A large-scale study in the US has found that teenagers who publicly pledged not to have sex before marriage lost their virginity about 18 months later than their peers.
Among those who formally promised to avoid unmarried sex, about half remained virgins until they were 20. Among non-pledgers half were no longer virgins by 17.
"The average delay among pledgers is 18 months," said Peter Bearman, a Columbia University sociologist and co-author of the study, which was published in the American Journal of Sociology. "That is significant. And that is a pure pledge effect."
Bearman analysed the effect of the pledges on the behaviour of teenagers enrolled in a massive federal-funded survey of adolescent health.
The study suggests that by 1995 a church-led effort had prompted about 2.5 million teenage boys and girls to pledges to refrain from sex until marriage.
Bearman acknowledged that some factors that led teenagers to make the pledge - such as belonging to a religious school - would have led them to avoid sex anyway. But even adjusting for these, there was an extra delay purely as a result of the pledge, he said.
For details see www.nichd.nih.gov newreleasesvirginity.htm