of leadership, the other 10 articles write about leadership in general and not about a specific leadership style, there for these are not included in the research. The precision is 97% [20].
The research synthesis involves the effort to discover patterns, consistencies, differences and explanations for this analysis and overview, with the aim of generalizing [17]. In this study the different types of leadership were collected,  showing how often the different types of leadership were mentioned together and how often the types of leadership were mentioned. A systematic literature review is a time-consuming and work intensive research method and as in all qualitative research processes, subjectivity plays an important role. The literature reviewed can be interpreted differently.

Results & Discussion
The journals in which the articles are placed are very diverse. However, a few journals were mentioned more often, namely the "Journal of Business Ethics" (32), "Social Behavior and Personality" (16), and "New Directions for Student Leadership" (8).
Of the authors 524 (58%) are male and 354 (39%) are female, with a total of 22 missing values. Finally, the region in which the authors worked is analyzed. As can be seen in Table 3 most of the articles came from North America, Asia/Pacific and Europe. The reason for this Western focus of the region the authors are located, are assumptions. It for example can be, because in Western countries there are more resources to publish, or there is a greater importance for publishing. Another reason can be that curiosity and the need for knowledge about leadership is greater in the Western countries. What the reason may be, it remains speculation.   Transformational is also combined with other adjectives to describe leadership, for example creative (1), CEO (1), team (1), shared (1) and transformational school leadership (2).
Effective leadership is the type of leadership mentioned most after transformational leadership. This is not so much a leadership style, but more as a desired outcome of leadership. However, Gandolfi and Stone [21] define five key attributes of leadership from the literature of Kouzes and Posner [22]: 1) to model the way, 2) to inspire a shared vision, 3) to challenge the process, 4) to enable others to act, and 5) to encourage the heart [22].
As effective leadership can be seen as a type of leadership one wishes to reach, this is the same for outstanding leadership and successful leadership.
Another notable result is the number of times leadership specific to the educational industry is mentioned, for example school leadership (26), principal leadership (6), university leadership (13), student leadership (17), and teacher leadership (57).
In the articles analyzed, leadership combined with the adjective female was mentioned 22 times and women's as adjective to leadership was mentioned 12 times, as can be seen in Table 4. These articles are mostly written by women, especially the ones on women's leadership, female leadership is more often discussed as well by men, but almost always in the combination with a female author as can be seen in the table below.
As mentioned in the introduction, in the literature there are three major theories on leadership, the theory of Bass [3] with transformational, transactional and laissez-faire leadership, the theory of Ha-Vikström [9] shows three basic paths to think about leadership; relation-oriented or people-oriented leadership, task-oriented or goal-oriented leadership and laissez-faire leadership and the leadership model of Lewin, Lippit and White [11], which describes autocratic, democratic and laissez-faire leadership.
In the results the model of Bass [3] is mentioned the most by far. The developed scale of Bass [3], the Multifactor Leadership Questionnaire, is also an often-used scale to measure leadership.
Even though Bass [3] is mentioned often in the literature, the model of Lewin, Lippit and White [11], which describes autocratic (18), democratic (26) and laissez-faire leadership (36) is mentioned less often. Of the 18 times autocratic leadership is mentioned, 15 times it is mentioned in combination with democratic leadership and 10 times in combination with laissez-faire leadership. Democratic leadership is mentioned 15 times in combination with laissez-faire leadership of the total of 26 times it is mentioned. This also shows the model is not often discussed as a complete model in the articles, but more often as separate types of leadership.
The same goes for the leadership model of Ha-Vikström [9] and the model of Lewin, Lippit and White [11] are not mentioned that often. The three basic paths of Ha-Vikström [9] also know a lot of variations, but relation-oriented (11) or people-oriented leadership (8), task-oriented (17) or goal-oriented leadership (2) and laissez-faire leadership (36) are not mentioned that often for a well-known leadership theory. Task-oriented leadership and people-oriented leadership are mentioned 7 times together, task-oriented leadership and laissez-faire leadership are mentioned 5 times together and people-oriented leadership and laissez-faire leadership are mentioned 3 times together. This shows the model is not often discussed as a complete model in the articles, but more often as separate types of leadership. There is a large overlap in the different leadership styles, as can be seen from the model of Ha-Vikström [9] with for example people-oriented or relation-oriented leadership, also mentioned as employee-oriented leadership. The same goes for shared and distributed leadership, which share a lot of common points in their definitions. Or the different leadership styles that can be mentioned as part of the transformational leadership style, for example charismatic, motivational or visionary leadership.

Conclusions
In total 658 different types of leadership were mentioned in the 380 analyzed articles. Transformational, effective, transactional, participative and ethical leadership is the most discussed type of leadership in the literature analyzed. The model of Bass [3] is the most discussed theory in the leadership literature in the past five years. Next the model of Lewin, Lippit and White [11] is also mentioned often, however, it does not come close to the popularity of the model of Bass [3].
Effective leadership is the type of leadership mentioned most after transformational leadership. However, it can be discussed if this really is a style of leadership or more a desired outcome of the leadership process. In the articles ana- As for example the model of Bass [3] leaves shared leadership or servant leadership aside, these are leadership styles often discussed in the literature in the past five years. This shows the need for an updated and transformed model.
One of the characteristics of the study is that it is explorative and a large range of articles have been included in the study. Therefore the selection was broad and inclusive. For further research it is advisable to narrow this down and be more focused, for example on specific journals, the journal of leadership studies of the university of Phoenix. The journals can for example also be selected because they are known to include either empirical studies or literature surveys, and to be used as sources for other systematic literature reviews related to leadership. Also, the search process and assessment of quality of the articles should be more precise and defined in further research.
Another limitation is that the correlation between the different leadership styles does not say that much. As leadership styles mean (almost) the same, it can be that both styles will be mentioned and only one will remain to be mentioned in the article, for example with laissez-faire leadership and passive leadership. It can also be that only one of the types of leadership is mentioned, in this case the different types of leadership discussed will be not mentioned together in articles, where other articles would do so, for example with people-oriented and relation-oriented leadership.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.