Trends of Positive Youth Development Publications (1995–2020): A Scientometric Review

In recent decades, the positive youth development approach has gained momentum among researchers and practitioners who focus on youth potential and adolescent well-being. In this study, 1435 academic works on positive youth development (PYD) are collected from the Web of Science using CiteSpace, reviewed and analyzed. First, the number, subject distribution, country, and institution of published papers are presented, and the author collaboration network involved in PYD is analyzed using network analysis. The results show that authors and institutions in Western societies are the main contributors to PYD research and that interdisciplinary collaboration is gradually growing. Using document co-citation analysis, research hotspots in the field were investigated. Furthermore, the research frontiers and trendy topics in PYD from 2010 to 2019 were found by using burst detection, and research gaps were identified. The findings provide research directions for future PYD studies in the field of applied quality of life research.


Introduction
Adolescent development involves a complex interaction of biological, psychological and social relations (Waid and Uhrich 2019). Adolescence is generally regarded as a period during which a person is at an increased risk of developing unhealthy and unsafe behaviors. Drug abuse, alcohol abuse, smoking, lack of exercise, unsafe sexual behavior, violence and injury and disability are common risks during adolescence (Johnson and Jones 2011;Kann et al. 2016). Scientific research on problems in adolescent development has been guided by a deficit model based on the notions of storm and stress (Hall 1904). According to this approach, adolescents are at risk, troubled by problems and threatened by inevitable psychological turmoil (Anthony 1969;Freud 1969). However, a new perspective based on positive youth development (PYD) began to emerge in the 1990's (Lerner et al. 2019). PYD stems from the criticism of the longstanding deficit model of adolescence and highlights healthy and holistic development rather than an emphasis on the reduction of problem behaviors, thereby redefining youth development from a more positive perspective (Lerner 2002(Lerner , 2004. Several PYD models have been proposed in the scientific literature . All PYD models focus on human potential, individual capability and plasticity ), but they take slightly different forms; such models include Benson's model on external and internal developmental assets (Benson 2007), Lerner's 5Cs/6Cs model (Lerner 2006), social-emotional learning (SEL) and Catalano's 15 PYD constructs (Catalano et al. 2003). The 15 PYD constructs identified from effective PYD programs presented by Catalano et al. (2003) include several domains of psychosocial competencies, resilience, spirituality and prosocial involvement. PYD theories, research and applications are important for the promotion of quality of life in adolescents.
Over the past twenty-six years, PYD has inspired many studies, and researchers have carried out a great amount of research on the connotation, impact and measurement of PYD as well as PYD interventions in the field of applied quality of life Zhou et al. 2020a). The PYD approach in the 1990s has had a far-reaching impact on the development of theory and practice in psychology, education, social services, social work, and public health. In addition, the growing interest of international organizations in PYD Research (UNICEF 2005(UNICEF , 2017World Bank 2000 confirms the important role of PYD theory in global youth development (Lerner et al. 2019). Hence, an analysis of the collaboration network, hotspots, and frontiers in this field can inform scholars of the current state of science and help practitioners to develop programs that promote quality of life in young people, particularly through the promotion of psychosocial competence and life skills (Shek 2020). The related findings can also help policymakers to prioritize youth development policies and services. Therefore, the present study attempted to conduct a scientometric analysis on the development of the knowledge base of PYD field.
Although some review articles have provided an overview of PYD studies, they are limited in terms of region and research topics (Maslow and Chung 2013;Shek et al. 2019;Waid and Uhrich 2020). Moreover, traditional reviews cannot quantitatively analyze the development of knowledge in a specific field from the vast literature. Furthermore, some of these reviews are qualitative in nature and thus prone to subjectivity. PYD is an interdisciplinary research field covering disciplines including psychology, education, social services, social work, and public health. Therefore, it is important to conduct a scientometric analysis based on a large bibliometric database that encompasses studies from the beginning of PYD research.
To address the abovementioned gaps, in this study, an in-depth scientometric review of PYD research is performed to map the knowledge structure and development trends of the PYD field. Scientometrics is the study of the inputs (e.g., researchers, research funds), outputs (e.g., papers, patents, periodicals) and processes (e.g., information dissemination, communication network) of scientific activities by using statistical and computational methods. Scientometric methods have been applied to knowledge structure analysis, discipline evolution analysis, scientific output evaluation, and other methods in many disciplines (Aleixandre et al. 2019;Ekundayo and Okoh 2018;Wang et al. 2018). In recent years, the knowledge map combining information visualization technology with the citation analysis of traditional scientometrics has been widely used. Through data mining, information processing, and graphic production, the development of and relationships within scientific knowledge can be explored (Chen et al. 2008;Shiffrin and Borner 2004). In addition to giving researchers an overview of the work in a particular field, the findings can also provide possible directions for future research and PYD program development by referencing the gaps and deficiencies in the field.
The present study aims to answer three important questions through a scientometric analysis of PYD research. The first research question is as follows: Which countries/ regions, institutions, and authors were most productive and influential in the field of PYD research? This question aims to determine the differences in progress between countries and regions and whether PYD is dominated by publications and academics in particular areas of the globe. The answer to this question is important because research in the social sciences has been criticized as "WEIRD" (participants are educated and from Western, industrial, rich democratic societies) (Nielsen et al. 2017). The second question is as follows: What are the research hotspots in this field, and how have they developed? This question aims to determine the topics and programs that have attracted the most attention in this area. The third question is as follows: What are the recent developmental trends and frontiers in PYD? This question aims to find the most popular articles published in recent years that will continue to impact the field in the near future. Answers to these questions are important for academic and applied research related to quality of life.
To answer the above questions, our review is guided by three key goals. The first goal is to determine the main contributors to PYD research by country/region, institution and author. The second goal is to make a knowledge map based on keywords and co-citation documents to understand the research hotspots in the field of PYD. The third goal is to make a knowledge map based on the perspective of burst references to reveal the research frontier and trend of PYD. Our study is different from previous review on PYD field in the following ways. First, we reviewed PYD research in a holistic manner rather than emphasizing on certain subfields. Second, we analyzed almost all relevant articles and reviews in the literature databases instead of focusing on key articles from specific journals. Third, to complement previous reviews, we employed a bibliometric visualization method to review PYD research.

