School’s back! To celebrate, FiveThirtyEight’s science crew figured it was time to talk about sex. Sex ed, that is. Over the next few days, we’ll be discussing why sex education is such a flashpoint and what role science can play in figuring out what kind of sex ed kids should get.

Debates over how (and even whether) sex ed should be taught in schools have raged for more than a century, with no end in sight. Those debates are fought on both scientific and cultural grounds — they’re about what works to prevent teen pregnancies and STDs and what’s appropriate for American kids to be taught in school, and at what age they should learn it.

The latter questions are important — and also outside the scope of science. They’re rooted in values, not evidence. We can answer the more concrete questions, though. What do we know works about sex ed? And what does it mean for something to “work” in the first place?

Hundreds of studies on sex ed and teen pregnancy prevention programs have been conducted, and what they show is that teaching kids about sex doesn’t make them start breeding like rabbits. Instead, it may prod them to delay having sex. Comprehensive sex education programs that include information about contraception have also been shown to reduce rates of teen pregnancy and sexually transmitted diseases and to increase the use of condoms and other contraceptives.

Those are the broad lessons. Where it gets messy is when you try to tease out which programs work best. Much of the research focuses on three common objectives: delaying the onset and frequency of sexual activity; preventing sexually transmitted diseases; and avoiding teen pregnancy. “I wish I could tell you that there was a study that showed that this program taught in this school is going to help kids throughout the United States fulfill all the outcomes that people want to fulfill, but there isn’t such a study,” said Cora Collette Breuner, chair of the American Academy of Pediatrics’s committee on adolescence. Bruener is the lead author on an AAP clinical report on sexuality education for children and adolescents that advocates “developmentally appropriate and evidence-based education about human sexuality and sexual reproduction.” She said the challenge is that “there are so many variables that go into when and how someone is going to have sex” — such as social norms among peer groups — and these variables can differ among various populations and communities.

So too can the way that sex ed is presented and received. “The schools are different, the teachers who teach the stuff are different, the parents are different, the students are different, the access to a clinic is different,” Breuner said. Studies have shown that a program that, say, reduces teen pregnancies in one setting may not have the same effects in another.

So it’s clear that there’s no one-size-fits-all way to deliver sex ed. But researchers are closing in on some of the essential components.

In 2007, renowned sex education researcher Douglas Kirby and two colleagues published a seminal review of 83 studies from across the globe measuring how sex education and HIV education programs influenced sexual behavior among people younger than 25. Their analysis concluded that these programs “do not hasten or increase sexual behavior” and that some of them could “delay or decrease sexual behaviors or increase condom or contraceptive use.” In all, two-thirds of the programs included in the review showed benefits like those, and Kirby’s team identified 17 characteristics shared by the effective programs.

Those characteristics were varied, but they fell into three categories: the process used to develop a curriculum, the curriculum’s contents and its implementation. For example, effective programs assessed the needs and assets of the target group and included pilot testing, and they focused on clear goals like preventing STDs and creating safe spaces for youth to take part. About 90 percent of the beneficial programs in the Kirby analysis included at least two interactive activities to help participants engage with the lessons via acting out scenarios or other exercises.

“At the end of the day, what young people need is the interpersonal skills to negotiate and communicate and to refuse. And to teach that, you’re going to spend an awful lot of time role-playing and not a lot of time labeling body parts,” said Leslie Kantor, chair of the Department of Urban-Global Public Health at the Rutgers School of Public Health. Nearly all of the effective programs in the Kirby review discussed specific sexual and protective behaviors. Usually this included encouraging abstinence as well as the use of condoms or other contraception if and when a person chose to become sexually active. Finally, 90 percent of these programs provided training to the educators who delivered the curriculum.

Kirby’s 17 characteristics were identified more than a decade ago, and since then, some of the most robust research on sex education has come from studies of federally funded programs. Researchers have a list of programs (not all of them school-based) that have been shown to have a positive effect on at least one of the following things: frequency of sexual activity, the number of sexual partners, use of contraception, rates of sexually transmitted diseases and pregnancies. There are currently 48 programs that have met these criteria for effectiveness.

What’s striking about the list is how varied the programs are. One showing results on multiple measures is delivered over the course of only one day, while another is administered over time periods spanning seven years. Some offer condom demonstrations, some don’t. The list itself was created with the intention of collating evidence on different program models. “Now we’re trying to unpack what makes a program effective,” said Meredith Kelsey, a researcher at Abt Associates who has worked on a U.S. Department of Health and Human Services contract to analyze the evidence on agency-funded teen pregnancy prevention programs.

While researchers seek to identify the things that make programs effective, studies of federally funded programs have identified some programs that don’t seem to produce their intended results. A government-sponsored analysis of four federally funded abstinence-only programs found that youth who were given abstinence-only sex ed were no more likely to remain abstinent than those who didn’t, nor were abstinence programs more likely to raise the age at first intercourse or lower the number of sexual partners or rates of unprotected sex.

At the same time, there wasn’t any evidence that youth from abstinence programs were any more likely to have unprotected sex than those who didn’t attend such programs, which was a concern among some critics of abstinence-only education.

So where does all of this leave us? Without definitive answers to the question of what’s the best way to teach sex ed — because “best” may have different definitions to different communities and parents. The HHS Office of Adolescent Health has amassed a list of programs shown to have achieved at least one objective like a reduction in pregnancy and STDs, but most of the effects are fairly modest, and they don’t always replicate from one program to the next. How to take this evidence and use it to find the best program for any given school is a decision that can’t be made with science alone, but requires value judgments too, which means that the studies probably won’t stop the squabbling.