Introduction

Human trafficking, the recruitment and transportation of persons by use of force, coercion, or deception for exploitation (United Nations, 2000), is one of the largest illicit industries worldwide. However, most victims remain unnoticed and merely a handful of cases result in a conviction (US Department of State, 2020). To address this challenge, international efforts have led to the creation of laws and guidelines, such as the Palermo Protocol (United Nations, 2000) adopted by the United Nations and signed and ratified by (most) countries in North America and Europe. In addition to the Palermo Protocol, both North America and Europe need to comply with obligations stemming from regional legal documents. For example, most European countries signed and ratified the Convention on Action Against Trafficking in Human Beings by the Council of Europe (2005), and most European countries also adhered to the Eu-Anti-Trafficking Directive (European Parliament, 2011). In addition, Europe established the Common European Asylum System (CEAS) in 1999 to manage asylum applications collectively. In North America, additional legislation resulted in the Trafficking Victims Protection Act (TVPA) in 2000 and the comprehensive Criminal Code (2010) penalizing human trafficking in Canada. In addition, the United States and Canada, sharing the longest undefended border globally, collaboratively implemented the Canada-US Safe Third Country Agreement in 2004 to manage their refugee systems. Citizens from these countries can travel visa-free for stays under six months. A downside to this freedom of movement is the perceived increased risk of human trafficking as open borders negate the need for regulations on individuals entering the country and thus controlling for human trafficking (Bell & Frank, 2022). These geographical differences in legislation are noteworthy since it provides North American and European countries with different frameworks to tackle human trafficking.

The implementation of anti-trafficking policies is entrusted by nation-states to local criminal justice agencies, emphasizing the critical role these agencies play in investigating and prosecuting human trafficking offenders and ensuring legal protection for victims (Farrell et al., 2014; Leser et al., 2017). In many countries criminal justice agencies hold the exclusive authority to determine the legal status of individuals as trafficking victims, influencing who is entitled to legal protection (Musto, 2013). However, challenges highlighted by Farrell et al. (2014) underscore the difficulty in correctly identifying trafficking victims due to the clandestine nature of the crime, leading to potential misclassifications with serious consequences such as criminal prosecution, deportation, or denial of protection (Villacampa & Torres, 2017).

Even when human trafficking is correctly identified, legal protection for victims is often limited to the duration of criminal investigations, ceasing once the investigation concludes (Dettmeijer-Vermeulen et al., 2017). Consequently, decisions made by criminal justice actors become essential not only for the identification and prosecution of traffickers but also for the ongoing protection of victims (Van der Leun & Van Schijndel, 2017). The challenges faced by criminal justice actors in recognizing and safeguarding trafficking victims emphasize the need for comprehensive and sustained efforts in anti-trafficking policies.

The current systematic review aims to contribute to a better understanding of the factors that impede the identification, investigation, and prosecution of human trafficking by criminal justice actors and will make a distinction between the challenges experienced by criminal justice actors in North America or Europe in the identification of human trafficking on the one hand (pre-identification), and those that influence the decision-making process during the investigation and prosecution phase (post-identification) on the other hand.

Responding to human trafficking: Street level bureaucracy theory

Police officers and prosecutors who deal with human trafficking cases exercise wide discretionary powers when deciding whether a person meets the criteria to be processed through the criminal justice system as a victim or as a perpetrator (Leser et al., 2017). Discretionary decision-making, the freedom to exercise one’s job (Evans, 2010), is inextricably linked with Lipsky’s (1980) theory on street-level bureaucracy. Street-level bureaucrats interact directly with citizens and thereby represent the frontline of government policy. According to Lipsky (1980), street-level bureaucrats are required to follow certain rules and procedures and treat similar cases in the same manner as much as possible but are often forced to depart from these rules and procedures. Time constraints, for instance, may force them to reach an efficient but imbalanced decision. Criminal justice actors handling human trafficking cases can also be considered street-level bureaucrats (Bjelland, 2017; Leser et al., 2017). They come directly into contact with possible victims and offenders and must make instant decisions about whom to protect and detain. Several extra-legal factors may force these officials to deviate from prescribed rules and procedures. According to Farrell et al. (2014) and Farrell et al. (2014), these factors can be subdivided into four general categories. The first category entails definitional factors that stem from uncertainty about the legal definition of human trafficking. This uncertainty leads to difficulties in interpreting certain components of this definition such as the concept of ‘coercion’. If prosecutors are uncertain about the legal definition of human trafficking, they will be less likely to process a case as human trafficking. The second category encompasses institutional factors that determine the readiness of criminal justice agencies to engage in certain types of crime, such as having sufficient resources, such as collaborations with other agencies and qualified expertise. The third category, attitudinal factors, is related to criminal justice actors’ beliefs and perceptions about human trafficking and its victims regarding the severity of human trafficking and certain case characteristics. Research by Farrell et al. (2014) adds environmental factors as a fourth category. This category refers to challenges outside criminal justice agencies, such as geographical or social and political influence, such as prioritizing cases because of pressure from the media. However, which factors dominate the decision-making processes of criminal justice actors in tackling human trafficking and whether these factors can be divided into definitional, institutional, attitudinal, and environmental factors is unknown, for a systematic review that summarizes and synthesizes previous research on this topic has never been conducted. Categorizing and understanding which types of factors are the most important in impeding a human trafficking response by criminal justice actors will also give policymakers more insight into which factors need to be addressed.

The current study

Given the aforementioned, the current study aims to systematically review previous research on extra-legal factors that influence the identification, investigation, and prosecution of human trafficking. We will use Farrell’s categorization to synthesize findings and provide suggestions for policy, practice, and future research.

