Social Entrepreneurship Research: Intellectual Structures and Future Perspectives

: Social entrepreneurship (SE) is an emerging research ﬁeld that has received much scholarly attention in recent years. Given the global scope of this attention, this review explores the existing scientiﬁc literature on social entrepreneurship to contribute to a systematization of the research ﬁeld. Based on the publications in Web of Science and Scopus, a total of 1425 scientiﬁc articles were analyzed. We used the bibliometric method to describe the evolution of social entrepreneurship research (e.g., evaluation by years, authors, scientiﬁc journal articles, and countries in the SE literature that have had the greatest impact in terms of production). In addition, we used the mapping of knowledge networks through the citations and co-citations analysis to identify schools of thought. A keyword co-occurrence analysis was performed to detect key research topics over the years. The results show that, although the research is still in a nascent phase, it has a multidisciplinary character. Furthermore, social entrepreneurship appears to be a concept closely linked to three schools of thought: commercial entrepreneurship, sustainable entrepreneurship, and social innovation. The keywords analysis allowed us to isolate the constructs that the literature has considered antecedents (e.g., socio-psychological factors) and accelerators (e.g., education, network, culture, and gender) to the development of social entrepreneurial intention. We will further discuss the ways researchers can explore this research ﬁeld and contribute to the global literature.


Introduction
This article deals with the issue of social entrepreneurship (SE), the study of entrepreneurial initiatives aimed at exploiting opportunities to generate social value [1][2][3]. In the practice, there is a growing diffusion of forms of social entrepreneurship, both in the non-profit sector, in which innovative approaches tend to spread that allow the generation of revenues, allowing the sustainability and development of individual initiatives, and in for-profit organizations that identify and pursue business opportunities aimed at jointly generating economic value for shareholders and social value. This has aroused the interest of scholars from various disciplines, who have tried to define the boundaries and distinctive characteristics of social entrepreneurship and to understand its determinants, success factors, and criticalities.
Social entrepreneurship is an emerging but rapidly growing field [4,5] involving diverse sectors such as innovation, technology, public policy, community development, social movements, and non-profit organizations [6]. The main objectives of social entrepreneurship are the reduction of poverty and illiteracy, the improvement of collective well-being and the quality of life of the community, the overcoming of social injustice, the conservation of the environment for future generations [7][8][9][10].
Several authors agree that the SE literature is still in an early stage of development [2,11,12]. This is evidenced by the fact that a universal definition has not yet been reached [3,13], partly as a consequence of the definition difficulties that characterize the more general field of entrepreneurship studies, and in part for the heterogeneity of the contributions that have addressed the issue from different points of view, often focusing on single specific areas.
In general, social entrepreneurship can be considered as a form of social change by means of innovative ideas or actions to achieve social objectives and create new value [14][15][16] through an organization that is financially independent and self-sufficient [17,18].
Despite this growing interest from the scientific community, analysis of the state of SE literature has shown that scholars struggle to determine a coherent and non-fragmented theoretical framework [19,20], due to uncertainty and confusion about who a social entrepreneur is and what he/she does. This justifies the need for this study.
This article uses the scientific mapping review methodology [21,22] to analyze the intellectual structure of the SE knowledge base.
Specifically, the review addresses the following research questions: RQ1: What are the main growth trends in SE research? RQ2: Which authors, scientific journal articles, and countries in the SE literature have had the greatest impact in terms of production and citations? RQ3: What is the intellectual structure of the knowledge base on social entrepreneurship? RQ4: Which research topics in the SE literature have been studied most frequently and are currently attracting the greatest attention?
Through these research questions, this review aims to guide researchers who are new to the social entrepreneurship field, but also to lay some foundations for future research.

Literature Review
Social entrepreneurship scholars have adopted different approaches to define the construct. However, a common element to many of the definitions in the literature is the creation of social value [1,2,19,[23][24][25][26] or social wealth [3].
More generally, it can be said that in all the definitions analyzed, there is an explicit reference to the social dimension expressed through the creation of social value or to the pursuit of a social objective or mission [27] or the impact of social entrepreneurship in terms of social benefits [28], mitigation or solution of social problems [29], satisfaction of social needs [30], social justice [11] or social change [31][32][33].
For example, Dees [1], considers the social entrepreneur as a change agent who works through a mission to create social value and the search for new opportunities to achieve that mission. Where others see problems, social entrepreneurs see opportunities. The will to innovate is part of the entrepreneurs' modus operandi, and it should not be understood as a sudden explosion of creativity, but as a continuous process of exploration and learning. Furthermore, entrepreneurs tend to have a high tolerance for ambiguity and learn to manage the risks associated with it. They see failure as a learning opportunity and act responsibly, using scarce resources efficiently, calculating risks so as to reduce the harm that will result from failure. This is an idealized definition, generally, the more an individual meets these requirements, the more he or she will be considered a social entrepreneur. As Dess [1] argues, in reality, many social entrepreneurs present these characteristics in different ways and to different degrees, and very few fit exactly this definition of social entrepreneur.
