American teenagers who take the pledge to remain virgins until they marry have almost the same rate of sexually transmitted disease as other young people, a new study of adolescent behaviour says.
The finding destroys a key rationale for the abstinence crusade - that it prevents disease - and poses a strong challenge to a social engineering project that has been embraced by the White House.
The eight-year study of 12,000 young people by two American sociologists found that the graduates of abstinence programmes were nearly as likely as other young people to catch sexually transmitted infections such as gonorrhoea or chlamydia.
Other findings, yet to be published, also suggest that abstinence programmes do not prevent early pregnancy, Hannah Bruckner, a sociologist at Yale University and co-author of the study, said.
That challenges the very underpinnings of a movement that has attracted 2.5 million American teenagers in recent years, and which is endorsed by church organisations and the Christian right.
Few of those teenagers continue to save themselves for marriage - 88% have sex before they reach the altar. However, the study found they start having sex later and have fewer partners than other teenagers.
Even so, Dr Bruckner said she was initially surprised to discover that there was virtually no statistical difference in their susceptibility to infection. That was because such teenagers are less likely to use condoms, and are less aware of sexually transmitted infections, largely because they have been indoctrinated to believe they are not going to have sex.
Under US law, abstinence programmes risk losing federal funding if they stray into the realm of sex education. Church-based abstinence programmes are openly hostile to condoms and preach that they do not guard against disease.
They can deny reality as loudly as they like, but sex among head-burying ostriches still carries a big whoops! factor.