Methodology

This review investigated studies that explored the decision-making processes of criminal justice actors dealing with human trafficking. The inclusion criteria encompassed 1) studies with empirical results on factors influencing criminal justice responses (interviews, experimental data, or analyses of statistics; literature reviews/studies were excluded), 2) focusing on participants tasked with criminal justice 3) in Western countries that signed the Palermo Protocol. The review considered research 4) published in academic journals or doctoral dissertations 5) after the implementation of the Palermo Protocol (United Nations, 2000), 6) and conducted in English.

The authors acknowledged adherence to the general principles of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) standard in this review. The followed protocol can be requested from the first author.

The search for relevant studies was conducted in Web of Science (all databases), Ebscohost (Academic Search Premier, Criminal Justice Abstracts, Ebook academic collection, Ebook collection, ERIC, MEDLINE, PsycARTICLES, PsycINFO, Psychological and Behavioural Sciences Collection) and ProQuest (Ebook Central, ERIC, Periodical Archive Online, Periodical Index Online, PILOTS, ProQuest Dissertations and Thesis Global, Sociological Abstracts, Worldwide Political Science Abstracts). A combination of keywords, clustered into three blocks containing synonyms of terms closely related to 1) human trafficking 2) decision-making and 3) criminal justice, was used to find relevant articles. The three blocks were combined using Boolean operators “AND” and “OR”. The complete search process is shown in Table 1.

Table 1 Search strategy
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Results

The first search pertained to the period from 8th of May 2018 until the 1th of June 2021 and because of the edits after review a second search took place which lasted from the 5th until the 15th of January 2024. A total of 6.473 results were generated during these searches. The initial screening involved collaboration between the first author and a research assistant to assess the relevance of titles and abstracts against inclusion criteria. Both researchers independently screened articles and resolved disagreement trough discussion. Full-text analysis of the 95 potentially relevant studies led to the exclusion of two due to not being set in North America or Europe, seven due to a lack of empirical data, twenty for not focusing on criminal justice actors, and thirty-one for not including variables directly affecting criminal justice responses to human trafficking (Fig. 1). The most important information of the final set of 35 studies such as study design, sample characteristics, and legal frameworks were placed in an extraction form.

Fig. 1
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Flow chart

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Inspired by the work of Farrell et al. (2014) and Farrell et al. (2014) and building on the theoretical framework of Lipsky (1980), the following section will discuss the factors that influence the response to human trafficking by criminal justice actors. The discussion categorizes these factors into five overarching groups: 1) definitional, 2) institutional, 3) attitudinal, 4) environmental and 5) victim. This last category emerged out of the results and will be discussed in depth down below. Given the predominant focus on North America, distinctions are made between North America and Europe. Thereafter distinctions were also made in the pre-identification phase (identification of human trafficking) and post-identification phases (investigation and prosecution), to give a more specified overview of the challenges experienced by criminal justice actors.

Characteristics of the included studies

The evaluation process identified 35 studies meeting the inclusion criteria (Fig. 1.), with over half (20) conducted in North America (19 in the United States, 1 in Canada) and 15 in various European countries. However, in both regions only a limited number of countries have been researched (see Table 2a/b). Most studies referenced relevant human trafficking statutes, except for Barrick et al. (2014), Leser et al. (2017), Verhoeven and van Gestel (2011) and Wells et al. (2012). Fifteen studies focused solely on sexual exploitation, of which eight specifically on the sexual exploitation of female domestic minors, four on labour exploitation, two on criminal exploitation and one on organ trafficking. Thirteen studies considered multiple forms of trafficking, however primarily covering sexual exploitation.

Table 2 a. Characteristics of included studies set in North America. b. Characteristics of included studies set in Europe
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In terms of study design, fourteen relied on interviews, twelve used mixed methods, five exclusively analysed criminal justice cases, and four employed surveys. Further details on sample characteristics, methodology, types of human trafficking, and factors influencing criminal justice responses are provided in Tables 2a/b and 3a/b (in appendix A)Footnote 1.

Factors associated with human trafficking case processing

Category 1: Definitional factors

Although human trafficking is not a new phenomenon, anti-trafficking laws are relatively new, complex, and largely untested in court. Criminal justice actors implementing and exercising these laws are therefore faced with several challenges when wanting to respond to human trafficking. Research stated that definitional challenges, such as not knowing what certain elements of the legal definition precisely entail hinder the response to human trafficking.

North American studies

Thirteen North America studies investigated whether and how definitional factors affect the criminal justice response to human trafficking, identifying three problems because of ambiguities in interpreting definitions. First, five studies found that this led to difficulties in distinguishing human trafficking from other crimes, such as labour violations and prostitution (Barrick et al., 2014; Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Nichols & Heil, 2015). Second, five studies observed challenges in defining concepts of anti-trafficking laws, such as ‘coercion’ and when labour violations rise to labour exploitation (Barrick et al., 2014; Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell & Pfeffer, 2014; Kaye & Hastie, 2015). Lastly, nine studies found that this led to challenges in obtaining evidence and not knowing what evidence is needed to obtain (Fahy, 2015; Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell et al., 2014; Nichols & Heil, 2015; Wells et al., 2012; Kaye & Hastie, 2015; Potts, 2018; de Vries & Farrell, 2022).

Seven studies researched three factors on how definitional problems hinder investigation and prosecution in the post-identification phase. First, six studies stated these problems led to cases being tried for other, lesser, or familiar offences such as prostitution or labour violations for an easy win and the fear of losing a case (de Vries & Farrell, 2022; Farrell et al., 2014; Nichols & Heil, 2015; Nelms, 2020; Kaye & Hastie, 2015; Farrell, Bright, et al., 2019). Additionally, one study found that cases not meeting a high legal threshold weren’t prosecuted, resulting in only the most extreme cases of human trafficking being prosecuted (Farrell, Bright, et al., 2019). Lastly, two studies found that definitional factors also lead to unclarity in what evidence is needed for prosecution (Farrell et al., 2014; Farrell, Bright, et al., 2019). Contrary to these studies five studies found that clarity exists and that corroborating evidence, such as having multiple victims or witnesses, digital and physical evidence does foster prosecution (Fahy, 2015; Farrell et al., 2014; Farrell, Bright, et al., 2019; Nelms, 2020; Wells et al., 2012). However, one study nuanced this finding by stating witnesses aren’t likely to make a statement since they have an interest in keeping the crime hidden (Farrell, Bright, et al., 2019).