Scientometric Analysis in CiteSpace
CiteSpace is a scientometrics tool that can be used to visualize knowledge. Compared with other visualization tools, it provides clearer and more interpretable visualizations by using various visualization analysis functions (Chen 2006). In recent years, CiteSpace has been widely used, not only in the information science field but also in psychology, education science, sociology, medicine, environmental science, and other disciplines (Chen et al. 2012;Guo et al. 2019;Jiang et al. 2019;Taskin and Aydinoglu 2015;Xu et al. 2020). It can provide an extensive and in-depth understanding of the relevant research in a specific period to help researchers gain a better understanding of the overall nature of the field and to identify future research directions. Despite its popularity, CiteSpace has not been used to map global PYD research. To systematically review the research of PYD, three technologies of CiteSpace are used in this study: a) collaborative analysis, b) document citation analysis, and c) keyword co-occurrence analysis. These techniques are commonly used in other research in the field (Chen 2006;Wang et al. 2019).

Data Collection
According to previous research using CiteSpace visual analysis software (Chen et al. 2012;Niazi and Hussain 2011;Song et al. 2016;Yu 2015), the data collection process included two steps. The first step was to determine the data source. Web of Science (WoS) is considered to be the ideal data source for scientometric research (Van 2006). Therefore, we chose to search the core collection of WoS. The core collection included six online databases: SCI Expanded (1900-present), SSCI (1900-present), A & HCI (1975-present), CPCI-S (1990-present), CPCI-SSH (1990-present), andESCI (2015present). By searching the database using the term "positive-youth-development" from January 1900 to April 26, 2020, 1585 papers related to PYD were obtained. We further refined the search results by applying the following conditions: language = English; document type = article + review. The refined search returned 1436 documents, and these results were downloaded as text files of the form "full record and quoted documents" on April 26, 2020. After the 1436 pieces of literature were imported into CiteSpace, 1435 records were obtained as the final data of the knowledge map by using CiteSpace's weight removal function. The references in these 1436 papers created a citation dataset of 47,635 records, including 34,735 original articles, 390 book chapters, 2210 review articles, and 10,690 other types of articles. The 47,635 citation dataset is used in the subsequent document co-citation analysis.

