Research trends on human trafficking: a bibliometric analysis using Scopus database

Background Human trafficking is a crime against humanity. It is also a serious threat to global health and security. Globalization has made human trafficking an easier task for the criminal organizations. No data are available on the volume, research trends, and key players in this field. Therefore, the aim of this study was to assess the research activity and research trends on human trafficking. Methods A bibliometric method was adopted. Literature published in academic journals indexed in Scopus database was retrieved. The study period was set from 2000 to 2017. Results Two thousand forty-four documents were retrieved. The average number of authors per document was 1.9. Over one third (n = 771; 37.7%) of the retrieved documents were about sex trafficking, 616 (30.1%) were about labor trafficking/forced labor, 199 (9.7%) were about child trafficking, and 138 (6.8%) were about organ trafficking. One third (n = 707; 34.6%) of the documents were in health-related fields while 1526 (74.7%) were in social sciences and humanities. The USA ranked first (n = 735; 36.0%) regarding the number of published documents. Geographic distribution of the retrieved document showed that world regions with a high prevalence of human trafficking had the least research contribution. International research collaboration has a limited contribution to the retrieved literature. The Harvard University (USA) was the most active institution (n = 39; 1.9%). International Migration (n = 35; 1.7%) was the most active journal in publishing documents on HT. Documents published in Transplantation journal received the highest number of citations per document (25.5) and two of the most cited documents were about organ trafficking. Conclusion There was an under-representation of health-related literature on human trafficking. Literature on sex trafficking dominated the field of human trafficking. Research networks and research collaboration between the source and destination countries is important. Future research plans need to focus on health issues and on exploited/trafficked laborers. Electronic supplementary material The online version of this article (10.1186/s12992-018-0427-9) contains supplementary material, which is available to authorized users.

countries developed their own national policies and protocols to criminalize and fight HT [10].
Sex trafficking is one of the most common forms of HT with more than half a million women being trafficked every year [6]. Forced labor, child labor, child soldiers, debt bondage, involuntary domestic servitude, and organ/tissue removal for transplantation are other common forms of HT [11]. Forced prostitution represents the highest percentage of HT victims with the majority being women from Eastern Europe, Central Asia, and North and South America [8,12,13]. In contrast, the majority of victims of forced labor are men from Africa, the Middle East, and Asia [8,12]. International organizations distinguish between HT and smuggling. In smuggling, the actor takes part in the process while HT involves some deception or coercion [14]. Unlike HT, migration and smuggling are less often involved with a criminal organization [14].
Human trafficking has a global dimension despite that trafficking could occur within the same country [8]. Human trafficking affects people of all races, religions, social class, and education. It often results in mental health disorders and life-threatening infections [8,[15][16][17][18][19][20]. For example, a study of 207 trafficked women from 14 countries reported that 95% of women had experienced physical and/or sexual violence [21]. A study on trafficked Nepalese girls and women found that 23% of them tested positive for HIV [22]. Despite its negative social, health, and legal consequences, HT is a growing business for criminal organizations [21,23]. Poverty, social injustice, disasters, substance abuse, family breakdown, and homelessness are major push factors for victims [24][25][26]. Globalization has created a larger gap between developing and developed countries with poverty and marginalization being major push factors for victims [1].
The  [28]. Out of the 24.9 million people trapped in forced labor, 16 million people in the private sector such as domestic work, construction or agriculture; 4.8 million persons in forced sexual exploitation, and 4 million persons in forced labor imposed by state authorities [27]. The ILO considers the Asia-Pacific region to have the largest number of victims of HT followed by Africa and Latin America [28]. It was estimated that over 40% of trafficked victims were detected in their own countries and that 71% of the trafficked victims were women while 28% were children [8].
Bibliometric analysis is a research method used to shed light on research activity [29][30][31]. It differs from systematic reviews, which aim to answer a specific research question based on a selected group of articles [32,33]. It also differs from scoping reviews, which aim to identify the nature and extent of research evidence [34,35]. Bibliometric studies were carried out to provide a snapshot of national and international contribution to literature [36] and to advance information and science [37]. Bibliometric indicators assess national and international efforts carried out to achieve a particular goal. Several bibliometric studies on migrants and refugees have been published [38][39][40]. However, none was carried out on HT. Therefore, the aim of this study was to analyze published literature on HT. Such a study will shed light on the global research activity and discussed themes in literature on HT. Bibliometric analysis may be useful for health authorities and UN agencies interested in mapping and identifying research gaps within the HT research landscape, which is important for advancing an evidence-informed research agenda. Findings of a bibliometric analysis are difficult to obtain by other research methods such as systematic or scoping reviews. For example, a recent systematic review of research methods on HT and health concluded that despite the presence of various quantitative and qualitative data collection and analysis methods, the implemented methodological approaches have limitations that affect what is known about HT and health [41]. Most available literature on HT is based on emotional stories rather than on empirical research [42]. Bibliometric analysis would give an accurate quantitative analysis of literature on HT that would serve as a basis for future studies. The findings of a bibliometric study on HT will shed light on the evolution, volume, and scope of research on HT and will help identify countries and regions lagging behind in this field [5]. A bibliometric analysis of literature on HT will also give detailed information on the number of publication on each type of HT and therefore a direct academic and research effort as well as funding towards lessresearched types of HT.