European studies

Ten European studies investigated whether and how definitional factors affect the criminal justice response to human trafficking. Five studies identified two challenges during the identification-phase. First, four studies found that this led to difficulties when defining (elements of) human trafficking laws such as 'coercion,' or 'forced labour,' (Ambagtsheer, 2021; Cockbain & Brayley-Morris, 2018; Leser et al., 2017; Matos et al., 2018). Additionally, one study stated that these definitional challenges led to legislation being underused and sometimes abused when clauses in victim protection are being misused by offenders (Olver & Cockbain, 2021). Unlike North American studies, European studies did not report difficulty in distinguishing human trafficking from other crimes, with one study stating that human trafficking was easily identified (Ambagtsheer, 2021).

Nine European studies researched two factors on how definitional problems hinder investigation and prosecution in the post-identification phase. First, two studies found that definitional challenges resulted in prosecutions for other (familiar) offences when prosecutors were uncertain of the outcome or when human trafficking cases were deprioritized and refined (Matos et al., 2018; Villacampa & Torres, 2017). Second, eight studies found that definitional problems led to difficulty obtaining and knowing what evidence is needed to obtain (Bjelland, 2017; Cockbain & Brayley-Morris, 2018; Leser et al., 2017; Matos et al., 2018; Matos et al., 2019; Pajón & Welsh, 2020; Vidra et al., 2018; Villacampa & Torres, 2017). However, two studies found two opposing factors; enough evidence and knowledge of a potential suspect’s identity improved the likelihood of investigation and prosecution (Bjelland, 2017; Matos et al., 2019). Additionally, two studies stated that the absence of a victim’s testimony, or it being the sole evidence, decreases this likelihood (Matos et al., 2019; Olver & Cockbain, 2021).

Summary

Overall, definitional factors did not appear to vary between North American and European studies with one notable exception where European studies didn’t report difficulties in distinguishing human trafficking from other crimes in the pre-identification phase.

Category 2: Institutional factors

Challenges that criminal justice actors experience often stem form how these agencies are equipped in terms of intelligence and personnel for tackling human trafficking; especially having sufficient resources and qualified expertise.

Sufficient resources

North American studies

Thirteen North American studies investigated whether and how sufficient resources affect the criminal justice response to human trafficking. Twelve studies found three factors which interacted with the identification of human trafficking. First, eleven studies found that the lack of resources and capacity could lead to the reluctance to proactively investigate cases, adoption of reactive strategies, and thereby relying on reports by victims or third parties (Fahy, 2015; Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell, Bouché, & Wolfe, 2019; Farrell, 2014; Farrell et al., 2014; Farrell & Pfeffer, 2014; Wilson & Dalton, 2008; Barrick et al., 2014; de Vries & Farrell, 2022; Mitchell et al., 2010). Second, four studies stated that the lack of collaboration between criminal justice agencies and other institutions hampered the identification of trafficking since fewer cases were being detected or referred to the police and less information was being disclosed (Barrick et al., 2014; Farrell, Bright, et al., 2019; Farrell & Pfeffer, 2014; Potts, 2018). Third, six studies found that the lack of collaboration with care providers hampered the identification of trafficking since these organisations often identify victims and can stop victims from disappearing and help them overcome challenges (Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Potts, 2018; Wilson & Dalton, 2008).

Nine studies investigated five factors hindering the response to human trafficking in the post-identification phase. First, six studies stated that cooperation with care providers positively influenced a victim's ability and willingness to cooperate with criminal justice actors and prosecute cases (Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Potts, 2018; Wilson & Dalton, 2008). Contradicting, two studies found that this cooperation negatively influenced prosecution since these victims are sometimes coached by care providers which undermines their credibility in court and efforts to obtain information about the trafficking situation (Farrell, Dank, et al., 2019; Wilson & Dalton, 2008). Third, three studies researched the willingness to commit resources (Fahy, 2015; Farrell et al., 2014; Farrell, Bouché, & Wolfe, 2019; Nelms, 2020). Since these cases are time-consuming and they lack resources, agencies are not always willing to commit resources impeding investigation and prosecution. The third and fourth factors hindering investigation and prosecution were found by two studies and stated difficulties in jurisdictions (Nelms, 2020) and a lack of resources to collaborate with police officials (Farrell et al., 2014). Additionally, the fifth, and sixth factors found by two other studies stated that having sufficient resources (Farrell, Bouché, & Wolfe, 2019) and improved collaboration between local and federal criminal justice agencies lead to the contrary (Wilson & Dalton, 2008).

European studies

Twelve European studies investigated whether and how sufficient resources affect the criminal justice response to human trafficking. Seven studies found interacting factors regarding sufficient resources and the identification of human trafficking. First, five studies stated that a lack of resources and capacity could lead to reactive strategies and the lack of identification of human trafficking (Leser et al., 2017; Matos et al., 2018; Pajón & Welsh, 2020; Vidra et al., 2018; Villacampa & Torres, 2017). Second, two studies stated that cooperating with other (non-) criminal justice agencies, especially community police, and Ngo’s positively influenced the identification of human trafficking although this wasn’t frequently done (Olver & Cockbain, 2021; Verhoeven & Van Gestel, 2011).

