Oh, that's why some do that

Chicago streetwalkers earn $25 to $30 an hour, four times what they'd get in other jobs available to them. Venkatesh has also tried to assess the value of having a pimp versus self-managing, which he calls "a classic business school, industrial organization question." Prostitutes who work with pimps, it turns out, appear to earn more and get arrested less frequently. The Chicago prostitution study, which continues, will be written up in the sequel to Freakonomics, to be published in a year or so.