Source of information
In the current study, SciVerse Scopus was selected to accomplish the objectives. Scopus is a bibliographic database created by Elsevier in November 2004 [43]. With 22,800 titles from over 5000 international publishers, Scopus delivers the most comprehensive view of the world's research output in the fields of science, technology, medicine, social science and arts and humanities [43]. Scopus database includes all MEDLINE journals and allows for citation analysis [44,45]. Scopus database has several operating functions that facilitate bibliometric analysis. Such operating functions include journal name, type of document, year of publication, authors and their affiliations, the number of citations, and h-index metrics for documents [46,47].

Study design
A bibliometric method was implemented. The study period was set from 2000 to 2017. The study period was determined based on the assumption that HT has attracted a lot of public attention after the introducing the UN Trafficking Protocol ratified in December 2000.

Search strategy
In bibliometric analysis, selection of keywords is of utmost importance, as these keywords have a direct impact on the findings and results. In the current study, search strategy was based on title or title/abstract search with certain constraints to minimize false-positive results ( Table 1). Keywords were obtained from previously published literature on HT [20,[48][49][50][51]. The search strategy was limited to documents published in academic journals but was not restricted to any language. The online search was performed on June 03, 2018.

Estimation of the number of health-and non-healthrelated documents
Using the function designated as "subject area"; it was possible to estimate the number of HT documents in health field, defined as documents in the following subject areas: medicine, nursing, psychology, pharmacology, neuroscience, general health, microbiology/immunology, biochemistry, and dentistry. The total number of health and non-health documents was greater than the total number of the retrieved documents because some journals are indexed in both medicine and social sciences; e.g. the Journal of Social Medicine which is categorized in both social science and medicine. Health-related documents were further divided into different domains such as mental health, health policy and systems, maternal and reproductive health, non-communicable diseases, and infectious diseases [38]. Title search TITLE("modern slavery" or "trade in human being" or "human trafficking" OR "trafficking in human" OR "lab*r trafficking" OR "forced lab*r" OR "compulsory lab*r" OR "sex* traffick*" OR "sex* trade*" OR "trafficking in persons" OR "traffick* person*" OR "traffick* wom*" OR "traffick* people" OR "traffick* girl*" OR "wom*n traffick*" OR "child* traffick*" OR "organ traffick*" OR "transplant tourism" OR "trafficking in wom*" OR "traffick* in organ*" OR "forced prostit*" OR "trafficking of child*" OR "trafficking of wom*" OR "sex* slavery" OR "commercial sexual exploitation" OR "lab*r traffick*" OR "organ trade" OR "forced marriage" OR "bonded lab*r" OR "child harvesting" OR "modern slavery" OR "child prostitut*" or "forced prostitut*" or "domestic servitude" or "debt bondage" or "debt slavery" or "bonded lab*r" or "child pornography") NONE AND NOT TITLE-ABSTRACT (insect or droso* OR sexta OR cell OR molecular OR biology OR leukocyte* OR DNA) OR Title search TITLE ("tissue removal" OR "ova removal" OR slave* OR slavery OR servitude OR "child lab*r" OR "cross border traffick*" OR "child soldier*" OR anti-trafficking OR "human abduction") AND ALL ("human trafficking" OR "trafficking in human" OR "forced lab*r" OR "forced sex*" OR "forced prostitution" OR kidnap* or abduct* OR "child sex* or exploitation") AND NOT TITLE-ABSTRACT (insect or droso* OR sexta OR cell OR molecular OR biology OR leukocyte* OR DNA) OR Title-Abstract search TITLE-ABSTRACT (traffick*) AND TITLE-ABSTRACT ("in human" OR "human being*") AND ALL ("human trafficking" OR "trafficking in human*")

Bibliometric indicators, analytics, and mapping
In the current study, bibliometric indicators were presented as ten most active countries, institutions, journals, authors, and ten most cited documents. The choice of number ten as a threshold to list the results was an arbitrary selection that has been used in previously published bibliometric studies [40,[52][53][54].