In the post-identification phase, eight studies researched four factors as challenges or advantages of having sufficient resources for investigating and prosecuting human trafficking. First, six studies found that willingness to commit resources isn’t a given because of the magnitude of the phenomenon and therefore the immense costs thus impacting prosecution (Bjelland, 2017; Cockbain & Brayley-Morris, 2018; Matos et al., 2018; Matos et al., 2019; Meneses-Falcón et al., 2022; Vidra et al., 2018). Second, two studies stated that officials prioritize other cases when tasked with multiple duties leading to fewer prosecutions (Lynch & Hadjimatheou, 2017; Meneses-Falcón et al., 20220., and relatedly, two studies found that pressure from higher authorities to complete quotas influences the decision to prosecute negatively and put the effort into other crimes less time-consuming (Meneses-Falcón et al., 2022; Vidra et al., 2018). Fourth, three studies found that collaboration with non-criminal justice partners enhanced victim cooperation in investigations and thus positively influenced the post-identification phase (Matos et al., 2018; Matos et al., 2019; Pajón & Welsh, 2020). However, three studies stated that this doesn’t apply to foreign authorities since delays, lack of responses, translation issues, confidentiality barriers, and coordination difficulties hinder efficient investigations and prosecutions (Ambagtsheer, 2021; Matos et al., 2018; Matos et al., 2019). Lastly, one study found resources in local courts hinder investigations of complex cases (Ambagtsheer, 2021).

Qualified expertise

North American studies

Sixteen North American studies investigated whether and how qualified expertise affects the criminal justice response to human trafficking. Fourteen studies researched four factors that were found to interfere in the identification phase. First, three studies found that the lack of tools and guidelines, such as a checklist, hampered the identification of human trafficking since this resulted in the adaptation of existing models, particularly the model for enforcing prostitution, leaving labour exploitation cases less proactively identified (Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell & Pfeffer, 2014). Second, eight studies researched the effect of specialized or vice units and six studies found that these significantly contribute to identifying human trafficking, particularly when they have adopted programmatic innovation in other areas (Fahy, 2015; Farrell, 2014; Farrell et al., 2010; Farrell et al., 2014; Farrell & Pfeffer, 2014; Wilson & Dalton, 2008). However, one of these studies also found two contradicting factors stating that having a vice unit may decrease the likelihood of an agency adopting a human trafficking response and that lacking specialized personnel often reflects broader institutional and community prioritization issues (Farrell et al., 2014). Relatedly, four studies found that vice units only increased identification in sex trafficking, because of their history of prosecuting sex-related crimes leaving other forms unidentified (Barrick et al., 2014; Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell & Pfeffer, 2014). Additionally, one study found that prioritization didn’t result in the identification of human trafficking (Farrell et al., 2010).

Third, ten studies researched the effect of knowledge and training on the identification of human trafficking. All studies stated that a lack of knowledge and training, for instance in language skills and cultural knowledge, especially concerning immigrant groups, hinders the identification of human trafficking cases (Belin, 2015; Fahy, 2015; Farrell et al., 2010; Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Jones, 2010; Wilson & Dalton, 2008). Two studies stated that this is more prevalent in recognizing labour exploitation than sexual exploitation (Farrell, Bright, et al., 2019; de Vries & Farrell, 2022). This was further illustrated by four studies which found that not all agencies provide training, leaving officers, especially those likely to encounter victims, untrained and resulting in unidentified victims (Farrell, Bright, et al., 2019; Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Jones, 2010).

The last factor in interacting with qualified experts is having protocols and policies. Seven studies stated that agencies having policies and protocols in place to combat human trafficking are more likely to identify human trafficking (Belin, 2015; Fahy, 2015; Farrell, 2014; Farrell et al., 2010; Farrell, Bright, et al., 2019; Halter, 2010; Wilson & Dalton, 2008). Two studies illustrated this point further by stating that less than ten percent of agencies in the US have implemented these protocols, especially on labour trafficking (Farrell, 2014; Farrell et al., 2010; Farrell, Bright, et al., 2019).

In the post-identification phase, ten studies researched two factors as challenges in having qualified expertise for investigating and prosecuting human trafficking. First, nine studies stated the first factor was missing tools to prosecute human trafficking, such as enforcing antitrafficking states (i.e. jury line instructions or motions), required evidence or how to perform victim-centred interviews, resulting in officials arresting victims to keep them safe (Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Farrell, Bright, et al., 2019; Wilson & Dalton, 2008; Belin, 2015; Fahy, 2015; Musto, 2013; de Vries & Farrell, 2022; Farrell et al., 2014). One study stated that agents able to conduct successful interviews obtained useful information for building cases (de Vries and Farrell, 2022). Furthermore, one study found that a lack of training on prosecuting in general prohibited prosecution (Nelms, 2020).

European studies

Eight European studies investigated whether and how qualified expertise affects the criminal justice response to human trafficking. Three factors, similar to North American studies, were found to interact with having qualified expertise in seven studies. First, four studies found that a lack of knowledge and training impeded the identification of human trafficking (Ambagtsheer, 2021; Matos et al., 2018; Pajón & Welsh, 2020; Villacampa & Torres, 2017). Additionally, one study found that having prior knowledge of human trafficking encouraged the recognition and prosecution of specific forms of trafficking (Ambagtsheer, 2021) although this was contradicted by another study stating that sexual exploitation is still the most thought of form of human trafficking (Villacampa & Torres, 2017). Second, questioning was found to impede the identification of human trafficking where language barriers and type of questions lead to evasive responses and limited information (Lindholm et al., 2015; Pajón & Welsh, 2020). Lastly, two studies found protocols and policies enhanced the identification of victims and the absence of these led to treating victims as offenders (Lynch & Hadjimatheou, 2017; Vidra et al., 2018).

