For some time, scholars and partisans have been fighting over two ways to reduce the abortion rate. One side favors contraception; the other favors abstinence. Each side has its logic. If you adequately design and deploy technology to block conception, it’ll work. And if you don’t have sex, you won’t get pregnant. But the devil is in the “ifs.” If you fall off the abstinence wagon, and if you don’t take your pill or properly use your condom, you’re not just screwed. You’re knocked up.

So the debate boils down to this: Which approach can overcome the weakness of human nature? Can the abstinence crowd find a way to keep people chaste? Can the contraception crowd find a way to make people stick to their birth control? Can either side deliver the bottom line: fewer abortions?

A study published Thursday in the New England Journal of Medicine (and outlined in Slate by Darshak Sanghavi and Amanda Marcotte) strikes a major blow for the contraception camp. It shows that women prefer long-acting reversible contraceptives (LARCs), that these methods can almost eliminate birth-control failures, and that they shrink the abortion rate by a margin that far exceeds anything offered by the other side.

Here’s how the study fits into the debate. Conservatives often cite the failure rate of contraception to discredit it as a means of preventing abortions. They don’t deny that programs to promote contraception can increase its use or that when it’s used perfectly, it works. What they question is the reliability of the whole chain of events. They point out, for example, that 99 percent of women who have undergone an abortion have, at some point, used contraception and that half of U.S. pregnancies are unintended even though 89 percent of women who might get pregnant claim to use birth control. Just because somebody tries the pill or buys a condom doesn’t mean they’ll stick to it.

The same is true of abstinence. Conservatives have studies indicating that when teens are taught not to have sex, many of them say they’ll abstain and some follow through. They also have data to show that when teens postpone or avoid sex, the pregnancy rate goes down. But no study confirms the integrity of the whole causal chain. Abstinence, after all, is just another method of birth control. And it’s the hardest one to comply with. Why should we believe that people who can’t stick to pills or condoms can stick to chastity? And even if abstinence-only education reduces the likelihood of sex, how do we know that this effect isn’t washed out by the higher probability of pregnancy (because the sex is unprotected) in the resulting encounters?

To settle this argument, we need the whole sequence, from beginning to end. We need to see an abstinence or contraception program that cuts the abortion rate.

That’s what the new study delivers. More than 1,400 teenage girls in the St. Louis area were offered a range of free contraceptives. Seventy percent chose LARCs. The beauty of LARCs is that they bypass the problem of inconsistent use. Once the implant or IUD is inserted, you don’t have to think about it every time you have sex.

After three years, researchers counted the pregnancies. For hormonal IUDs and injections, the annual failure rate was five per 1,000 women. For hormonal implants and copper IUDs, the failure rate was zero. These methods wildly outperformed contraceptive rings (52 failures per 1,000), pills (57 per 1,000), and patches (61 per 1,000).

During the study, the abortion rate among teenage girls nationwide was 15 per 1,000 women. Among sexually experienced teens, it was 42 per 1,000. But among the girls in the study—99 percent of whom were sexually experienced—it was 10 per 1,000. That’s a 75 percent reduction in the expected abortion rate.

You can argue with the methodology. There was no control group, and the sample wasn’t random. But the odds were stacked against success. The girls were found at or through community clinics. Nearly three-quarters (compared with one-quarter of teenage girls nationwide) said they’d had intercourse in the previous month. Forty-eight percent had been through an unplanned pregnancy, 25 percent had given birth, and 18 percent had undergone an abortion. Sixty percent had been relying on condoms, withdrawal, or no birth control at all.

Abstinence proponents have nothing like this. Not one study has shown an effect of abstinence education on the rate of abortion. The best that’s been offered is a decade-old rough calculation that states that accepted federal money for abstinence-only education had greater reductions in teen pregnancy. But that crude index was trumped by a study that measured what the states were actually teaching. It turned out that the correlation between abstinence education and teen pregnancy was, if anything, positive.

Pro-lifers also claim that the laws they’ve enacted in many states—parental involvement, waiting periods, restrictions on public funding—have prevented abortions. But even using their methodology and calculations, from ages 13 to 17, the asserted reduction is just four abortions per 1,000 girls. The LARC study beats that result by a factor of eight.

This study won’t end the policy debate. It certainly won’t silence the ideologues. But for sensible people who consider themselves pro-life, it ought to inspire reflection. Contraceptive advocates are offering you a 75 percent cut in the abortion rate. What are advocates of abstinence offering you? For that matter, what are you getting from any of the laws enacted by the right-to-life movement? For 40 years, activists and politicians on the right have sold you an agenda of piety without results. Now you have another choice.