Those public pledges that adolescents make to delay sex until marriage seem to work, major government-sponsored research shows.

On average, adolescents who took a pledge delayed having sex about one-third longer — 18 months more — than peers who did not, says research in the American Journal of Sociology.

However, the effectiveness of the pledges was affected by age and environment in the students' schools.

Among teens 18 or older, the pledge made no difference, says Study co-author Peter Bearman of Columbia University. Among 16- and 17-year-olds, pledgers delayed sex significantly.

Among the youngest adolescents, either too many or too few pledgers in school negated the pledge effect, Bearman says. To be influenced, young pledgers must feel there are neither too many nor too few of them so that they can feel part of a special group.

The research shows 2.5 million teens made such pledges in the 1990s. The "virginity pledge" movement began with organized religious support, Bearman says. It now is loosely organized around about 80 independent organizations. "The typical pledger is not some far-right religious kid," Bearman says. "These are the kids you see at the mall."

The pledgers do share characteristics with those who are less likely to have sex in the first place, he says. Many are more religious than their peers, of Asian ancestry and are less advanced in their pubertal development.

But the researchers controlled for those three key factors, Bearman says. "The pledge effect is in addition." The report states: "The delay effect is substantial and almost impossible to erase."

Researchers used the National Longitudinal Study of Adolescent Health, a comprehensive, congressionally mandated survey of 90,000 students in grades seven through 12. They were questioned about their personal beliefs and various health issues. The chief sponsor is the National Institute of Child Health and Human Development.

Students were surveyed in 1994 and 1995. This portion of the Research focused on 5,679 pledgers who said they had never had intercourse. Jacqueline Darroch of the Alan Guttmacher Institute says the Research shows "how important it is to help young people understand that they don't need to become sexually active at a young age. Not everybody is doing it." The non-profit institute focuses on reproductive health issues.

But Darroch has two concerns. The findings show that among teens who eventually did have intercourse, pledgers were less likely to use contraception than non-pledgers. "That leaves them at a higher risk of pregnancy and sexually transmitted diseases," she says.

And she notes that the goal of the pledge, abstinence until marriage, hasn't necessarily been met. "We are still talking about delaying the onset of intercourse," not waiting until marriage.