See the following literature (and add more to it!)
- Cho, Seo-Young, Dreher, Axel, & Neumayer, Eric. (2013). Does Legalized Prostitution Increase Human Trafficking? World Development, 41, 67-82. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1986065
- Huisman, W, & Kleemans, E.R. (2014). The challenges of fighting sex trafficking in the legalized prostitution market of the Netherlands. Crime, Law, and Social Change, 61(2), 215-228.
- European Institute for Gender Equality (2018) Gender-specific measures in anti-trafficking actions. Luxembourg: Publications Office.
- Jiang, Bo, & LaFree, Gary. (2017). Social Control, Trade Openness and Human Trafficking. Journal of Quantitative Criminology, 33(4), 887-913.
- Cho, S. (2016). Liberal coercion? Prostitution, human trafficking and policy. European Journal of Law and Economics, 41(2), 321-348.
Gathered data and solid scientific evidence drawing on empirical and comparative analysis indicate that those countries which have legalized prostitution are associated with higher human trafficking inflows, in opposition to those countries where prostitution is prohibited.