The following correction was printed in the Guardian's Corrections and clarifications column, Thursday April 26 2007

The article below about abstinence programmes in the US was accompanied by a photograph of teenage girls holding silver rings. The caption to the photograph suggested that the Silver Ring Thing was one of the programmes included in the research mentioned in the article, but this was not the case. This photograph has now been deleted.

It's been a central plank of George Bush's social policy: to stop teenagers having sex. More than $1bn of federal money has been spent on promoting abstinence since 1998 - posters printed, television adverts broadcast and entire education programmes devised for hundreds of thousands of girls and boys.

The trouble is, new research suggests that it hasn't worked. At all.

A survey of more than 2,000 teenagers carried out by a research company on behalf of Congress found that the half of the sample given abstinence-only education displayed exactly the same predilection for sex as those who had received conventional sex education in which contraception was discussed.

Mathematica Policy Research sampled teenagers with an average age of 16 from a cross-section of communities in Florida, Wisconsin, Mississippi and Virginia. Both control groups had the same breakdown of behaviour: 23% in both sets had had sex in the previous year and always used a condom, 17% had sex only sometimes using a condom; and 4% had sex never using one. About a quarter of each group had had sex with three or more partners.

Since his days as governor of Texas, George Bush has been a firm advocate of abstinence education programmes, which teach that keeping zipped up is the only certain way to avoid unwanted pregnancy and sexually transmitted diseases, and that to deviate from the norms of human sexual activity is to risk harmful psychological and physical effects. "Abstinence hasn't been given a very good chance, but it's worked when it's tried. That's for certain," he said.

But even in 1990s Texas, where Mr Bush spent $10m a year on abstinence education, the state had the fifth highest teen pregnancy rate in the US. Over the past six years he has stepped up the programme to more than $100m a year. He recently braved ridicule by extending it to adults aged 20-29, an age range in which 90% of people are sexually active.

In the Mathematica survey, which was released by sex education activists after the health department sat on it, the mean age at which the control group, that had been taught about contraception, lost their virginity was 14.9 years. That seems strikingly low, until you look at the mean age of first sexual experience for the abstinence control group - 14.9 years.

In the context of findings like this, health workers and statisticians conclude that it is far better that children have safe sex, with knowledge of and access to contraception, than that they are preached a message of abstinence only to ignore it.

Federal funding for abstinence education began as a small part of Mr Clinton's welfare reforms but was stepped up substantially by the Bush administration. Its supporters claim that the fact that though teenaged pregnancies have fallen in the US from a high of 62.1 per 1,000 women aged 15 to 19 in 1991 to 41.1 births per 1,000 in 2004 shows the campaign is working.

But the Mathematica findings, building on earlier research, cast that optimism in doubt. Anti-abstinence activists have long argued that the movement is dangerous because it leaves young people exposed to the risk of teen pregnancy and infection because the teaching shuns any mention of condoms or contraception. Of about 19m new STD infections in the US each year, almost half are recorded among people aged 15 to 24.