The study also questions whether teens' self-reported sexual histories are a good way of gauging how well they adhere to these pledges, reports foodconsumer.org.
Virginity pledges usually take the form of public or written declarations to remain a virgin until marriage.
"A better and more reliable measure than adolescents' self-reported sexual history might be the straightforward results of medical STD (sexually transmitted disease) tests," said researcher Janet Rosenbaum, a doctoral student in health policy at the Harvard School of Public Health in Boston.
Rosenbaum analyzed data from 13,568 U.S. adolescents.
She found that 73 percent of those who signed a virginity pledge and then went on to have premarital sex disavowed ever having signed such a pledge.
The teens were initially surveyed in 1995, and again a year later.
She also found that adolescents who'd had premarital sex and then decided to make a virginity pledge were highly likely to misreport their earlier sexual history, which makes it difficult to accurately assess virginity pledges' effects on early sexual intercourse.
Of the teens who reported a sexual experience in the first survey, those who later took a virginity pledge were four times more likely to retract their reports of sexual experience than teens who had not take a pledge at the time of the second survey.
Rosenbaum said the fact that 52 percent of adolescents who made virginity pledges recanted their vows within a year suggests that these programs have a high drop-out rate and that adolescents don't make a strong connection with the pledge.
She concluded that using teens' self-reported history of sexual intercourse is an unreliable measure for studies examining the effectiveness of virginity pledges.