Three studies researched one factor interacting with having qualified expertise in the post-identification phase, namely lack of training and knowledge. Prosecution was prevented when lack of training and knowledge led to insufficient evidence for prosecution and a high workload for those with training since agents solely bring cases to these courts (Matos et al., 2018; Meneses-Falcón et al., 2022; Pajón & Welsh, 2020).

Summary

Overall, North American, and European studies did not appear to vary greatly in which factors interacted with institutional factors. Most importantly, the lack of resources and the lack of knowledge and training hindered the combatting of human trafficking in both areas all through the judicial chain. Two notable exceptions are the collaboration with foreign authorities which is not seen as a positive influence in Europe and that specialized, and vice units were solely mentioned in North America.

Category 3: Attitudinal factors

Even with clear legal definitions and adequate resources, the perception of the prevalence, severity, and specific characteristics of human trafficking can impede its identification, investigation, and prosecution. For instance, how human trafficking is perceived is found to impede the identification of other forms of trafficking than sexual exploitation and how victims are viewed. Two challenges related to attitudinal factors were detected, namely 1) perceptions about the prevalence and severity of human trafficking and 2) perceptions about case characteristics.

Perceptions about the prevalence and severity of human trafficking

North American studies

A total of eight studies in North America investigated whether and how perceptions about the prevalence and severity of human trafficking affect the criminal justice response to human trafficking. Research six studies found three factors interacting with perceptions of the prevalence and severity impeding the identification of human trafficking. First, four studies researched the type of trafficking influencing the perception of human trafficking as serious and thus increasing the identification (Barrick et al., 2014; Farrell, Bright, et al., 2019; Farrell & Pfeffer, 2014; Kaye & Hastie, 2015). These studies stated that sex trafficking is often prioritized since other forms of trafficking are seen as less serious since these are seen as less harmful, involving less vulnerable and forced people. Second, one study found that criminal justice attitudes influenced perceiving human trafficking as serious increased identification, with larger communities perceiving human trafficking as more serious (Farrell et al., 2010). However, two studies noted that this perception of seriousness may not be universally evident (Barrick et al., 2014; Fahy, 2015). Third, one study found that cases of minors in prostitution were also less prioritized by criminal justice actors since these cases were often considered less important than other crimes like drugs and homicide since they were perceived as affecting only the individual in question and not the lives of other people such as agents impeding identification (Fahy, 2015).

Two studies researched one factor regarding perceptions about the prevalence and severity of human trafficking impeding the investigation and prosecution of human trafficking in the post-identification phase. This is because prosecutors fear that potential jury members might not understand the phenomenon of human trafficking with misconceptions leading to victims being seen as prostitutes or solely as drug users impeding prosecution (Farrell et al., 2014; Farrell, Bright, et al., 2019; Nelms, 2020).

European studies

One study in Europe investigated whether and how perceptions about the prevalence and severity of human trafficking affect the criminal justice response to human trafficking. One factor was identified to interact with the identification and investigation of human trafficking, namely the prioritization of sex trafficking cases over other forms based on perceived severity impeding investigation of other forms of human trafficking (Villacampa & Torres, 2017).

Perceptions about case characteristics

North American studies

A total of eleven studies in North America investigated whether and how perceptions about case characteristics affect the criminal justice response to human trafficking. Research by eleven studies found three factors interacting with perceptions of case characteristics impeding the identification of human trafficking. First, eight studies stated that negative attitudes towards victims such as blaming victims for not leaving the trafficking situation, victims involved in crime or prostitution or victims with disabilities or who run away were not seen as credible by criminal justice actors and thus not identified as such (Farrell, Dank, et al., 2019; Wilson & Dalton, 2008; Farrell et al., 2014; Nichols & Heil, 2015; de Vries & Farrell, 2022; Farrell & Pfeffer, 2014; Fahy, 2015; Musto, 2013). Second, six studies relatedly found that victims aligning with perceptions of criminal justice actors, such as being underage, female, domestic minors, frightened, cooperating, born in the US, increased identification, and case processing (Fahy, 2015; Farrell & Pfeffer, 2014; Halter, 2010; Mitchell et al., 2010; Musto, 2013; Wells et al., 2012). Third, three studies researched the way of reporting affecting police officials’ perceptions of victims stating that offenders are seen more often as victims if they learn about the crime through police initiation, third-party reports, or arrested exploiters (Halter, 2010; Mitchell et al., 2010; Wells et al., 2012).

Eleven studies researched three factors regarding perceptions of case characteristics impeding the investigation and prosecution of human trafficking in the post-identification phase. First, seven studies stated that negative attitudes towards victims by judges and juries, for instance, because of credibility issues regarding the changes in testimony, involvement in crime, and use of substances (e.g. drugs) lead to the hesitancy to prosecute cases, the denial of victimhood and treated as criminals (Farrell & Pfeffer, 2014; Farrell et al., 2014; Nichols & Heil, 2015; Farrell, Dank, et al., 2019; Musto, 2013; de Vries & Farrell, 2022; Wilson & Dalton, 2008). Second, seven studies relatedly found that victims aligning with perceptions of criminal justice actors, for instance, forced into prostitution or perceived as sympathetic were more likely to be seen as credible by the jury and cases more likely to be processed (Fahy, 2015; Farrell et al., 2014; Farrell & Pfeffer, 2014; Halter, 2010; Mitchell et al., 2010; Musto, 2013; Wells et al., 2012). Lastly, two studies found that a lack of understanding of human trafficking especially non-sexual cases, and the specific legal definitions involved hampered prosecution of cases without strong corroborating evidence (Farrell et al., 2014; Farrell, Dank, et al., 2019).

