I said upthread that I am hesitant about surveys on either side of this argument however I just read this interesting article about surveys of prostitution:
www.theguardian.com/society/2013/sep/21/united-nations-prostitution-sex-trafficking
Interestingly, the article refers to a London School of Economics report last year by a development professor, Eric Neumayer who claimed that legalisation of prostitution in the Netherlands, Germany and New Zealand and the subsequent growth in demand had led to increases in human trafficking, or coercion of people into the industry.
Here is that report:
eprints.lse.ac.uk/45198/1/Neumayer_Legalized_Prostitution_Increase_2012.pdf
and here is the summary of it:
eprints.lse.ac.uk/48483/1/__libfile_REPOSITORY_Content_LSE%20British%20Politics%20and%20Policy%20Blog_2013_Jan_2013_TO_DO_blogs.lse.ac.uk-There_is_a_complex_relationship_between_legalised_prostitution_and_human_trafficking.pdf
Eric Neumayer's bio is at:
www.lse.ac.uk/researchAndExpertise/Experts/[email protected]
The study investigates the impact of legalized prostitution on human trafficking inflows. The study used an empirical analysis for a cross-section of up to 150 countries and showed that, on average, countries where prostitution is legal experience an increase in demand for prostitution and larger reported human trafficking inflows.
In particular, the study found that trafficking in Denmark (where prostitution is decriminalised) is four times higher than in Sweden (where purchase of sex is criminalised) even though the population of Sweden is 40% higher.
This is consistent with the accounts in the very grim article about legalisation in Germany:
www.spiegel.de/international/germany/human-trafficking-persists-despite-legality-of-prostitution-in-germany-a-902533.html
There is also a very interesting section on the difficulty finding reliable data due to the hidden nature of the population of trafficked people. The report explains how the researchers tried to address this problem.
Funny how this survey is never brought up by the survey-obsessed punter lobbyists.