But Lisak soon discovered something startling: a small number of serial offenders were responsible for a vast majority of these crimes.
In one early study, out of a sample of 1882 male college students, 120 admitted to committing rape. Of those 120 rapists, 76 were serial offenders, who committed an average of six rapes each. None of these men had been detected or disciplined by campus authorities.
In total, 76 men admitted to 439 rapes and 49 sexual assaults. They represented only four percent of the sample, but these men were responsible for over 90% of the rapes. As Lisak explained, these numbers resembled those of convicted rapists more than "the still-prevalent image of a male college student who...mistakenly crosses the line between sexual pressure and rape" (Lisak & Miller 81).
Lisak has since suggested that each report of rape represents an opportunity "to identify a person who is very likely to be a serial offender" (Harwell & Lisak, 1), and should be followed up by a thorough investigation of the suspect. Removing serial offenders from the campus environment may go a long way to reducing sexual violence on campus.
Schools could also find useful allies in other men, encouraging them to intervene when they witness sexual violence. Most men do not support beliefs and attitudes that condone sexual violence. But they are often afraid to speak up when confronted with offensive language or behavior, because they hold mistaken beliefs about their peers' attitudes and beliefs concerning gender and sex. In their desire to conform, they stay silent.
Indeed according to one study, the sole predictor of men's willingness to intervene in a potential sexual assault was their perception of other men's willingness to intervene.
Disturbingly, researchers also suggest that when men perceive rape-myths to be widely accepted, they report a greater "willingness to engage in sexually aggressive behavior."  Thus silence itself encourages harmful behavior.
Campuses should break that silence and make heard the voices of the majority of young men who oppose harmful attitudes and behaviors.
For more information about perpetrators of sexual violence see the CDC's page, "Sexual Violence: Risk and Protective Factors."
For more on Lisak's research see:
"Myths That Make It Hard To Stop Campus Rape," NPR, 4th March 2010"
Q&A with David Lisak, a leading expert on non-stranger rape," The Star-Telegram, 20th August 2012.
 Lisak, D., Miller, P. (2002). Repeat Rape and Multiple Offending among Undetected Rapists. Violence and Victims, 1, 73-84.
 Harwell, M.C., & Lisak, D. (2010). Why Rapists Run Free. Sexual Assault Report, 14, 17-18 & 26-27, esp. 17.
 Berkowitz, A.D. (2011). Using How College Men Fell about Being Men and "Doing the Right Thing" to Promote Men's Development. In Laker, J.A., and Davis, T. (Eds). Masculinities in Higher Education: Theoretical and Practical Considerations (pp. 161-176). New York, NY: Routledge.
 Fabiano, P.M, Perkins, W.H., Berkowitz, A, Linkenbach, J., Stark, C. (2003). Engaging Men as Social Justice Allies in Ending Violence Against Women: Evidence for a Social Norms Approach. Journal of American College Health, 52, 105-112.
 Bohner, G., Siebler, F., and Schmelcher, J. (2006). Social Norms and the Likelihood of Raping: Perceived Rape Myth Acceptance of Others Affects Men's Rape Proclivity. Personality and Social Psychology Bulletin, 32, 286-297, esp. 293. Eyssel, F., Bohner, G., Siebler, F. (2006). Perceived Rape Myth Acceptance of Others Predicts rape Proclivity: Social Norm or Judgmental Anchoring? Swiss Journal of Psychology, 65, 93-99.