Analysis Plan
Based on the three research objectives of this paper, combined with the visualization map of CiteSpace, the collaborative network analysis, document co-citation analysis and keyword co-occurrence analysis are completed, and the research contributors, hotspots, frontiers and trends are obtained. The specific research plan is shown in Table 1.

Research Outputs
The level of development of a discipline can be represented by the number of papers published in the area; in other words, an increase in the number of publications demonstrates an increase in the scientific knowledge of the field. Based on the literature retrieval process, which returned 1436 documents, a histogram of papers published in the PYD field was created by using Excel (Fig. 1). Figure 1 shows the progress of papers published in the 26 years from 1995 to 2020 related to PYD. According to the annual change in the number of published papers, we divided the development of PYD into two stages. The first stage was from 1995 to 2005, when scholars had just begun to pay attention to PYD, and the number of published papers grew slowly. The second stage was from 2005 to 2020, when the number of published papers increased rapidly as PYD researched gradually deepened and expanded; this stage also promoted the development of the PYD field. The number of publications reached 194 in 2018 and 233 in 2019, indicating that PYD research gained momentum, and it seems like it will remain active in the next few years.

Research Categories
According to the analysis of the WoS database, we evaluated the publishing trends of disciplines involved in PYD research. We selected the top ten disciplines in terms of their involvement in the publication of PYD research (Table 2); the top disciplines are ranked as follows: psychology (n = 731); social sciences (other) (n = 209); public, environmental, and occupational health (n = 193); social work (n = 175); education To make a knowledge map based on the perspective of burst references to reveal the research frontier and trend of PYD document co-citation burst analysis Identify the surge in the frequency of co-citation literature in a short period of time.
research frontiers and trends (n = 147); family studies (n = 139); science/technology (other) (n = 96), sports sciences (n = 87), environmental sciences/ecology (n = 75) and pediatrics (n = 70). The distribution of subjects shows that psychology, social sciences, public, environmental, and occupational health, and education research were the most active in this field.

Research Journals
By searching PYD publications in WoS, we observed that more than 100 journals had published research related to PYD. We listed the ten journals with the most articles in

The Collaboration Network of Positive Youth Development
Scientific collaboration analyzes scholars who cooperate to create scientific knowledge (Sylvan and Ben 1997). CiteSpace provides three main levels of collaboration analysis: macro-national collaboration (co-country/region), meso-institutional collaboration (coinstitution), and micro-author collaboration (coauthor). In this study, the evolution of interdisciplinary collaboration in the past 25 years was further analyzed. The collaboration network represents the degree of refinement of a research field. The more frequent the collaboration, the deeper the development. In practice, the collaboration network can serve as a useful guide for new researchers and for those seeking potential collaboration opportunities in the research areas of PYD.

Country/Regional Collaboration Network
To analyze collaboration among countries and regions, the parameters of the collaboration analysis in CiteSpace were set as follows: (1) time slices: 1995 to 2020, by year; (2) node type = country; (3) top 50 most frequent nodes from each slice; (4) pruning = default settings. The collaboration analysis focused on the principal countries/regions of research in the PYD field. The results showed that the number of papers published in PYD research differed by country/region. We found the top ten countries in terms of the number of published papers; the results are as follows: America, China, Canada, the United Kingdom, Australia, the Netherlands, Portugal, Italy, New Zealand, and Spain (Table 4). The United States, as the largest contributor, published 956 papers, followed by China, with 191 publications. Figure 2 shows a country collaboration network with 33 nodes and 91 edges, which means that 33 countries/regions have participated in the collaboration of PYD research. The node size in the collaboration map indicates the number of papers published by country/region, and the edges between nodes reflect collaboration between countries. A node with a purple outer ring has a high betweenness-centrality (BC) index, which indicates that the node has made significant collaborative contributions. BC ranges from 0 to 1, and the higher the value, the higher the contribution of the node. Nodes of high BC are usually regarded as key points in the field (Chen 2006). As shown in Fig. 2, the United States, Canada, and the United Kingdom had purple outer rings, which indicated that the three countries had a high impact on research in the field of PYD (BC > 0.1). In general, the development of PYD research requires global collaboration.

