I don't know all the studies Yvain might have in mind, but at least one of them is "Pornography and sex crimes in the Czech Republic" http://www.researchgate.net/publication/49644341_Pornography... - one of a few papers exploiting various lapses in nations' child porn laws and observing not an increase in child sex crimes but rather decreases. You can probably find more studies by looking at related papers (http://scholar.google.com/scholar?cites=14325274059754433423...) and reverse citations (http://scholar.google.com/scholar?q=related:j-vH2duYzcYJ:sch...) of that paper.

(This is broadly similar to other correlations you may've heard about with more regular porn: porn seems to substitute for sex crimes, and not increase crimes like rape.)