You know how sometimes you read a headline about a scientific study and the finding is so obvious you wonder "why did they even bother?" Enter: a comprehensive review of studies published Tuesday that found teaching teens only the "wait-until-marriage" kind of sex ed doesn't work.

Try not to faint from shock.

What's important here, though, is that researchers have been racking up data and evidence for decades showing that abstinence-only sex education isn't effective. It not only fails to delay the age when teens first have sex, but also fails to reduce unintended pregnancies and the spread of sexually transmitted infections. Sadly, there is a need to continue this research because, in spite of all the evidence, some lawmakers still love abstinence-only sex education, and it's common—and increasing—in schools across the United States.

Part of the problem with abstinence-only sex ed is that it doesn't take into consideration that abstinence needs to be perfect to be effective, according the review published Tuesday in the Journal of Adolescent Health. Most birth control methods have two rates of effectiveness: one for perfect use, and one for "typical" use, (that is, how people actually behave). The pill, for example, is 99 percent effective if you are a robot who never forgets your purse, but 91 percent effective for the average human who sometimes misses a pill or takes it late. Likewise, abstinence is a 100 percent effective form of birth control with perfect use, but you only need to slip up once or twice for that effectiveness to plummet to zero.

Adolescents who try to be abstinent often fail, and when they do have sex, they often don't use condoms or other contraception, according to the review. The authors report that as Americans get married later, the longer the window for them to try to be abstinent: the current gap between the age young women first have sex and get married is 8.7 years; for young men it's 11.7 years. Other studies have shown that teen girls who've taken a virginity pledge end up having a higher rates of HPV and unintended pregnancy.

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Conversely, comprehensive sex ed programs—which include lessons on how to have safe sex as well as information on abstinence—have been shown to be much more effective.

"In general, comprehensive sex education includes lesson plans on abstinence," says John Santelli, a population and family health professor at Columbia University and lead author of the study. "Many of these programs are effective in helping adolescents delay initiation of sex. The paradox is the comprehensive programs are effective in promoting abstinence, while abstinence-only programs are not."

Santelli and his colleagues already did a systematic review of the scientific literature on this topic, back in 2006. They found similar results, but Santelli told me it's important to continue studying this as long as abstinence-only education prevails in the US.

"A lot of people are surprised that the federal and state governments are still funding abstinence-only," Santelli said via email. "There is considerable support among conservative legislators for abstinence only until marriage."

The federal government currently spends $85 million a year on abstinence-only programs; it spent more than $2 billion between 1982 and 2017.

Maybe with enough painfully obvious headlines about scientific studies, our legislators will finally get the message. But the Trump administration in June appointed abstinence-only supporter Valerie Huber to a position in the Department of Health and Human Services and cut more than $200 million in teen pregnancy prevention funding last month. We just might see another comprehensive review of abstinence-only sex ed, with exactly the same results, in a few short years.