Institutional Collaboration Network
For the analysis of institutional collaboration, the parameters from the country collaboration network analysis were used; however, in this case, the node type was the institution. According to the results, 206 institutions have published research papers related to PYD. In Table 4, we list the top ten research institutions according to the number of published papers. Tufts University was the largest contributor, with 106 papers published, followed by 103 papers published by The Hong Kong Polytechnic University. Figure 3 shows the institutional collaboration network, which has 206 nodes and 457 edges. The larger the node, the more studies published by the institution. According to Fig. 3, there were many institutional collaborations within the United States and within East Asian countries/regions, but there were relatively few institutional collaborations between the Eastern and Western worlds. In addition, Tufts University, Boston University, and Pennsylvania State University had thick purple outer rings (BC > 0.1), which means that these institutions played an important role in the collaboration network.

Author Collaboration Network
For the analysis of author collaboration, the parameters from the country collaboration network analysis were used; however, in this analysis, the node type was the author. Table 4 lists the top 10 authors in PYD sorted by the number of published papers. Table 4 shows that Daniel T. L. Shek was the most prolific author in the field of PYD, contributing 118 articles, followed by Richard M. Lerner (n = 80), Jacqueline V. Lerner (n = 35), Rachel C. F. Sun (n = 35), Edmond P. Bowers (n = 25), and Peter C. Scales (n = 22). By analyzing the co-occurring author network, we found that scholars collaborate closely in this field (Fig. 4). The author's collaboration network, which consists of 255 authors and 552 links, is presented in Fig. 4. Three typical author groups, those led by Daniel T. L. Shek, Richard M. Lerner, and Martin Camiré, made up a portion of the research on PYD (Fig. 4). The research group led by Daniel T. L. Shek launched a long-term PYD research project named "P.A.T.H.S. to Adulthood" to promote youth development in Hong Kong . To assess how young people in Hong Kong develop during the project period, they developed a Chinese Youth Positive Development Scale (CYPDS) with 80 items ) based on the 15 PYD structures validated in successful PYD research projects in the United States (Catalano et al. 2004). It was found that CYPDS has good psychometric quality . This scale is an attempt to apply PYD to Hong Kong, China, and it has been verified based on its applicability. The group also conducted longitudinal study on PYD development in adolescents in Mainland China (Zhou et al. 2020a, b).
The research team led by Richard M. Lerner and colleagues focused on the positive development of American adolescents and evaluated and revised the scale for measuring the Five Cs (Bowers et al. 2010;Geldhof et al. 2014;Lerner et al. 2005;Phelps et al. 2009). Lerner's model used five characteristics related to positive youth functioning as Five Cs: competence, confidence, character, caring, and connection (Lerner et al. 2005). Lerner and colleagues then found another C, contribution, can be fostered in youth when they possess the characteristics of 5Cs (Lerner et al. 2005). The 5Cs were also found to be correlated with behavior problems in early adolescence (Jelicic et al. 2007).
Martin Camiré and colleagues integrated a PYD framework into physical activity programs. Through his research, Camiré examined how PYD could be facilitated in the context of sports (Bean et al. 2018;Camiré and Santos 2019;Santos et al. 2018). Camiré also studied the role of coaches in the promotion of positive development and the transfer of life skills (e.g., leadership and decision-making) to young sports participants . From a PYD perspective, Camiré's focused on intervention, health promotion, and research methods and successfully applied PYD to sports.