European studies

A total of five studies in Europe investigated whether and how perceptions about case characteristics affect the criminal justice response to human trafficking. Research by five studies found two factors interacting with perceptions of case characteristics impeding the identification of human trafficking. First, five studies stated that negative attitudes towards victims such as blaming victims for not leaving the trafficking situation, victims belonging to a marginalized group or showing inappropriate emotions hinder the identification of human trafficking (Leser et al., 2017; Matos et al., 2018; Pajón & Welsh, 2020; Vidra et al., 2018; Villacampa & Torres, 2017). Second, relatedly, two studies found that victims aligning with perceptions of criminal justice actors such as being female, undocumented, and showing non-verbal signs of victimhood increased their credibility and in turn their identification of victimhood (Matos et al., 2018; Villacampa & Torres, 2017).

Six studies researched three factors regarding perceptions of case characteristics impeding the investigation and prosecution of human trafficking in the post-identification phase. First, six studies stated that negative attitudes towards victims by judges and juries, for instance, because of victims changing testimonies over time due to initial fear of criminal justice actors, or not mastering the language, impacts their credibility leading to the denial of victimhood or the treatment of victims as offenders for other crimes instead of prosecution for human trafficking (Cockbain & Brayley-Morris, 2018; Leser et al., 2017; Matos et al., 2018; Olver & Cockbain, 2021; Vidra et al., 2018; Villacampa & Torres, 2017). Second, one study found that understanding of human trafficking impeded prosecution since prosecutors hesitated to put cases on trial especially when it came to non-sexual cases of trafficking (Cockbain & Brayley-Morris, 2018). Lastly, two studies researched the way of reporting and stated that more cases led to investigation or prosecution than cases that were filed by victims or third parties if they were initiated by the police (Bjelland, 2017; Matos et al., 2019).

Summary

Overall, North American, and European studies did not vary in factors regarding the perceptions about the prevalence and severity of human trafficking. Stating that positive and negative attitudes towards victims greatly influenced the identification, investigation, and prosecution of human trafficking. Notably, only one study in Europe researched factors regarding perceptions of severity making results based on North American studies more reliable.

Category 4: Environmental factors

Lastly, environmental factors influence the decision-making process of criminal justice actors in identifying and prosecuting human trafficking cases. Farrell cooperated with various authors across different articles and found several environmental factors such as population size, geographical location, and external influences from the public and politics. These factors can be summed up into two factors, namely 1) geographical factors and 2) social and political factors.

Geographical factors

North American studies

Two North American studies researched whether and how geographical factors affect the criminal justice response to human trafficking. Four factors were identified as being of a positive influence in fostering a response, namely areas containing larger communities, better-equipped agencies, agencies in border states and agencies in higher foreign-born populations (Farrell et al., 2010; Farrell, Bouché, & Wolfe, 2019). One study nuanced these results stating these results might be attributed to the higher likelihood of encountering traffickers in densely populated areas (Farrell, Bouché, & Wolfe, 2019).

During the post-identification phase, one study found that having more women acting in the state legislature fostered prosecutions (Farrell, Bouché, & Wolfe, 2019).

Social and political factors

North American studies

Three studies in North America researched whether and how the influence of public pressure (i.e. advocacy groups, the media, or the public) and politics affect the criminal justice response to human trafficking. Two factors were identified by these studies to hamper the identification, (investigation and prosecution) of human trafficking. First, the prioritization of cases, resulting in the alignment of cases with the popular image of human trafficking and the pressure of the public to respond to these cases, and the deprioritization of cases like labour exploitation or those involving male victims (Farrell & Pfeffer, 2014; de Vries & Farrell, 2022). Additionally, one study found modest negative correlations between the presence of advocacy groups or media coverage and the influence of responses by agencies (i.e. adopting training, developing protocols, or assigning specialized personnel) (Farrell, 2014).

Two studies in North America researched three factors interacting with political factors that affect the criminal justice response to human trafficking. First, in the pre-identification phase, two studies researched the passage of legislation which initially didn’t show a strong influence (Farrell et al., 2014) however later research found a strong correlation Farrell, Bouché, and Wolfe (2019). Second, in the post-identification phase, one study researched the funding by states which led to increased arrests and more trials (Farrell, Bouché, & Wolfe, 2019). Lastly, tougher criminal statutes, such as increasing sentences by one year were associated with a higher likelihood of arrests.

European studies

Two studies in Europe researched whether and how political factors influence the criminal justice response to human trafficking. Two studies found three factors that interacted with this response. First, one study found that the prioritization of cases aligned with crimes that generated more social-political alarm and therefore prioritized by police officers higher up in the chain influenced the identified cases (Villacampa & Torres, 2017). Second, one study in the post-identification phase identified corruption and not acknowledging countries as sovereign states as further hindering investigations since no evidence could be gathered (Ambagtsheer, 2021).

Summary

Overall, North American, and European studies varied greatly in environmental factors. European studies solely focused on social and political influences whereas North American studies also took geographical factors into account. Nevertheless, research is limited in both areas and therefore results must be carefully interpreted.

Category 5: Victim factors

Our findings indicated that adding a fifth category would be fitting given the found barriers in the criminal justice response to human trafficking in the included articles. Additional hurdles exist in getting victims to cooperate again influencing the response to human trafficking.

North American studies

Eleven North American studies investigated whether and how victim cooperation affects the criminal justice response to human trafficking. One factor was identified by two studies in affecting victims’ cooperation in the identification of human trafficking. First, one study found that victims withhold from reporting their victimization, for instance, because of being complicit in illicit behaviour or having a huge debt to the trafficker (Farrell & Pfeffer, 2014). Leading to victims reporting after the statute of limitation has expired (Nichols & Heil, 2015).