When systematizing the numerous contributions that characterize social entrepreneurship, in general, two lines of research distinguish the field in the literature. Indeed, many authors have limited the scope of social entrepreneurship to the non-profit sector [34], a sector on which the studies had initially focused, through the analysis of the differences between social enterprises and commercial enterprises. Other authors, especially recently, have extended the scope of social entrepreneurship to include hybrid organizations that combine economic and social goals [12,35,36], generating social change through sustainable Sustainability 2021, 13, 7532 3 of 21 business models. In this context, the concept of social innovation becomes central as a balance between social entrepreneurship and other forms of entrepreneurship.
According to Christopoulos and Vogl [37], while commercial entrepreneurs approach the problem from a purely economic point of view, social entrepreneurs are motivated by social needs. Of the same opinion is Olsen [38], according to whom the social entrepreneur uses the same tools that are usually used in the traditional sector, but applies them to solve social problems.
Austin et al. [8] used four variables to compare social and commercial entrepreneurship, trying to establish differences. The first variable used is market failure, which is a situation that describes an inefficient distribution of goods and services in the free market. In this sense, a problem for the commercial entrepreneur becomes an opportunity for the social entrepreneur. The second variable is the mission defined as the values and visions that guide the entrepreneur. In principle, the basic purpose of social entrepreneurship is the creation of social value for public welfare, while the entrepreneur seeks the creation of profitable operations that result, in the first instance, in private profitability for the shareholders.
A third variable is resource mobilization, which refers to the set of activities put in place to ensure new and additional resources for the organization. On the one hand, "the nondistributive restriction on surpluses generated by nonprofit organizations and the embedded social purpose of for-profit or hybrid forms of social enterprise limits social entrepreneurs from tapping into the same capital markets as commercial entrepreneurs" [8] (p. 371). Finally, the fourth variable is defined as performance measurement, a process by which an organization monitors important aspects of its systems. Data are collected to reflect how its processes work, and this information is used to guide the organization's decisions over time. This represents a limitation for social enterprises, which, unlike commercial enterprises, encounter great difficulties in evaluating performance due to the impossibility of measuring the social impact [23].
Weerawardena and Sullivan Mort [39] described opportunity identification as a separate activity in which social entrepreneurs actively seek opportunities to create social value. According to their study, the process of identification of opportunities and evaluation is simultaneously influenced by the social mission and by organizational and environmental sustainability. Regarding the sources of identification of opportunities, Thompson et al. [40] found that opportunities could arise from an individual's vision or necessity.
Grayson and Hodges [41] coined the term "corporate social opportunity" to designate these opportunities, which correspond to the possibility of combining the creation of economic value for the company with a benefit for society. The authors defined these opportunities as "commercially viable activities which also advance environmental and social sustainability" [41] (p. 11). These activities with economic and socio-environmental significance typically involve some form of innovation.
From this perspective, the social component of entrepreneurship consists of the ability to identify innovative solutions for specific social problems. Therefore, innovation is another key element of social entrepreneurs because it involves novelty for a relevant company in society [42], and it is what has distinguished social entrepreneurship from other forms of entrepreneurship, especially in recent years.
Bloom and Chatterji [29], for example, explicitly identified social entrepreneurs as individuals who seek to solve or mitigate a social problem by developing change strategies that differ from those that have been used to address the problem in the past. The distinction with respect to other initiatives with social purposes lies in the innovativeness of the solutions adopted, which-by definition-can correspond to very different organizational forms, corporate forms, management practices, and business models: hence the considerable heterogeneity of social entrepreneurship initiatives and the difficulty of describing its boundaries.
This approach reflects that of several works on entrepreneurship that attribute a central role to innovation and interpret entrepreneurship as the ability to recombine resources in an innovative way. Furthermore, similar to what has been found with regard to entrepreneurship in general, the definition of social entrepreneurship often emphasizes the change generated by the innovative action of the entrepreneur.
The Organization for Economic Cooperation and Development (OECD) [43] defined entrepreneurs as agents of change in growth in a market economy; similarly, some authors have identified social entrepreneurs as agents of change in the social sector [1,24].
Social entrepreneurship is explicitly indicated in several contributions as a harbinger of social transformations [11,31,32]. In this regard, Martin and Osberg [25] underlined the ability of social entrepreneurs to identify stable equilibrium situations characterized by social inequity and to change them, generating a new equilibrium that corresponds to better conditions for a group of people. These authors therefore recognized a role of creative destruction for social entrepreneurship, which involves changing the status quo and redefining social balances, as in the Schumpeterian vision the innovation brought by the entrepreneur undermines the market rents.
Furthermore, the definition of Martin and Osberg [25] emphasized how the new equilibrium generated for the benefit of one group can be extended to others as a result of imitation. This is an interesting aspect because, even if the actual impact dimension of social entrepreneurship is still to be evaluated, we note how it can be linked to two aspects: the direct effect, i.e., the immediate change generated by the business social entrepreneur; the indirect effect related to the attention that this activity arouses toward specific social problems and toward innovative ways to deal with them [31].
This emphasis on the extent of the effects that can be generated by social entrepreneurship and on the supremacy of this form of social change corresponds to that which, with prevalent regard to environmental sustainability issues, Hall et al. [44] defined as a "panacea hypothesis", i.e., the idea that through entrepreneurship it is possible to reach a solution to the problems of society. At present, however, this actually turns out to be just a hypothesis, yet to be adequately tested and against which, as mentioned above, important questions also arise about the direction of social change and the values that guide it.
