One of the most common rationalizations you'll hear for criminalizing prostitution is that permitting it would lead to more sexual violence. It's a tale shared by anti-prostitution feminists and Christian conservatives alike, though the former may see its roots in patriarchy and the latter immorality. Still, both believe that permitting people to pay for sex—a situation which, for their purposes, always involves a man paying for a woman's affections—somehow encourages men to rape.
I'm not quite sure how this argument is supposed to work—does the legal status of fruit encourage people to steal kiwi? Should we criminalize barber shops to stop people from forcing staff into free buzz-cuts? It's a silly idea, that prostitution encourages rape—and also one that's been routinely repudiated by researchers.
The latest study to show a correlation between decriminalizing prostitution and declining sexual violence comes from Rhode Island. A loophole in Rhode Island law effectively decriminalized indoor prostitution there for a several year period ending in 2009. During that time period, the state also saw significant decreases in both sexual violence and cases of gonorrhea, according to data analyzed by the National Bureau of Economic Research.
"The results suggest that decriminalization could have potentially large social benefits for the population at large–not just sex market participants," wrote economists Scott Cunningham and Manisha Shah in a working paper about their research. Between 2004 and 2009, they estimate that decriminalization led to a 31 to 39 percent per-capita decline in rapes and a 39 to 45 percent decline in cases of female gonorrhea in Rhode Island.
Perhaps, however, something else was happening that could explain both declines? After all, the overall gonorrhea rate in the United States did decline around the same time period as this study covers. According to the U.S. Centers for Disease Control and Prevention, the gonorrhea rate decreased 15.8 percent overall during 2006–2010. And the number of rapes reported in the U.S. as a whole also declined somewhat over this time period.
But Cunningham and Shah used several economic models to track decriminalization's effects versus other possible causes and compare Rhode Island to other states. "Robust evidence" across models points to decriminalization as the cause, they write. Basically, while other states saw some decreases post-2003, Rhode Island exhibited much sharper declines.
Numerous studies from other parts of the world have showed a correlation between decriminalizing the sex trade and lowering rape rates. A 2004 working paper from The Independent Institute's Kirby R. Cundiff concluded that rape rates were "correlated with the homicide rate and anti-correlated with the availability of prostitution. It is estimated that if prostitution were legalized in the United States, the rape rate would decrease by roughly 25% for a decrease of approximately 25,000 rapes per year," Cundiff wrote.