. >> clinical psychologist david lesak travels the country training prosecutors and police on sex offenders. his research revealed a fact. >> the vast majority on college campuses are by serial offenders. >> in his study in 2002 he asked 2,000 male students in massachusetts at a college about their sex life. 6% said their encounters in a way that meant the legal definition of rape, meaning they had sexual intercourse without the consent of the woman, often using force or alcohol. of that group a majority had assaulted multiple women. >> most serial offenders were prolific, so the average number of rapes for each serial offenders was six. >> serial rapists? >> these are the serial rapists, yes. >> even more disturbing he found the serial rapists admitted deliberately taking advantage of vulnerable women. they were cold and calculations. >> serious rapists have perfected techniques and perfected ways of identifying who on campus, for example, are most vulnerable and the individuals to target as they call them to prey on. >> to prove his point he showed us a video based on the interview conduc