Eleven studies researched three factors regarding the lack of victim cooperation impeding the investigation and prosecution of human trafficking in the post-identification phase. First, eleven studies found that victims refused to testify, for instance, due to trauma, fearing their exploiter or distrusting criminal justice (Belin, 2015; Fahy, 2015; Farrell et al., 2014; Farrell, Dank, et al., 2019; Farrell & Pfeffer, 2014; Jones, 2010; Musto, 2013; Nichols & Heil, 2015; Potts, 2018). Victims' testimonies are hard to obtain but often a primary, overstressed, source of evidence (Farrell et al., 2014; Kaye & Hastie, 2015; Nelms, 2020). Not having victims’ testimonies leads to the dismissal of the case or a plea for lesser offenses (Nelms, 2020). Second, four studies identified victims going missing during the trial as an impeding factor for cooperation and investigation and prosecution of cases, for instance, because of lack of access to comprehensive services or going back to the trafficker (Fahy, 2015; Farrell et al., 2014; Nelms, 2020). In addition, one study stated that victims only cooperate if they are ready to get out and get into a recovery programme (Farrell, Dank, et al., 2019).

European studies

Nine studies investigated whether and how victim cooperation affects the criminal justice response to human trafficking. Two factors were found to interact with victim cooperation in the post-identification phase. First, eight studies found refusing to testify, for instance, because of trauma or being controlled by their exploiter (Cockbain & Brayley-Morris, 2018; Leser et al., 2017; Matos et al., 2018; Meneses-Falcón et al., 2022; Olver & Cockbain, 2021; Pajón & Welsh, 2020; Vidra et al., 2018; Villacampa & Torres, 2017). Since the need for a victim’s testimony is overstressed for again mostly being the primary and sometimes sole source of evidence the absence of victim cooperation leads to the dismissal of cases (Leser et al., 2017; Matos et al., 2018; Matos et al., 2019; Meneses-Falcón et al., 2022; Pajón & Welsh, 2020; Vidra et al., 2018; Villacampa & Torres, 2017).

Second, one study found that not being able to locate victims impedes the obtainment of testimonies and further trials (Ambagtsheer, 2021).

Summary

Overall, studies in North America and Europe did not vary greatly in factors impeding combatting human trafficking regarding victim cooperation. Both regions found that victims refusing to testify and victims going missing during trial hinder the investigation and prosecution. Additionally, North American studies found factors impeding identification of human trafficking.

Synthesis

Criminal justice actors in North America and Europe encounter similar challenges in combating human trafficking. First, both regions face definitional issues, struggling to interpret anti-trafficking laws and defining elements of these laws. Next, institutional factors included having insufficient resources and knowledge and training gaps, which were found in both areas with one overall notable exception that vice units were solely mentioned in North America. Furthermore, attitudinal factors involved criminal justice actors' perceptions of trafficking prevalence and severity and case characteristics, impacting the identification of specific types of trafficking and questioning victims' credibility. Environmental factors, predominantly studied in North America, highlight how geographical, social, and political influences affect human trafficking identification. Only one study in Europe researched social challenges in combating trafficking. Individual factors focus on obtaining victims' cooperation, especially in providing testimonies and presence during trial hindering prosecutions. In summary, these challenges collectively impede efforts to combat human trafficking in both regions, with North America reporting more challenges due to the abundance of research in this area making this research more reliable and consistent.

Discussion

Nation-states have assigned a central place to criminal justice agencies to execute their anti-trafficking policies (Farrell et al., 2014; Leser et al., 2017; Musto, 2013). However, research revealed challenges faced by these agencies and prosecutors experience reflected in a consistently low success rate in prosecutions (42,6% of the agencies indicated the charges ended in a conviction) and identified victims of human trafficking (Farrell et al., 2010). To strengthen the response by criminal justice actors, understanding their challenges is crucial. Accordingly, his systematic review aimed to build on the work by Farrell and collect and synthesise empirical literature on factors that affect the response of criminal justice actors to human trafficking.

This review shows that, despite their central role, criminal justice actors tackling human trafficking face dilemmas like those described by Lipsky (1980) in his theory on street-level bureaucracy. Although they have an obligation to handle their cases uniformly, they experience numerous challenges affecting both the identification (pre-identification phase) and investigation and prosecution (post-identification phase) of human trafficking. As a result of these challenges, and in line with Lipsky's theory criminal justice actors use their discretionary powers in deciding if a crime constitutes as human trafficking and if this is investigated and prosecuted as such.

The study reveals that the perceived likelihood of success is a central factor in selecting human trafficking cases. Consistent with Lipsky (1980), this study supports the conclusion that criminal justice actors make decisions about cases based on expectations of the outcome, with a higher likelihood of investigation and prosecution in case expected to succeed (Farrell et al., 2014; Farrell, Bright, et al., 2019; Nichols & Heil, 2015). Therefore, police officials investigate more cases they are familiar with (higher chance of winning) or for which specific routines and strategies have been developed to identify victims or prosecute offenders over others. In practice this means that cases of sexual exploitation are considerably more often being identified and processed than cases of labour exploitation and criminal exploitation (Farrell, Bright, et al., 2019).

