He surveyed about 1,800 men, asking them a wide range of questions about their sexual experiences. To learn about sexual assault, he asked things like, "Have you ever had sex with an adult when they didn't want to because you used physical force?" When the results came back, he was stunned.
All told, 120 men in the sample, or about 6 percent of the total, had raped women they knew. Two-thirds of those men were serial rapists, who had done this, on average, six times. Many of the serial rapists began offending before college, back in high school.
Other studies at colleges and in the military have since found similar numbers — usually somewhere around 10 percent of men admitting to either an attempted rape or a rape, with a significant proportion of them reporting a history of repeated offenses.
"I was forced, really, to accept that these are college students, but there is this small percentage of college students who are sex offenders," says Lisak. "They are behaving like sex offenders. They are sex offenders."