A Systematic Review and Methodological Critique of Human Trafficking Prevalence Studies

BACKGROUND AND PURPOSE: Although human trafficking has received a great deal of attention over the past decade, little is known about the prevalence of the problem both in the U.S. and globally. To date, the U.S. federal government has allocated over 500 million dollars to combat human trafficking domestically and internationally; however, only 2,515 human trafficking incidents have been investigated by law enforcement from 2008 to 2010. This systematic review sought to answer three questions: 1) What studies on human trafficking prevalence have been conducted to date 2) What methodologies have been employed to estimate the problem and which yield the most successful findings 3) What can be learned from the strengths and limitations of these studies to inform future research on human trafficking? Studies will be compared to critically evaluate their similarities and differences, research methodology and design, findings, strengths, and limitations. 

METHODS:  A systematic review was conducted to identify, analyze, and critique existing prevalence studies on human trafficking. Littell, Corcoran, and Pillai’s (2008) framework to systematic reviews and meta-analysis was used to guide the current study. Studies published between the years of 2000 and 2014 were selected for review. Studies published before 2000 would not have been able to include data on cases recognized under the Trafficking Victims Protection Act of 2000. Databases used to locate peer-reviewed articles included EBSCOhost, Google, Google Scholar, PsycINFO, Sociological Abstracts, Social Sciences Citation Index, and Social Services Abstracts. Combinations of the following keywords were used: “human trafficking,” “labor trafficking,” “sex trafficking,” “estimate*” “prevalence” “prevalence data” and “measurement.” 

RESULTS: A total of 7 human trafficking prevalence studies were identified in the search. One study appeared in a peer-reviewed journal and the remaining 6 studies were published in government and non-governmental reports. Methodologies employed included respondent-driven sampling, capture-recapture methods, economic modeling techniques, and estimates derived from open data sources. Overall, the estimates are vastly disproportionate from one another, consist of single and range estimates, and measure different types of human trafficking within specific populations. Studies produced the following estimates: labor trafficking victims globally; child sex trafficking victims in the U.S.; child sex trafficking victims in the U.S., Mexico, and Canada; total human trafficking victims in the U.S.; adult and child sex and labor trafficking victims in the U.S.; total human trafficking victims in 37 European countries; and estimates of the dollar amounts produced by the unlawful commercial sex economy in eight major U.S. cities. 

CONCLUSIONS AND IMPLICATIONS: There have been several attempts to measure the extent of human trafficking but most studies fall short in producing rigorous and accurate estimates of the problem. Flawed prevalence data hinders anti-trafficking efforts and can lead to inappropriate allocations of funding for victim services, trafficking prosecutions, service delivery evaluation, training and education, and research. Indeed, human trafficking is a hidden and clandestine activity that is extremely difficult to measure. Still, more reliable estimates are necessary in order to equip local and federal governments, accurately inform practice and policy efforts, and provide direction for future studies on human trafficking.