Furthermore, criminal justice actors' decision-making regarding human trafficking hinges on perceptions of victims and the crime's nature. Leser et al. (2017) highlight that officials' views often clash with victims' actual background and behaviour due to preconceived notions of a ‘genuine victim’. Victims not conforming to this stereotype, especially in cases like forced prostitution, may be misclassified, and denied protection whilst harmful stereotypes in anti-trafficking efforts are reinforced. Victims aligning with these stereotypes of a ‘genuine’ or ‘ideal victim’ have a higher likelihood of being identified. The concept of an ‘ideal victim’ involves weakness, respectability, and blamelessness whilst having a big and bad offender unknown to the victim (Christie, 1986). Victims of human trafficking do not always comply with this image; for example, women being forced into prostitution already do not fit this image since they are not carrying out ‘respectable projects’ by being involved in prostitution and are not seen as blameless if they don’t leave the situation (O’Brien, 2013). Additionally, human trafficking victims often personally know their offenders. Ideal victimhood also depends on their perceived credibility. Victims often have a history of crime, substance use or lack a residential status due to their human trafficking situation, which compromises their perceived credibility (Farrell et al., 2014). Even if a prosecutor finds a victim to be trustworthy, they are hesitant to prosecute a case since they fear a lack of understanding by the court and judges. Our findings thereby support Frohmann’s (1991) concept of downstream orientation, which states that the prosecutor’s decision to bring a case to court highly depends on their predictions of the way a victim’s background and behaviour may be assessed by decision-makers up the judicial chain.

This review has identified several challenges that hamper the response by criminal justice actors to human trafficking. However, we argue that, rather than operating in isolation, these challenges interact and even amplify each other. Prevailing stereotypes about what human trafficking constitutes and how legitimate victims are expected to behave, cause criminal justice actors to funnel most of their efforts into cases they perceive to be the most harmful forms of human trafficking, involving the most deserving victims. Domestic minor female victims of sexual exploitation are therefore more likely to be identified than victims who do not meet the criteria of the ‘ideal victim’, such as victims of labour or criminal exploitation. Consequently, criminal justice actors gain expertise on how to successfully address these types of human trafficking cases while they, simultaneously, remain inadequate to address other types. Furthermore, results showed that access to cooperation with criminal justice actors is not automatically guaranteed for every victim or every form of human trafficking which complicates the issue of protection. This challenges the conventional understanding of cooperation as a condition for protection, thus emphasizing the need for universal access and questing whether this connection adequately meets the proactive obligation to protect victims of human trafficking as set in European guidelines by the Council of Europe and by TVPA (2000) in America.

Lastly, this study has a few limitations that must be taken into consideration. First, most of the studies included (57,2%) were conducted in the United States and less than half in Europe (42,8%). Second, the included studies mostly focused on sexual exploitation (42,9%). This even accounted for the studies that indicated to focus on the broad spectrum of human trafficking. Consequently, the results presented are more applicable to the response of criminal justice actors towards sexual exploitation. Finally, this study did not make use of a quality assessment regarding the underlying studies since the aim of this review was to provide a complete overview of the findings of empirical studies that focussed on the response of criminal justice actors to human trafficking and contributes to science in that way.

Recommendations for policy and future research

Since the response by criminal justice actors to human trafficking is subject to a variety of factors that are connected and mutually reinforcing, simply attempting to influence these factors in isolation will unlikely foster change. Instead, a multi-layered approach is required in which several measures should be taken simultaneously to strengthen criminal justice agencies’ response to human trafficking. We therefore conclude with three recommendations that aim to improve the response by criminal justice.

First, it is imperative that criminal justice actors invest in proactive strategies to identify human trafficking (Barrick et al., 2014; Farrell et al., 2014; Farrell, Bright, et al., 2019; Farrell & Pfeffer, 2014). Criminal justice agencies still heavily rely on victims for information and on information received by third parties. Since victims face many barriers that prevent them from reporting to the police and victims therefore seldom self-report (Farrell & Reichert, 2017), these reactive strategies have not been very successful. Deploying more proactive enforcement strategies, for example by including frontline criminal justice actors in training programs on human trafficking, are therefore needed. In addition, they should benefit from stronger partnerships with stakeholders who are likely to encounter victims of human trafficking, such as care providers, NGOs with ties to migrants communities, or labour regulation agencies. This may also include concerted multi-agency approaches that do not primarily rely on criminal justice actors and that also consider the strengthening of the legal framework to protect the rights of (migrant) workers. Finally, in recent years, several initiatives have been developed that can be used to proactively identify human trafficking, for example by tracking suspicious financial transactions (Coster van Voorhout, 2020) or the detection of indicators of human trafficking in online adverts through text analysis (Volodko et al., 2020). It is important that criminal justice agencies embrace and utilize these innovative measures. Additionally, a broader perspective beyond cooperation is needed to protect victims of human trafficking, thus highlighting the necessity for comprehensive policy reevaluation. Unravelling these complexities is crucial to developing more effective, inclusive strategies against global human trafficking.

Second, this review shows that prosecution of human trafficking cases is still highly dependent on the willingness or ability of a victim to cooperate, which is often not the case. Based on insights gained from this review, two alternative strategies are therefore recommended. First, criminal justice actors should invest in strategies to investigate and prosecute human trafficking cases without the need for a cooperating victim, for example by investing more time and expertise in finding evidence that stems from financial, digital, and medical sources (Farrell et al., 2014; Farrell, Bright, et al., 2019). Second, a more victim-centred approach is needed to increase victim cooperation (Belin, 2015; Farrell et al., 2014; Farrell, Dank, et al., 2019). This requires a more integral approach in which criminal justice actors, receive more training on how to deal with vulnerable populations, and victims' services, such as secure housing, specialized health care, legal advocacy, and education, are intensified to ensure and maintain victims’ cooperation.

Third, it's crucial to increase expertise about the extent and varied nature of human trafficking and its victims in the judicial chain (Fahy, 2015; Farrell, Bright, et al., 2019; Villacampa & Torres, 2017). This accounts especially for knowledge about non-sexual forms of human trafficking, such as labour and criminal exploitation, and about victims who do not live up to the stereotype. Not only does the current status quo lead to unequal treatment and access to justice for certain victims, but it also prevents criminal justice actors from gaining much-needed experience with non-sexual cases of human trafficking. Specialized agents were therefore highly recommended by Matos et al. (2018). The combat against human trafficking and the research in this field deserve and require a much broader perspective.