Despite the differences that characterize the different approaches, the authors who are engaged in the study of social entrepreneurship have highlighted common themes that need to be taken into consideration. Social entrepreneurship combines the passion of a social mission with the entrepreneurial approach [45]. Social entrepreneurship uses the principles of entrepreneurship to organize, create, and manage a business to bring about social change. They are change agents with a problem-solving mission. The social mission is the core of what distinguishes social entrepreneurs from classical entrepreneurs [1]. Social entrepreneurs are like entrepreneurs only they are driven by social improvement and not by profits [42].
To overcome the problem of a universal definition, we accepted the suggestion of Choi and Majumdar [46], namely, of considering social entrepreneurship as a concept of cluster, formed by sub-concepts: creation of value by the social entrepreneur, SE organization, and social innovation.
According to Choi and Majumdar [46], this would help overcome the different perspectives present in the literature in two ways. First, it would force researchers to explicitly state which of the sub-concepts they emphasize in their understanding of the concept. Second, conceptualizing social entrepreneurship as a cluster concept could serve as a broad research agenda for the field of social entrepreneurship and, at the same time, help organize and locate existing work within the field.
In this context, social value is configured as the search for social progress, removing the barriers that make inclusion difficult and helping those who are temporarily weakened. Contrary to what happens with the economic value in which the consumer is captured autonomously by beneficiaries and remunerated with his or her own resources in a commercial exchange, in this case the social entrepreneur helps the beneficiary to obtain a value which, for different reasons, would have been out of reach [47].
This definition not only allows us to understand why social enterprises are primarily responsible for generating social value, but also underscores why academics refer to social entrepreneurship as creating positive social change, regardless of the structures or processes through which is implemented [48].

Materials and Methods
This analysis was conducted with the aim of contributing to the systematization of intellectual structure in the field of social entrepreneurship research, and to answer the four research questions explained in the introduction. In this sense, the objectives can be summarized in two fundamental points: identifying the evolution of the object of study as well as emerging developments, and secondly, creating food for thought for future investigation.
Scopus and Web of Science databases were used to search for scientific articles. They are two widely known databases in the literature that index 22,878 peer-reviewed journals, which is why they are currently considered the largest and most important multidisciplinary bibliometric databases [49]. The literature search ended in January 2021 and generated a total of 4752 documents.
In the first phase, we introduced the search term "Social Entrepren*", limiting the search to "titles, abstracts and/or keywords". Articles published in 2021 were excluded to focus the analysis on full calendar years.
In the second phase, to reduce the risk of including false positives with no complementary value to the constructed data set, further investigations were conducted and inclusion criteria agreed upon. The inclusion criteria were: (i) scientific articles published in peer-reviewed journals, including articles in print, as scientifically valid sources of knowledge [50] (ii) where it was possible to demonstrate studies on social entrepreneurship through the inclusion of the words in the titles, abstracts, and/or author keywords, (iii) written in English, and (iv) published through the year 2020 (Table 1). From the analysis, therefore, chapters of books, books, conference proceedings, notes, etc., written in a language other than English, in which it was not possible to demonstrate an investigation into entrepreneurship in the social field, were eliminated.
The authors read the abstracts, and when it was not possible to isolate the inclusion criteria from the abstract, the entire article was read. This selection phase produced the final result of 1425 scientific articles.
To minimize the subjective component, possible attribution risks, or bias errors, and to be able to replicate the study, the guidelines of the PRISMA method were used [51][52][53] ( Figure 1).
The 1425 articles identified were exported to a .csv file. This file included author names, years of publication, titles, affiliations, author keywords, abstracts, citations, and references. Additionally, another copy of the file was saved in Excel for use in descriptive data analysis. To minimize the subjective component, possible attribution risks, or bias errors, and to be able to replicate the study, the guidelines of the PRISMA method were used [51][52][53] ( Figure 1). The 1425 articles identified were exported to a .csv file. This file included author names, years of publication, titles, affiliations, author keywords, abstracts, citations, and references. Additionally, another copy of the file was saved in Excel for use in descriptive data analysis.
Descriptive statistics were created to describe the landscape of the social entrepreneurship knowledge corpus. VOSviewer software 1.6.10 [54,55] was used to analyze the citation patterns, intellectual structure, and research front in the social entrepreneurship literature. It is a statistical tool that, unlike the qualitative method, allows the subjective component of the authors to be kept under control, which is why it is increasingly used in entrepreneurial review [56,57].
Specifically, citation analysis (e.g., total citations, citations per document) was conducted to identify high-impact journals, documents, and authors. Author co-citation analysis (ACA) and keyword co-occurrence analysis [21] were used to identify the most prevalent "schools of thought" and the emerging topics within the social entrepreneurship knowledge base.
It should be emphasized that, unlike the co-citation analysis in which the SE knowledge base is deduced from the relationships among the authors (authors who are frequently co-cited are considered to share a conceptual perspective), keyword analysis uses highly frequent terms extracted from the articles to identify the most developed topical themes [22]. In this review we visualized the results of keyword analysis in a temporal display that highlights the topics of current interest among scholars in the field and allows Descriptive statistics were created to describe the landscape of the social entrepreneurship knowledge corpus. VOSviewer software 1.6.10 [54,55] was used to analyze the citation patterns, intellectual structure, and research front in the social entrepreneurship literature. It is a statistical tool that, unlike the qualitative method, allows the subjective component of the authors to be kept under control, which is why it is increasingly used in entrepreneurial review [56,57].
