Why Static Analyses of Social Problems Don't Work

From the generally liberal Atlantic Monthly, on why Amsterdam is beginning to tighten regulation of prostitution:

Holland legalized prostitution in 2000 as a way of stopping exploitation, but evidence suggests that more women than ever are being forced into brothels against their will. A study from the London School of Economics published this winter found that in countries where selling sex was decriminalized, human trafficking has increased. While the number of women entering prostitution voluntarily grows under legalization, demand grows yet further, creating a shortfall filled by women trafficked and run by pimps.

Yup.  No surprise.  Make something legal, and the price drops.  Price drops create increased demand for the service or good.  Increased demand leads to more supply by profit-maximizing entrepreneurs (in this case, by trafficking of women and often, girls).