International collaboration
Research collaboration was assessed using author affiliation. For example, documents with authors having different country affiliations represent international research collaboration while documents with authors having the same country affiliation represent intra-country collaboration. Scopus allows for segregation of documents based on authoraffiliation. Therefore, for each country, the number of documents with international authors was calculated as a percentage of the total number of documents published by that country. Documents with international authors were referred to as multiple country publications (MCP) and represented the extent of international collaboration [55][56][57][58].

Bibliometric visualization maps
Co-authorship analysis, international collaboration, and keyword analysis were presented as network visualization maps using VOSviewer (Leiden University, Leiden, Netherlands) [59,60].The Statistical Package for Social Sciences (SPSS) (SPSS, Chicago, Illinois) was used to generate line presentation of the annual growth of publications. ArcGIS 10.1 software (Esri; USA) is a geographic information system used in bibliometric studies to map worldwide research contribution. The GIS map allows for a better understanding of regional contribution to a specific scientific topic.

Growth of publications and typology of documents
In total, 2044 documents were retrieved (Additional file 1  Figure 1). If this trend continues, it was estimated that 293 documents would be published in 2020.

Mapping the most frequent keywords and typology of trafficking
Most frequent keywords were visualized (Fig. 2). The map showed five clusters: (1) organ trafficking; (2)   sex trafficking, 616 (30.1%) were about labor trafficking/ forced labor, 199 (9.7%) were about child trafficking, and 138 (6.8%) were about organ trafficking. The remaining documents were general documents or ones that discussed multiple types of trafficking.

Research domains
In total, 707 (34.6%) documents were in the health field while 1526 (74.7%) were in social sciences and humanities taking into consideration certain limited overlap between the health and health research domains. Analysis of the retrieved showed that 971 (47.5%) documents were about law and criminology, 238 (11.6%) were about health policy and systems, 200 (9.8%) were about social work, and 183 (9.0%) were about mental health (Fig. 3).

Top 10 active countries
Authors from 91 different countries participated in publishing the retrieved documents. The top 10 active countries participated in publishing 1478 (72.3%) documents ( Table 2). The United States ranked first (735; 36.0%) regarding the number of published documents. Five countries in the top 10 active list were in Western Europe, two were in Northern America, one in the Western Pacific region, one in Southeast Asia, and one in Latin America. The geographic distribution of the retrieved document showed that world regions with a high prevalence of HT, such as South East Asia, East Europe, Africa, and Latin America [8], had the least research contribution (Fig. 4).

International collaboration
Analysis of international research collaboration for the top 10 active countries showed that India had the highest percentage of documents (49%) with international research collaboration. In contrast, Brazil had the least percentage of documents (12%) with international research collaboration. For the US, which led in the number of publications, only 12.5% of documents with US authors included authors from other countries. Of the 1478 publications produced by the top 10 active countries, there were only 238 (16.1%) publications with international collaboration.

Top 10 active institutions
Harvard University (USA) (n = 39; 1.9%) was the most active institution in this field followed by University of British Columbia (Canada) and London School of Hygiene & Tropical Medicine (UK) ( Table 3). The top 10 active list of institutions included four in the USA, three in Canada, and three in the UK.

Research networks
Mapping research networks for authors with a minimum research output of five documents showed only two research clusters (Fig. 5). The first cluster (red) included six authors with Zimmerman, C. as a member in this cluster. The second cluster (green) included five authors with Silverman, J as a member of this cluster.

Preferred journals for publishing documents about HT
The retrieved documents were published in 998 different journals. The International Migration was the most active journal in this field. Top 10 active journals belonged to different fields including migration, criminology, medicine, social studies, gynecology, transplantation, violence, and human rights ( Table 5). The majority of active journals were based in the USA. Documents published in Transplantation (25.5) journal received the highest number of citations per article followed by those published in International Migration (22.5).

Citation analysis
The retrieved documents received 15,505 citations giving a mean of 7.6 citations per document. The h-index of the retrieved documents was 48. The document that received the highest number of citations was published in 2007 in Politics and Society. Top ten cited documents discussed topics about organ trafficking, mental health, and HIV infections of HT victims. Table 6 showed the list of top 10 cited documents [22,[61][62][63][64][65][66][67][68][69][70].

Volume and growth of publications
The current study aimed to assess and analyze published literature on HT. Quantitative analysis of literature on HT is complicated by the lack of academic and legal consensus on the definition of HT as well as the unclear distinction between trafficked victims, exploited people, and vulnerable migrants [71]. For example, some researchers consider illegal migrants who end up in prostitution as victims of HT [72,73]. Despite these technical difficulties, the current study was the first to analyze volume, growth, research trends, and research domains of literature on HT published in academic journals.
The findings of the current study showed that the total number of retrieved documents on HT was low when compared to the size of literature on the 21 million people described as refugees [74]. The number of publications on HT could be attributed to the methodological, operational, criminal, and hidden nature of HT [20]. The limited research collaboration, as evident from the mean number of authors per document and the percentage of documents with international authors, is another potential reason for the limited number of publications on HT. The finding regarding international research collaboration is difficult to explain but could be attributed to lack of communications, motivation, funding, or lack of international conferences that could help gather experts in this field [75,76].