Specifically, citation analysis (e.g., total citations, citations per document) was conducted to identify high-impact journals, documents, and authors. Author co-citation analysis (ACA) and keyword co-occurrence analysis [21] were used to identify the most prevalent "schools of thought" and the emerging topics within the social entrepreneurship knowledge base.
It should be emphasized that, unlike the co-citation analysis in which the SE knowledge base is deduced from the relationships among the authors (authors who are frequently co-cited are considered to share a conceptual perspective), keyword analysis uses highly frequent terms extracted from the articles to identify the most developed topical themes [22]. In this review we visualized the results of keyword analysis in a temporal display that highlights the topics of current interest among scholars in the field and allows comparisons between different years, which is what Price [58] defined the "scientific research front".  trend, which has increasingly captured the attention of scholars, as demonstrated by the increase in the number of publications, especially in the second decade of the 2000s, as well as the high number of citations that characterize the scholarship on social entrepreneurship. From 2012 to 2020, 1245 scientific articles were published, representing 87% of the total publications.

Results
These results justify the topicality of the research, an extremely current field that, over the years, has increasingly received the attention of the scientific community.  These results justify the topicality of the research, an extremely current field that, over the years, has increasingly received the attention of the scientific community.
Although the contributions on social entrepreneurship came from researchers located in 98 different countries, there was a significant geographical imbalance in this knowledge base (Figure 3). In fact, half of the social entrepreneurship studies (51%) came from scholars from the United States (n = 361), United Kingdom (n = 177), Spain (n = 82), Canada (n = 77), and Australia (n = 75).  European countries such as Germany (n = 65), Italy (n = 48), and the Netherlands (n = 45) have actively contributed to this literature, representing a further 17% of current knowledge. In general, the more in-depth analysis made it possible to observe that of the 1425 articles published on social entrepreneurship, 88% came from developed countries and only 12% from developing countries. This is a gap that should not be overlooked, especially given the potential positive impact that social entrepreneurship activities have in these contexts where problems are more likely to be solved by initiatives promoted by citizens [59].
In this context, India deserves to be mentioned, which with 71 articles was the only country among emerging economies to be in the top ten countries with greater scientific production. This was positive result considering that 22% of India's population is still below the poverty line [60]. European countries such as Germany (n = 65), Italy (n = 48), and the Netherlands (n = 45) have actively contributed to this literature, representing a further 17% of current knowledge. In general, the more in-depth analysis made it possible to observe that of the 1425 articles published on social entrepreneurship, 88% came from developed countries and only 12% from developing countries. This is a gap that should not be overlooked, especially given the potential positive impact that social entrepreneurship activities have in these contexts where problems are more likely to be solved by initiatives promoted by citizens [59].
In this context, India deserves to be mentioned, which with 71 articles was the only country among emerging economies to be in the top ten countries with greater scientific production. This was positive result considering that 22% of India's population is still below the poverty line [60].
The 1425 articles that make up the knowledge base on social entrepreneurship were published in 536 different scientific journals. This result indicated the multidisciplinary nature of social entrepreneurship, also confirmed by the analysis of the different research areas.
Specifically, the journals that exceeded the minimum threshold of 15 articles ( Table 2) represented 30% of the total articles. The list of remaining journals appeared to be more distributed. This indicated that social entrepreneurship is a very broad field of study affecting different areas, which are analyzed from different perspectives. This result, however, was in line with the distribution of the research areas, where it was noted that the categories that were most used in the study of social entrepreneurship mainly referred to the area of Business and Management and Economics. The more in-depth analysis indicated that in recent years the object of study has also started to receive attention from other research sectors, such as social science (education), environmental science and sustainability, engineering, psychology, law. These data underline the emerging multidisciplinary nature of the scholarship on social entrepreneurship, which in any case remains historically linked to the field of business economics [61].
Furthermore, the most productive scientific journals were characterized by high quality. Of the 15 selected journals, 13 scientific journals were placed in Q1 or Q2, thus underlining the quality that distinguishes this research topic.
One journal (Social Enterprise Journal) was, however, not ranked because it started to be indexed and covered by the Scopus Index in 2018.
The analysis of the journal quartile rankings as a proxy for research quality suggested that the most cited publications on social entrepreneurship were featured in Q1 and Q2 journals.
The analysis showed a total of 2909 authors, with an average of two authors for each article, more than half of the articles (n = 794) were written by one or maximum two authors, which was indicative of the fact that this research field is characterized by discrete collaborations, probably due to the recent interest and development of social entrepreneurship. Furthermore, it was also a very fragmented research field: 23 authors exceeded the minimum threshold of five published articles (Table 3), representing 30% of the scholarship on social entrepreneurship. The cross-analysis between the number of articles and the total number of citations showed that the most cited author was Mair, J. (2212). All the other authors showed a total of more contained citations, probably a result indicative of the emerging character of social entrepreneurship as a basis for scientific knowledge.
