Facing what many regard as an epidemic of campus sexual assault, some colleges have cracked down on binge drinking, others have reined in fraternities, while still others are training incoming students not to be passive bystanders when they see signs of trouble.

But the most talked-about new approach, adopted by many schools in the past year, is to require mutual “affirmative consent,” and not just passive acquiescence, before any sexual contact. California has raised the stakes becoming the first state in the country to pass a law obliging every college to have a consent policy or lose state financial aid.

And while advocates are nodding approval, experts — and many college administrators — say they have no idea if it will work any better than the other ways. In fact, they say, at a time when politicians from President Obama on down are drawing attention to campus sexual assaults, responses are hampered by a lack of hard data about what works, leaving colleges to rely on instinct and anecdote.

“In a lot of places, there is little to no evidence behind the measures being taken,” said Jane Stapleton, a researcher who advises colleges on sexual assault policies, and is co-director of the Prevention Innovations program at the University of New Hampshire. “That doesn’t mean they won’t work. It means we don’t know.”