""In some instances, you can only get high marks on performance if you perform" without using condoms or other protection, Graham said."

Back then, I didn't think about the effect recommendation systems would have outside filtering bad clients. This negative effect on positive reviews is totally believable after all the stories I've seen about people extorting eBay or Amazon store owners for good reviews. This needs further research to figure out how to reduce its effect. It's essentially a form of subversion in practice where prostitutes get positive reviews the more dangerous (i.e. STD's) they are for their more honest clients. On top of pregnancy risks.