Analysis of influential documents in the knowledge base on social entrepreneurship also revealed a dominance of scholars from Western societies. More specifically, the most cited documents in Table 4 came from the US, Canada, and Europe. Among the top 20 most cited documents, no documents came from developing countries.  The majority of highly cited documents on social entrepreneurship (Table 4) included conceptual (n = 6), but also qualitative (5) and review (5) method research. This series of influential papers focused on the definition and development of social entrepreneurship models [2,19,62] and on differentiating from concepts closely related to commercial entrepreneurship [8,63].
As for qualitative studies, they mainly used case studies, analyzing successful social enterprises and examples of social innovation (e.g., not-for-profit organizations and the creation of new organizational forms of social entrepreneurship). A qualitative study, on the other hand, applied the current bricolage theories in entrepreneurial studies with the aim of perfecting the framework of social entrepreneurship [64].
The results seemed to suggest that the knowledge base of social entrepreneurship is still in an early stage, where the most influential articles still focus on basic concepts and test the theoretical relationships between these key constructs, in an attempt to differentiate social entrepreneurship from the commercial one and to trace a specific theoretical framework of reference.

Intellectual Structure of the Social Entrepreneurship Knowledge Base
To answer the third research question, that is, to analyze the "intellectual structure" of social entrepreneurship research, author co-citation analysis (ACA) was carried out.
The intellectual structure refers to the different "schools of thought" that characterize the research topic, i.e., the main lines of investigation that define the field of study.
Co-citation analysis examined the frequency with which pairs of authors were cited together in the reference lists of the 1425 articles in the review database. Therefore, the co-citation analysis analyzed a much broader literature than the direct citation analysis. By examining the frequency of "co-citations of authors", the VOSviewer software was able to produce a network map that "visualizes the similarities" between the authors cited in our SE database [22]. Figure 4 shows the co-citation network for the selected articles. Articles that had at least 105 co-citation links were considered cited references. It turned out that of the 51,586 cited references identified by VOSviewer, 150 authors met this selection criterion. The most highly co-cited authors in the social entrepreneurship literature were Mair (1773), Dees (1127), Marti (882), Nicholls (861), and Tracey (715).
Publications are displayed by circles and labels. Their size depends on the total strength of the links between the different authors. The color of an element was determined by the cluster to which it belongs, which distinguishes the different schools of thought in the literature on social entrepreneurship. The distance between two elements indicates the strength of their relationship in terms of similarity, i.e., links of common citations. Figure 4 shows the co-citation network for the selected articles. Articles that had at least 105 co-citation links were considered cited references. It turned out that of the 51,586 cited references identified by VOSviewer, 150 authors met this selection criterion. The most highly co-cited authors in the social entrepreneurship literature were Mair (1773), Dees (1127), Marti (882), Nicholls (861), and Tracey (715).  The author co-citation map in Figure 4 shows that the intellectual structure of the social entrepreneurship literature is composed of three schools of thought: two are closely related to social entrepreneurship (red and blue cluster); the green cluster, instead, is made up of 47 authors associated with two research areas, that of "entrepreneurship" and "sustainability". Those associated with entrepreneurship research included: Lumpkin (537), Shane (363), Ajzen (220), Bandura (177), Kickul (171), Fayolle (135), Kuratko (128), and Audretsch (127). The presence of these authors in the map is justified by the fact that, in the initial stages of the development of social entrepreneurship research, scholars focused more on the comparison between commercial and social entrepreneurship, to develop alternative business models [65,66]. This is an area that can be considered as a starting point for studies on social entrepreneurship.
Scholars associated with the area of sustainability, such as McMullen (395), Hockerts (337), Venkataraman (315), Stephan (274), and Shepherd (230), are linked because their research has examined the impact of entrepreneurship on economic and social outcomes, generating an environmental value [67,68]. The emergence of this school of thought on the map reflects the frequent co-citation of entrepreneurship and sustainability topics by authors specializing in social entrepreneurship, indicating that research on the theme of social entrepreneurship is affected by the influences of the authors of the sub-theme of sustainable entrepreneurship.
The other two schools of thought (red and blue clusters) are closely linked to subthemes concerning social entrepreneurship.
In fact, its influence is quite substantial, as also indicated by the size of the nodes of the different authors. The scholars of the red cluster have tried to provide a definitive clarity of the construct [62,69] with the aim of arriving at new social business models [7,70].
Furthermore, the scholars of this school of thought have explored issues related to corporate social entrepreneurship [71] and social innovation [72].
Several authors in this group have written works that were among the earliest documents cited in Table 4. Based on their central location and close links with authors from all groups, Mair and Marti represent the "boundary-hugging" reference authors who link the social entrepreneurship cluster to the other two schools of thought. This may be due to their research focus on both traditional entrepreneurship and social entrepreneurship issues. Notably, this cluster also includes scholars known for publications on research methods, such as the qualitative case study [73] and the case method [74]. This result may mean that scholars who deal with analyzing social entrepreneurship frequently adopt these qualitative methods in their studies.