Category Collaboration Network
In addition to analyzing the overall interdisciplinary collaboration network, we were interested in determining how the interdisciplinary collaboration network evolved over time. Therefore, we evaluated the evolution of cross-disciplinary collaboration every five years based on the 200 most frequently occurring nodes. The parameters used in the collaboration analysis in this case were as follows: (1) time slices: 1995 to 2019, every 5 years; (2) node type = category; (3) top 200 most frequent nodes from each slice; (4) pruning = default settings. Figure 5 shows the cross-disciplinary collaboration of PYD in the past 25 years. The results showed that PYD research has become more interdisciplinary over time. Although PYD research has been predominantly performed in the area of psychology in the past 25 years, cross-disciplinary collaboration of PYD has changed over time. In the first five years of exploration of this field (1995)(1996)(1997)(1998)(1999)(2000), studies in psychology and other disciplines such as education and social work were relatively independent. Starting in the twenty-first century, researchers in psychology began collaborating with those in fields such as public, environmental and occupational health and pediatrics. From 2005 to 2009, the collaboration network became more complex and more diverse, and even small-scale discipline such as hospitality started to cooperate with other disciplines. With the rapid increase in the publication of PYD since 2010, the collaboration network has continued to expand and accelerate significantly. In the last five years, psychology, social work, environmental science and ecology, public, environmental and occupational health, and family studies were the most active areas in terms of interdisciplinary collaboration.

Intellectual Landscape of Positive Youth Development
This section introduces a rigorous analytical framework for integrating keyword cooccurrence and literature co-citation to determine research hotspots, research trends, and frontiers in the field of PYD.

Keyword Co-Occurrence Analysis
Keywords are used to summarize the content of a paper. Useful information, such as objectives, methods, and viewpoints, can be obtained from the keywords of a publication (Tian et al. 2018). Therefore, frequency analysis of keywords is critical for investigating hot topics and developments in a given field (Wang et al. 2018). The co-occurrence analysis of CiteSpace mainly included two steps: (1) extract keywords and calculate the frequency of keywords through classification, and (2) perform keyword co-occurrence analysis by obtaining the co-occurrence matrix of the extracted keywords (Chen 2004(Chen , 2006. The keywords used in the analysis included "Author Keywords" provided by authors and "Keywords Plus" provided by journals. The parameters of co-cccurrence analysis in this case were as follows: (1) time slices: from 1995 to 2020, by year; (2) node type = keyword; (3) the top 50 most frequent nodes from each slice; (4) pruning = default settings. The knowledge map generated under these settings consisted of 289 nodes and 2437 links (Fig. 6). The co-occurrence network of PYD keywords included two critical features: frequency and BC.
Frequently occurring keywords were represented by bigger nodes. A keyword with a high BC value plays an important role in linking other keywords in the network graph The top two keywords in terms of co-occurrence frequency (Table 5) were "positive youth development" (n = 843; BC = 0.12) and "adolescent" (n = 376; BC = 0.06). Teenagers were the main subjects of PYD research. The keywords "adolescent" "program" "behavior" "prevention" "risk" "school" and "participation" had high BC values which indicates that these nodes played an important role within the keyword Fig. 5 Category collaboration network. Each node indicates a category, and the larger the node, the more papers were published. Each edge indicates a collaborative relationship between two categories co-occurrence network. According to previous scientometric studies (Sun et al. 2020;Wang et al. 2019) high-frequency keywords can be regarded as PYD publication hotspots. However the hotspots identified by this method were macroscopic and this analysis is limited to find specific hot topics. To better present the topics of PYD research the present study conducted document co-citation analysis using Cite Space

