Promises can be hard to keep, especially when it comes to sex. U.S. teens who pledged to remain virgins until marriage had a bit of trouble keeping their word, according to a study of 12,000 adolescents by researchers at Columbia and Yale. In fact, 88% of them went ahead and had premarital sex anyway. What's more, those who made a vow of chastity were less likely to use condoms than other teens, contracted sexually transmitted diseases just as often and were less likely to know they were infected. It's difficult for teens to learn safe sex while saying they're not going to have sex, says study co-author Peter Bearman.