From the analysis carried out a picture emerged of social entrepreneurship as a cluster concept [46] that also embraces constructs related to describing commercial and sustainable entrepreneurship ( Figure 5). Furthermore, in the definition of social enterprise, the construct of social innovation appears important, which can be defined as the element that distinguishes social enterprise from other forms of enterprise. Social innovation in our study appeared as a tool that allows us to overcome the mechanisms that govern organizations for profit and their reinvestment of profits to provide positive changes for communities. Unlike traditional entrepreneurship, social entrepreneurs focus on the "double bottom line" [75], which is reaching both financial and social goals.
In fact, the graphical representation of the co-citation analysis also shows a fourth cluster (yellow cluster). It is more dispersed and poorly represented, thus, will not be considered as independent. Compared to the others clusters, the yellow one does not include many elements (only four authors) that are not well grouped, which makes classification difficult.

The Keywords Co-Occurrence Analysis
To answer the last research question, keyword co-occurrence analysis was used. A temporal keyword map should be interpreted in two ways. First, we must pay attention to the size of the keyword labels because it reflects the "relative frequency" with which the keywords occurred in our database. Larger labels indicate higher frequency. Second, the color of a label highlights the time period in which documents containing the keyword were concentrated. Keywords present in articles from recent years are indicated by the color yellow. Furthermore, in the definition of social enterprise, the construct of social innovation appears important, which can be defined as the element that distinguishes social enterprise from other forms of enterprise. Social innovation in our study appeared as a tool that allows us to overcome the mechanisms that govern organizations for profit and their reinvestment of profits to provide positive changes for communities. Unlike traditional entrepreneurship, social entrepreneurs focus on the "double bottom line" [75], which is reaching both financial and social goals.
In fact, the graphical representation of the co-citation analysis also shows a fourth cluster (yellow cluster). It is more dispersed and poorly represented, thus, will not be considered as independent. Compared to the others clusters, the yellow one does not include many elements (only four authors) that are not well grouped, which makes classification difficult.

The Keywords Co-Occurrence Analysis
To answer the last research question, keyword co-occurrence analysis was used. A temporal keyword map should be interpreted in two ways. First, we must pay attention to the size of the keyword labels because it reflects the "relative frequency" with which the keywords occurred in our database. Larger labels indicate higher frequency. Second, the color of a label highlights the time period in which documents containing the keyword were concentrated. Keywords present in articles from recent years are indicated by the color yellow. Figure 6 reveals three periods that describe the historical evolution of social entrepreneurship research. Although the research began to grow rapidly, the analysis showed us that in the initial phase (purple color), scholars were still struggling with the conceptual definition of social entrepreneurship, particularly through comparison with commercial entrepreneurship.
This particular group of works has been much cited and has formed the basis for further academic research on social entrepreneurship [2,63]. Along with these works, other scholars have also conceptualized social entrepreneurship in relation to third sector enterprises [76,77] and nonprofit organizations [39,78]. This is not surprising, since scholars who approached the study of social entrepreneurship have highlighted above all the non-profit aspects of the organization. In this period, scholars discussed the meaning and conceptualization of social entrepreneurship in terms of historical roots, characteristics, and future perspectives [79,80].
In the second phase (green color), scholars have focused on the concept of hybridity [81,82]. This marked a shift in focus from idealistic conceptualizations to more pragmatic aspects of social entrepreneurship.
The concept of hybridity was born with the double objective of the organization on the creation of social value and on economic purposes [83]. The pursuit of a dual mission is not exclusive to social entrepreneurship, but can be found in other forms of hybrid initiatives such as sustainable entrepreneurship, which, not surprisingly, appears in the map closely related to social entrepreneurship. Although the research began to grow rapidly, the analysis showed us that in the initial phase (purple color), scholars were still struggling with the conceptual definition of social entrepreneurship, particularly through comparison with commercial entrepreneurship.
This particular group of works has been much cited and has formed the basis for further academic research on social entrepreneurship [2,63]. Along with these works, other scholars have also conceptualized social entrepreneurship in relation to third sector enterprises [76,77] and nonprofit organizations [39,78]. This is not surprising, since scholars who approached the study of social entrepreneurship have highlighted above all the nonprofit aspects of the organization. In this period, scholars discussed the meaning and conceptualization of social entrepreneurship in terms of historical roots, characteristics, and future perspectives [79,80].
In the second phase (green color), scholars have focused on the concept of hybridity [81,82]. This marked a shift in focus from idealistic conceptualizations to more pragmatic aspects of social entrepreneurship.
The concept of hybridity was born with the double objective of the organization on the creation of social value and on economic purposes [83]. The pursuit of a dual mission is not exclusive to social entrepreneurship, but can be found in other forms of hybrid initiatives such as sustainable entrepreneurship, which, not surprisingly, appears in the map closely related to social entrepreneurship.
These hybrid initiatives act on a double level, supporting economic feasibility and environmental protection. This focus on sustainability, in addition to the creation of social value, has led scholars to question the initial idea of social entrepreneurship and to address certain issues such as the double identity of the entrepreneur [84] or institutional conflicts [81,[85][86][87], reasons why sustainable entrepreneurship can be considered a subset of the social entrepreneurship domain.
The introduction of the concept of hybridity marked a significant change in research by influencing scholars to also consider the ethical aspects of social entrepreneurship. In recent years, the number of empirical studies testing theoretical proposals for social entrepreneurship has increased [16]. This has led to focusing the attention of scholars on the study of the factors that stimulate social entrepreneurial intention.