Document Co-Citation Analysis
Co-citation occurs when two items from the early literature are cited by later research at the same time (Small 1973). Compared with citation analysis, co-citation analysis can provide more reliable insights into the knowledge field by excluding promising articles (Mustafee et al. 2014). The knowledge network of the references used in this study includes highly cited references, high-BC references, and strong citation burst references (i.e., burst citations). The parameters of document co-citation analysis in CiteSpace were as follows: (1) time slices: from 1995 to 2020, by year; (2) node type = reference; (3) the top 200 most frequent nodes were selected from each slice; (4) pruning = default settings. After running CiteSpace, a network of 2199 nodes and 10,154 links was created to visualize the 26-year period.
If the reference has a high BC, then it is important to the development of the research field (Chen 2006). Table 6 shows the top 10 references with the highest co-citation counts, and Table 7 shows the top 10 references with the highest BC values.
To better characterize the research hotspots of PYD, the co-citation network was divided into 27 clusters (Fig. 7) that were automatically marked by title terms. The clusters were numbered in a descending order of the cluster size, starting from the largest cluster 0, the second largest cluster 1, and so forth. A node represents one cited reference, and a line represents one co-citation relationship. The color represents the date of the publication, with yellow indicating that the study was recently published, and green and blue indicating that the study was published in the early years. The network has a Modularity Q of 0.7851 (Q > 0.7), which is very high, suggesting that the specialties in science mapping are clearly defined in terms of co-citation clusters. The Fig. 6 Keyword co-occurrence network. Each node represents a keyword. A larger node indicates a keyword with a higher frequency. Each edge indicates a co-occurrence relationship between two keywords average Silhouette S of 0.2601 (S < 0.3) is relatively low and due mainly to the numerous small clusters.
Burst detection is a useful research method because it can help scholars to identify topics that attracted special attention in a certain period of time (Zhou et al. 2018). Therefore, we generated a total of 220 burst articles by using the reference co-citation analysis of CiteSpace. Table 8 presents 20 references with the strongest citation bursts. A reference is said to burst if it experiences a sharp increase in citation frequency (Zhou et al. 2018). Table 9 presents references with recent bursts. Such references can be used to investigate research frontiers and predict research trends ).

Collaboration Network of Positive Youth Development Research
In recent years, the importance of scientific cooperation has been increasingly recognized. Through scientific collaboration, complex scientific problems can be solved and knowledge creation can be promoted (Sonnenwald 2007). Collaboration analysis is essential for understanding academic exchange and knowledge diffusion (Chen 2006). To provide a collaboration network and academic influences on PYD research, four types of collaboration analysis were applied in this study: country analysis, institution analysis, author analysis, and interdisciplinary analysis.
This study showed that scientific publications on PYD research were primarily performed in America, China, and Canada. The most productive institutions came from China and the US, confirming the importance of their contributions to PYD. This finding is consistent with the WEIRD phenomenon mentioned above. The Hong Kong Polytechnic University and Tufts University were the most productive and influential institutions because they published the most articles and had high BC values. Meanwhile, institutions from Asian countries have been increasingly Three prominent authors formed three leading research groups. However, we found that the collaborative relationships among the three research groups were not strong. Daniel T. L. Shek and Richard M. Lerner were the most productive and high-impact authors of PYD research, but there was no direct collaboration. Interdisciplinary cooperation was rare at the beginning of the period we investigated, but recently, it has begun to flourish and cover more areas. In the last five The contribution of the developmental assets framework to positive youth development theory and practice.
years, psychology, social work, environmental science and ecology, public, environmental and occupational health, family studies were active areas in terms of interdisciplinary collaboration. Interdisciplinary cooperation should remain a trend in future research on PYD.

Research Hotspots of Positive Youth Development
We identified the hot topics of PYD by analyzing keywords and co-citation clusters in the present study. The hotspots are discussed in terms of the definition of PYD, the evaluations of PYD, and the theoretical exploration of PYD.

