Teen birth and pregnancy rates have been in a free fall, and there are a few commonly held explanations why. One is that more teens are using the morning-after pill and long-acting reversible contraceptives, or LARCs. The economy might have played a role, since the decline in teen births accelerated during the recession. Finally, only 44 percent of unmarried teen girls now say they’ve had sex, down from 51 percent in 1988.

Teens are having less sex, and that’s good news for pregnancy-and STD-prevention. But paradoxically, while it’s good for teens not to have sex, new research suggests it might be bad for them to promise not to.

As of 2002, about one in eight teens, or 12 percent, pledged to be sexually abstinent until marriage. Some studies have found that taking virginity pledges does indeed lead teens to delay sex and have fewer overall sex partners. But since just 3 percent of Americans wait until marriage to have sex, the majority of these “pledge takers” become “pledge breakers,” as Anthony Paik, a sociologist at the University of Massachusetts–Amherst, explains in his new study, which was published in the Journal of Marriage and Family.

Paik wanted to see what happens if and when these teens break their pledges. He and his co-authors relied on interviews with thousands of teens conducted as part of the National Longitudinal Study of Adolescent Health in 2002 and 2008. The results showed that women who did or did not take abstinence pledges were equally likely to get HPV—about 27 percent of each group would eventually contract the virus, which causes genital warts. Among women who had multiple sexual partners, however, pledge breakers were more likely to get HPV.