Abstinence-only sex education is once again being called to the witness stand. Despite record lows in teenage pregnancy, the US still has some of the highest rates of adolescent births among western industrialised nations.

A meta-analysis now suggests that part of the problem has to do with the country's sex education, and specifically, its federal efforts to encourage abstinence.

Since 1996, the US Congress has funnelled over US$2.1 billion taxpayer dollars into abstinence-only-until-marriage programs. And despite a brief downturn in funding during the Obama administration, that steady stream of money continues to this day.

It's the perennial problem of politics. Both sides ultimately want the same thing, they just disagree on the best way to get there.

But while the road to teen pregnancy may be paved with good intentions, in the tug of war between morality and evidence, the former appears to be winning.

To figure out whether the current tactics are working, researchers compared federal funding for sex education and adolescent birth rates (number of births to women ages 15-19 per 1,000 women) between 1998 and 2016.

The findings confirm that federal abstinence-only grants have had no overall effect on the country's adolescent birth rates.

When examining the data state by state, abstinence funding was actually linked to higher adolescent birth rates in those areas that are more politically conservative.

In 2008 alone, for instance, conservative states received more than $71 million in abstinence funding, and the authors say this translates to 1,080 extra adolescent births than would have otherwise been the case.

This suggests that abstinence funding "may not simply be an ineffective policy but may also have perverse effects in these states."

The conclusion is backed by considerable scientific evidence, which has also found that abstinence education has no effect at reducing adolescent pregnancy.

"While abstinence is theoretically effective, in actual practice, intentions to abstain from sexual activity often fail," says John Santelli, a co-author of a 2017 review on this topic and an expert on family health at the Mailman School of Public Health.

"These programs simply do not prepare young people to avoid unwanted pregnancies or sexually transmitted diseases."

If there's a law of unintended consequences, abstinence-only education is the blueprint. It's essentially like trying to put out a fire with gasoline.

The good news is, 'safe sex' education acts more like a fire hydrant. In 2014, conservative states received only $13 million in pregnancy-prevention funding, but even that small amount was enough to avoid an additional 1,870 potential teenage births.

What's more, there is no evidence that teaching teenagers truthful and practical information about sexuality, contraception, and abortion, increases the frequency of sexual activity or leads to earlier sexual practices - two moral qualms that many conservative Americans hold.

Public health experts largely agree that abstinence-only education should be abandoned in favour of more evidence-based practices, like pregnancy prevention. But some policymakers are choosing not to listen.

Not only has the Trump administration proposed an additional $277 million in abstinence-only education, they recently cut more than $200 million to adolescent pregnancy-prevention programs.

While this may not effect every adolescent American, the current research suggests that these policies could be detrimental to those living in more conservative states.

This study has been published in the American Journal of Public Health.