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The University of Southampton
Mathematical Sciences

Population size estimation of human trafficking

Human trafficking is a major international crime. One of the main challenges faced by countries is in understanding its extent and prevalence within society. Through their research on capture-recapture methods, Professor Peter van der Heijden and Professor Dankmar Böhning developed new techniques to improve the accuracy of national-level estimates of the extent of human trafficking.


United Nations Office on Drugs and Crime

The eradication of human trafficking has been adopted as part of the UN Sustainable Development Goals (SDGs). Specifically, SDG target 16.2 asserts to ‘end abuse, exploitation, trafficking, and all forms of violence and torture against children’.

In support of this goal, the United Nations Office on Drugs and Crime (UNODC) has collected estimates of human trafficking since 2003. In 2013, UNODC approached Professor Peter van der Heijden to improve the methodology for making estimates of the number of undetected victims and to try this out in practice.

Research challenge

The lack of a sound estimate of the number of victims was due to difficulty to estimate the victims that are unobserved, making the number of victims a so-called dark number.

Van der Heijden used multiple systems estimation (MSE), in which more than one source of data is used to identify the observed size of the elusive target population. After linking the sources using individual data, the overlap between the different data sources then allows the construction of an estimate of the true size of the hidden population.

In the case of human trafficking, data from ngo’s, the police and border forces can be used to identify the observed number of human trafficking victims.

Global impact through UN Office on Drugs and Crime (UNODC) and Walk Free Foundation (WFF) to achieve Sustainable Development Goal Target 16.2

As a result of van der Heijden’s work, UNODC concluded that MSE should be officially adopted as a means to estimate the extent of human trafficking because it produces “a more realistic estimate of the size of any country’s human trafficking problem,” and a “clearer projection of the scale of the issue within that country”.

In 2016, UNODC and WFF signed an official agreement to work together to use MSE to develop estimates of human trafficking in The Netherlands, Ireland, Serbia and Romania.

In 2019, the UNODC commissioned van der Heijden to write a handbook on how to apply MSE to estimate the extent of human slavery that can be used globally.

Policy impact in The Netherlands

In 2016 and 2017, van der Heijden used the MSE technique to provide estimates of human trafficking in the Netherlands. These showed that only around 1 in 5 victims of human trafficking are detected each year in the Netherlands. Also, the number of children among the victims turned out to be much larger than expected.

As a result, the Dutch government released more than €50 million in an effort to curb the issue and launched a programme, ‘Together against human trafficking’. Several policies have arisen from the programme, and its progress is reported annually.

In 2019, the government enacted legislation which included an authorisation requirement for all forms of sexual services, and by criminalising facilitating the pursuit of profit from illegal prostitution – the so-called ‘pimp ban’.

As stated by the Dutch National Rapporteur, “knowing the number of unobserved victims is tremendously useful for us, as the size of the total population of victims reveals the extent of the problem that we face in the Netherlands. But more importantly … it enables us to develop more evidence-based policies, because it also provides insight into the different detection-rates of the different forms of human trafficking.”

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