As for the external variables that can support the development of social entrepreneurial intention, greater emphasis has been given to the role of education [98]. For example, Shahverdi, Ismail and Qureshi [98] identified the barriers of social entrepreneurial intention by moderating the role of education among research universities in Malaysia. Hockerts [99], in turn, focused on the relationship between the experiential learning process and the trend of the social enterprise institution. Furthermore, a number of external factors such as prior experiences [97,100], culture [101,102], and support from the social network [103], have been investigated empirically in relation to the social entrepreneurial intention. Within this phase, some articles have also explored "gender issues" by examining the role of gender in the formation of the social entrepreneurial intention [104,105].
For example, Notais and Tixier [106], through the analysis of six life stories of women from disadvantaged areas, studied the factors that push women to choose a social entrepreneurial career. Among these factors, the economic dimension and the attraction toward a social role were the most important. Lortie et al. [107], through gender selfschemas theory and social identity theory, explained the natural propensity of women toward social goals and the creation of social value.

Discussion and Conclusions
This review contributes to social entrepreneurship research by demonstrating a systematization of the knowledge base and useful insights into the intellectual structure.
Specifically, as the citation analysis shows, social entrepreneurship is an emerging research field, but in recent years it has increasingly interested researchers internationally. Furthermore, social entrepreneurial scholarship stands out for its high quality (for example, of the 15 selected scientific journals, 13 are placed in a Q1 or Q2 rank) and for a discreet collaboration between the authors (with an average of two authors per article), which is indicative of the importance that the scientific community attributes to this field of research.
The analysis of the most cited articles showed on the one hand a strong domination of Western society scholars, and on the other a scarce presence of quantitative studies.
In contrast to previous and important reviews on the topic (e.g., [108][109][110]), our study considered social entrepreneurship as a cluster concept (in line with the suggestions of Choi and Majumdar [46]), also analyzing the sub-themes that constitute the starting point for the study of social entrepreneurship. For this reason, together with the citation analysis, we considered it appropriate to carry out author co-citation analysis (ACA). This technique allowed us to analyze not only the citations that were part of the field of social entrepreneurship, but also all those authors who were most cited by researchers interested in social entrepreneurship.
Specifically, the results relating to the author co-citation analysis (ACA) indicated three schools of thought; one is related to entrepreneurship and sustainability (green cluster). This school can be considered as a starting point for research into social entrepreneurship. This is indicated in the first case, by borrowing some models of commercial entrepreneurship (for example, [111]) and in the second case by considering sustainable entrepreneurship as closely connected to social entrepreneurship.
The other two "schools of thought" are more specifically related to sub-themes of social entrepreneurship: definition and theoretical framework (blue cluster) and social innovation (red cluster), which are configured as elements that distinguish social entrepreneurship from other forms of business. The results showed that social entrepreneurship is configured as a cluster concept that embraces issues not only relating to other research fields such as commercial and sustainable entrepreneurship, but also issues that specifically concern the social dimension of entrepreneurship such as innovation.
The analysis of the keyword co-occurrence also made it possible to monitor the evolution of topics related to social entrepreneurship (research front). Specifically, there are three phases of development of research on social entrepreneurship. In the first period (2014-2016) the literature was based on the convergences between commercial entrepreneurship.
Scholars in this phase were more interested in analyzing the peculiarities between social and commercial entrepreneurship, placing emphasis on non-profit organizations and third sector. This is an extremely important phase because it constitutes the theoretical basis for the development of social entrepreneurship as an independent research field.
In the second phase (2016-2018), the concept of hybridity changed the focus of the attention of the scientific community. This change led to the proliferation of hybrid organizations and the birth of sustainable entrepreneurship, considered as a sub-theme in the study of social entrepreneurship.
Finally, in the third phase (2018-2020), the research shifted to the analysis of the factors that support the development of social entrepreneurial intention. From the documents that are part of our database it emerged that not only have personality traits specifically been studied (with reference to the Big Five Dimensions), but also psychological factors such as self-efficacy, resilience, moral obligation, empathy, and prosocial motivation. In addition, the articles that analyzed the factors that support the development of social entrepreneurial intention, specifically education, social network, culture and gender, are also part of this group.
Among these, gender deserves particular attention. In general, there is an imbalance between men and women in the choice of an entrepreneurial career, with a supremacy of men. Studies conducted on social entrepreneurship have shown that this gap is significantly reduced, further justifying the importance of this form of entrepreneurship in reducing prejudices and inequalities [57,112,113].
Based on the foregoing, we developed a conceptual model of the evolution of social entrepreneurship in the three phases that distinguish it (Figure 7). This conceptual model can be used for future research to deepen the knowledge base of social entrepreneurship. For example, our analysis showed that many researchers have undertaken to analyze the factors that stimulate the choice of a social entrepreneurial career. However, little is known about the interplay between the different factors and the mechanisms that come into play specifically. Furthermore, research on possible obstacles still appears to be underdeveloped [16]. We invite researchers to further investigate these aspects in their future research. Sustainability 2021, 13, x FOR PEER REVIEW 1 This conceptual model can be used for future research to deepen the knowledg of social entrepreneurship. For example, our analysis showed that many researcher undertaken to analyze the factors that stimulate the choice of a social entrepreneur reer. However, little is known about the interplay between the different factors a mechanisms that come into play specifically. Furthermore, research on possible ob still appears to be underdeveloped [16]. We invite researchers to further investigate aspects in their future research.