Health versus non-health related research
The current study showed that there was an underrepresentation of health-related research on HT despite the potential threat of HT to national health security [1]. Health is a subject that has been neglected in anti-trafficking work efforts compared to activities in the fields of immigration and law enforcement [77,78]. Many experts in health and HT agree that the health sector has had limited engagement in trafficking dialogues and research [78] despite that HT is being considered as a threat to global health security [79]. The presence of sexually transmitted diseases and tuberculosis among trafficked victims place the general population at risk [80,81].  The findings of the current study regarding the under-representation of labor trafficking relative to sex trafficking were in agreement several previously published studies [18,82]. Promotion and advancement of research on health aspects of HT require intensive involvement of public health experts in the global debate about HT. Public health experts and editors of medical and health journals need to create an international forum to encourage researchers from different parts of the world to get involved in health research about HT. This will benefit global public health agendas by shedding light on aspects related to HT such as labor exploitation and smuggling in Africa, Asia, and the Middle East where labor exploitation might not be considered illegal or a human right violation [83]. Establishing research networks that include scholars from destination and source countries will create a more detailed analysis of health aspects of HT.

The source versus destination countries
The current study showed that the bulk of the retrieved literature and the topics discussed represent the perspective of the destination rather than source countries. It is believed that the source countries are ones with no or minimum level of democracy, high levels of corruption, and poor economic growth, which make engagement in research and academic investigation an unaffordable luxury. The source countries might not have enough public health researchers or experts HT, which influence their    Documents with equal number of citations were given the same rank, and then a gap is left in the ranking numbers contribution to literature in this field. It is recommended that the source countries place migration and trafficking in their national and foreign policy agendas. Healthcare professionals, social care specialists, and experts in global health diplomacy in source countries need to get engaged in developing evidence-based information about methods used by traffickers in recruiting their victims from source countries [84]. Legal and labor experts need to provide detailed information for migrants to increase their awareness of labor exploitation and trafficking [85]. Awareness of being a potential victim to traffickers is important in source countries, particularly those with a humanitarian crisis or armed conflict where young desperate men and women will take a quick decision regarding migration in hope of a better life and better wages [86]. For destination countries, healthcare professionals, and public health experts need to identify working conditions and occupational health hazards of migrant workers who might end up being victims of HT. Policymakers need to develop strict regulation to provide migrants with legal and health protection equivalent to that of domestic workers [87]. Health policymakers need to develop mechanisms to discover victims of HT once they attend a healthcare facility [88] and offer them appropriate health services and social as well as financial support and compensation. Experts in criminology need to develop detailed criminal codes based on international diplomatic cooperation to fight organizations involved in HT [89,90].

Types of trafficking
In the current study, published literature on sex trafficking dominated the field of HT. The over-representation of sex trafficking suggests that research related to other forms of trafficking, particularly labor trafficking is being underresearched. Trafficked laborers live and work in harsh and mostly unhealthy conditions that might involve confiscation of passports, low wages, sex slavery, and deprivation of basic health need [91][92][93][94]. Research domain in labor trafficking needs to be strengthened, supported, funded, and encouraged to generate more evidence-based data in this field.

Study limitations
The current study has a few limitations. Using Scopus database to retrieve literature on HT created a certain bias toward countries with a large number of journals indexed in Scopus. Scopus is biased toward academic journals in which documents are published in English. Therefore, documents in HT published in non-English were not retrieved. The current study, like all other bibliometric studies, did not include grey literature. The search strategy used in this study might have led to some false-negative or false-positive results. Therefore, interpretation of the results should take into consideration these limitations.

Conclusion
The current study is the first to assess research activity in the field of HT. The current study showed that health aspects of HT were under-represented compared to criminal, legal, and social aspects of HT. Similarly, research on labor trafficking was under-represented compared to sex trafficking and exploitation. The literature on HT represents the agendas of destination countries with limited contribution of countries in Asia, Latin America, Africa, the Middle East, and Eastern European. Future research plans should shed light on trafficking for nonsexual purposes. Furthermore, research networks need to be strengthened by including scholar from source and destination countries to investigate health conditions of trafficking survivors or exploited migrants. Greater investments in international research collaborations and research networks should be encouraged to help prioritize research on HT in various world regions.

Additional file
Additional