As the new school year get underway, colleges around the country are hosting workshops to teach students about sexual assualt, violence, and consent.

One popular workshop is "Sex Signals," which uses improv-style sketches to teach college-aged audiences about the definition of consent, why not to blame victims for sexual assault, and similar concepts.  Used at more than 1,000 college campuses, it has become extremely popular and now has spread to the military, which has its own well-publicized sexual assault problem.

The appeal is obvious: it's hard to make a tough subject like sexual assault engaging and stop the audience from tuning out entirely. But the problem with Sex Signals, and other one-off programs meant to prevent sexual violence, is that the research shows they doesn't work. The bigger problem is we don't really know what does.

The evidence on sexual violence prevention

Earlier this year, the Centers for Disease Control and Prevention reviewed studies of programs meant to reduce the rate of sexual violence, particularly on college campuses.

They found only two programs with rigorous evidence to back them up, both intended for an audience of middle school students. One program, Safe Dates, is a 10-session curriculum meant to teach kids to recognize and prevent dating violence. The other, Shifting Boundaries, focuses on personal space and identifying and dealing with unsafe spaces within the school.

Programs for young teenagers aren't easy to scale up for college students. And while the report identified other promising approaches — including Coaching Boys Into Men, which deals with high school coaches — it pointed out that they aren't yet backed up by rigorous research.

But the researchers were clear about what doesn't work: interventions that only happen once, such as hourlong workshops. Those interventions are cheaper, they can sometimes be delivered online or by video, and they allow colleges to reach a lot of students at once. "These brief programs may increase awareness of the issue," they wrote, but it is "unlikely that such programs are sufficient to change behavioral patterns that are developed and continually influenced and reinforced across the lifespan."

What does work? We don't really know

Bystander intervention — training everyone in a campus community to step up to help victims or potential victims of sexual assault — is becoming the most popular approach to sexual assault prevention. The Obama administration's "It's On Us" campaign, which focuses on a societal responsibility, especially for men, to prevent sexual assault, is just the latest high-profile example. (Disclosure: SB Nation, owned by Vox's parent company Vox Media, is participating in the "It's On Us" campaign.)

Research on bystander intervention indicates that it does make students more likely to say they'll speak up if they need to, and more likely to report that they actually do. A study of Bringing In the Bystander, a program at the University of New Hampshire, found that two months after the training, students were less likely to believe in rape myths, more likely to say they would intervene to stop a sexual assault, and more likely to say that they actually had intervened.

Recent findings on the effects of Living the Green Dot, a bystander intervention program that the CDC described as "promising," suggest that it could be effective. The program was tested in 26 high schools in Kentucky. All students heard a speech about bystander intervention, and student leaders received additional intensive training. At the schools receiving the training, reports of violence declined significantly; at the schools that did not, violence increased slightly.

Green Dot has been introduced at more than 200 colleges in the US and Canada, though who is trained and how widely the program is implemented on campus varies.

The good news is more evidence may be available soon. The CDC is spending $1.8 million on grants to study effective programs to prevent sexual violence, particularly strategies that engage boys and men.