The Definition and Connotation of PYD
PYD is a broad concept with rich connotations. Several researchers have proposed definitions according to their different emphases. First, Damon (2004) pointed out that PYD focuses on the potential of individuals and pays attention to talent, strength, interest, and potential, instead of ability defects. Hamilton et al. (2004) developed and expanded this definition and expounded the PYD concept to include the development process, methods and principles, and practice. Catalano et al. (2004) combined existing research and proposed a definition for development goals/outcome orientation that included 15 aspects of goals: connection, resilience, behavioral capacity, social capacity, emotional competence, cognitive ability, moral capacity, self-determination, spiritual intelligence, self-efficacy, clear and positive identity, hope for the future, positive behavior recognition, prosocial activity participation, and prosocial norms. Lerner et al. (2005) also defined PYD as a developmental striving toward being full, healthy, and successful. However, conceptions of PYD are primarily based on Western concepts and there have been few attempts to integrate the related concepts across cultures.

Evaluation Methods of PYD
The evaluation of PYD has remained a concern of scholars. The evaluation of PYD has gone from the initial qualitative research and exploration of questionnaires on some dimensions of PYD to a mature measurement tool. However, because there are several definitions and structures of PYD, researchers often adopt different questionnaires. It is common now to use multi-index independent measurement or multidimensional comprehensive measurement. With multi-index independent measurement, two or more PYD indexes are adopted to conduct independent measurements. Among them, the 5Cs structure, which includes five dimensions (i.e., ability, self-confidence, connection,  character, and care), is one of the most commonly used tools (Lerner et al. 2005). The scale has a solid theoretical basis and a refined structure, with good reliability and validity that has been verified via long-term follow-up study. Furthermore, an abridged version and a super-abridged version have been developed, and these versions are very applicable and generalizable (Geldhof et al. 2014). In addition to assessment, evaluation studies of PYD programs in Hong Kong (Ma et al. 2019) and mainland China (Zhu and Shek 2020) are becoming more common.

Related Theoretical Exploration of PYD
As research continues to develop, a theoretical basis of PYD is being formed. Its theoretical core is based on the following three questions: how to conceptualize PYD; how to promote PYD; and the mechanism of promoting PYD. Therefore, the development of the relevant theories of PYD draws from the key concepts of developmental system theory and develops a series of key ideas covering the interaction between individual and environment and the realization of optimal development based empirical evidence. Resilience (Lerner et al. 2013;Leipold and Greve 2009;Masten 2007), developmental assets (Benson 2007;Sesma et al. 2005), developmental contextualism (Bronfenbrenner 2001;Lerner 2006) and relational developmental systems (Bateson and Gluckman 2011;Lerner 2006;Overton 2013) have been PYD research hotspots for the past few years.

Psychosocial Intervention and Educational Practice
The scientific intervention of PYD emphasizes the promotion of the resources of individual and environment (e.g., family, school, community) and the enhancement of the positive interaction between individual and environment. Youth development institutions have carried out influential long-term PYD promotion programs in recent years. The 4H program in the United States is the most influential of such programs. The program aims to improve civic awareness, leadership, responsibility and life skills and focuses on the positive development of the youth through long-term follow-up study ). In addition, the Changing Lives Program of Florida International University helped youth to deal with behavioral and emotional problems, to form perfect self-identity, and to set academic and career goals to promote positive development (Eichas et al. 2010). The program focuses on solving problems and fostering skills and advantages to promote full development. The P.A.T.H.S. program in Hong Kong is a PYD intervention program in the Chinese cultural context. The program has developed a curriculum-based PYD promotion program for junior high school students in Hong Kong, China. It integrated various resources including research institutions, all levels of government, schools, and nongovernmental institutions and highlights the 5 Ps (program, people, process, policy, and place) to ensure quality and implementation (Shek and Sun 2013). The program is a good example of the organic integration of PYD theory, scientific intervention, and educational practice.

