Teens who take public pledges to remain virgins wait about 18 months longer to have sex than those who don't, says a new study published in the American Journal of Sociology. "Adolescents who pledge are much less likely to have intercourse than adolescents who do not pledge," wrote Peter S. Bearman of Columbia University and Hannah Bruckner of Yale University, the authors of the study. "The delay effect is substantial and robust. Pledging delays intercourse for a long time. In this sense, the pledge works."

It works, they say, because taking a public stand for virginity helps to give teens a sense of identity and community. Unfortunately, if a school's students accept the pledge as a fashionable stance (with more than 30 percent pledging), the effect diminishes dramatically. The results come from the National Longitudinal Study of Adolescent Health, which surveyed about 90,000 American teens and found that by 1995 about 10 percent of teen boys and 16 percent of teen girls had taken virginity pledges—that's about 2.5 million teens.

The University of North Carolina's J. Richard Udry, who helped design the survey, told Canada's National Post, "We were cynical about the likelihood that the pledge would produce [significant results]. But we were wrong."