The results also showed low participation from developing countries. Indeed the exception of India, no emerging economy actively participated in the developm the knowledge base of social entrepreneurship. Given the importance that the lat sumes in the social change of a country, we believe that this is a gap that future re should fill. In fact, numerous studies have focused on entrepreneurs' efforts to in social impact and transformation, but there are some less explored areas under this t For example, the engagement of social entrepreneurs in important social problems r to poverty, health, education, and unemployment. Since the creation of social va driven by the mission and success of the enterprise, the strategy therefore depen factors very different from the commercial enterprise. This research area could be f enriched.
Our findings on the methods employed in social entrepreneurship research line with previous studies (e.g., [114,115]). This indicates that the social entreprene literature lacks empirical analysis, and, furthermore, the articles presenting empiri sults are predominantly based on qualitative methods. There is no doubt that case s and other forms of qualitative research, when used correctly, appear to be importan for improving understanding of the field [116]; however they are often used to exem the theoretical concepts of social entrepreneurship rather than for the construction adequate theory. Different research methods would help improve the field of soc trepreneurship because the field still lacks sufficient basis for large-scale quant studies.
Indeed, quantitative research on social entrepreneurship is very limited in scop focuses mainly on measuring social impact [117] or evaluating social enterprise fina [118]. Therefore, the development of quantitative measurement tools in social ent neurship is one of the most current research challenges [12]. The results also showed low participation from developing countries. Indeed, with the exception of India, no emerging economy actively participated in the development of the knowledge base of social entrepreneurship. Given the importance that the latter assumes in the social change of a country, we believe that this is a gap that future research should fill. In fact, numerous studies have focused on entrepreneurs' efforts to increase social impact and transformation, but there are some less explored areas under this theme. For example, the engagement of social entrepreneurs in important social problems related to poverty, health, education, and unemployment. Since the creation of social value is driven by the mission and success of the enterprise, the strategy therefore depends on factors very different from the commercial enterprise. This research area could be further enriched.
Our findings on the methods employed in social entrepreneurship research are in line with previous studies (e.g., [114,115]). This indicates that the social entrepreneurship literature lacks empirical analysis, and, furthermore, the articles presenting empirical results are predominantly based on qualitative methods. There is no doubt that case studies and other forms of qualitative research, when used correctly, appear to be important tools for improving understanding of the field [116]; however they are often used to exemplify the theoretical concepts of social entrepreneurship rather than for the construction of an adequate theory. Different research methods would help improve the field of social entrepreneurship because the field still lacks sufficient basis for large-scale quantitative studies.
Indeed, quantitative research on social entrepreneurship is very limited in scope and focuses mainly on measuring social impact [117] or evaluating social enterprise financing [118]. Therefore, the development of quantitative measurement tools in social entrepreneurship is one of the most current research challenges [12].
Other major challenges relate to the boundaries of social entrepreneurship compared to other more traditional forms of entrepreneurship. For example, with the development of the key concept of hybridization, the gap between the business world and the social world is narrowing, and hybrid organizations are starting to emerge in this context. The results indicate that hybridization is becoming an established concept, with the emergence of terms such as "social business" that conceptually mix the corporate and social worlds, it would be interesting to know who are the financiers of this type of organization and what are the reasons. These trends indicate future lines of research about which we still know very little. Some methodological limitations can be drawn from our study. In the first place, the use of VOSviewer and the choice of the analysis of the co-occurrences of the keywords imply that only those key words that are repeated several times in the scientific articles in our database should be used; considering the emerging social entrepreneurship phase, this could represent a limit because it does not give due importance to the whole panorama that describes the field of study, especially since it is an extremely varied and dispersive field of research. However, we overcame this possible limitation through the use of precise inclusion criteria, according to the recommendations of the PRISMA method, which allowed us to reduce the large number of studies on social entrepreneurship and to keep under control any attribution bias, analyzing only those articles relevant to our research objectives.
Secondly, although in our analysis we used two of the largest databases in the literature, it would also be interesting to use other databases to expand the body of the literature and highlight the differences and similarities regarding the analysis presented by us. It may also be useful to use different sources (e.g., books, book chapters, conference papers, etc.) as well as to include works written in a language other than English and different bibliometric indicators to continue studying the research field that appears to be constantly evolving.
Beyond the methodological implications, we believe that the present study makes a significant contribution to the literature in two different ways. First, by considering social entrepreneurship as a cluster concept. This prompted us to identify the relationships between the different sub-themes of social entrepreneurship and to broaden the body of knowledge. From this perspective it was possible to observe how the clusters are strongly related to each other (see Figure 4), significant proof that scholars who do research on social entrepreneurship dialogue together with authors interested in other forms of entrepreneurship, with the aim to build a common research pool and contribute to the development of this field of study. Furthermore, through this study, we identify a conceptual framework that summarizes all the most important stages in the evolution of social entrepreneurship. Knowing the current research trends, strengths, and weaknesses is important to inform researchers who are entering this field of study by leading the way in the development of this research topic.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.