Research Frontiers of Positive Youth Development
In this paper, the development trends and future directions of PYD research were evaluated by using literature co-citation analysis. The PYD research trends from 2005 to 2010 focused on the concept of PYD (Damon 2004;Lerner et al. 2005) and measurement evaluation methods . From 2011 to 2015, the focus shifted the evaluation of programs in different institutions, such as 4H ) and P.A.T.H.S. . From 2016 to 2020, the theory of PYD gradually diversified and focused on development assets (Benson et al. 2011), social emotions (Durlak and Dupre 2008), etc.; furthermore, through practical projects, the Five Cs model was developed (Bowers et al. 2010). In addition, some researchers have focused on the development of teenagers during sports programs (Bean and Forneris 2016). In short, PYD research has shifted from basic theoretical studies on the developmental resources and structures to practical applications of intervention programs. Moreover, we regarded references that underwent significant bursts within the past five years (2015-2020) as the newest PYD research frontiers. The 26 recent burst references in Table 9 include Holt et al. (2017), Jennifer et al. (2014, Camiré et al. (2013), Benson et al. (2011), andTolan et al. (2016); these references are expected to continue to be important in the future because bursts are usually a predictor of subsequent research trends (Fu et al. 2019). The focus on applied PYD is important because PYD attributes positively influence adolescent development (Shek 2020), including academic adjustment (Shek and Chai in press).

Research Gaps in Positive Youth Development Research
Although PYD research has been fruitful and has underwent theoretical refinement, there are two research gaps. First, the research has been dominated by Western researchers and institutions. The most productive authors and institutions were from Western countries, and the findings and evaluations were mainly based on Western children and adolescents. Because the PYD models were mostly developed in Western contexts ) and culture is an important dimension of positive youth (Benson et al. 2006), it is necessary to conduct more research in non-Western contexts.
According to Wiium and Dimitrova (2019), researchers should consider the context in which young people live to ensure the generalization of developmental assets. In recent years, a group of researchers in Hong Kong applied the PYD model in the Chinese cultural context Shek et al. 2019;Zhou et al. 2020a, b). However, research on children from other cultural backgrounds is still lacking. It is important to focus on cultural diversity in future PYD research if we wish to apply Western PYD theories and models to promote the quality of life in adolescents in non-Western contexts.
Second, knowledge from different disciplines must be integrated. Although PYD has been studied for more than 20 years, large-scale interdisciplinary collaboration began only in recent years. As Cantor et al. (2018) and colleagues argued, a holistic understanding of youth development requires the integration of psychology, biology, neuroscience, and other social sciences. Cross-disciplinary collaboration is needed to fully utilize the breadth and depth of the available knowledge. In the last decades, since the importance of interdisciplinary work has been increasingly recognized, interdisciplinary research is becoming more common (Noorden 2015). As Noorden (2015) pointed out, interdisciplinary research has a longer-term impact than noninterdisciplinary studies. Therefore, interdisciplinary collaboration and integration is crucial for PYD research.

Conclusion
To the best of our knowledge, this study is the first scientometric analysis of the global PYD research carried out over the past two decades. Using the databases from WoS, we found that PYD research has attracted increasing attention from researchers in different disciplines. We used several methods via CiteSpace to address three research questions, and the investigation included the following: (1) the countries/regions, institutions, and authors that most contributed to PYD research were recognized, and the development of interdisciplinary collaboration was analyzed by using the collaboration network method; (2) the combination of document co-citation analysis and keyword cooccurrence analysis revealed the hotspots and research trends of PYD research; and (3) the research frontiers and trends of PYD were examined by using document cocitation burst analysis. The visualization tools offered an in-depth understanding of the knowledge map of PYD. These results provide information that is valuable to PYD researchers and practitioners.
However, there are some limitations of this study. First, we used only six online databases and thus this study might not consider all PYD studies. Second, "positiveyouth-development" was used as the only search term; this method could be improved. For example, some researchers might use the term "social-emotional learning" instead of PYD in their studies, even though such studies are within the scope of PYD. Therefore, future studies should use more flexible keywords to search the databases. Third, the collaboration network analysis focused on journal articles and reviews, as books and book chapters were not covered in the database. Furthermore, some organizations have published work in the form of reports and web-based information. Therefore, future studies could incorporate more diverse sources for the scientometric analysis.
Funding This research was supported by "the Fundamental Research Funds for the Central Universities", Southwestern University of Finance and Economics (Grant Number: 330600004007000006).

Compliance with Ethical Standards
•Conflict of Interest The authors declare that they have no conflict of interest.
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