From Slate's William Saletan:
Findings: 1) 52 percent of pledgers denied a year later that they'd pledged. 2) Among pledgers who later admitted to having sex the year after the pledge, 73 percent denied they'd pledged. 3) Among pledgers who conceded in the first survey that they'd had sex, nearly one in three claimed a year later that they'd never had sex. 4) Pledgers were four times as likely as non-pledgers to recant previous admissions that they'd had sex. Researchers' conclusions: 1) Teens lie. 2) Pledgers lie more. 3) Born-again pledgers (those who pledge after having sex) lie the most. 4) Pledges fail. 5) We have no idea what works or what the truth is, because all this revisionism makes the data worthless.
Not surprisingly, conservative Christian groups claim that the study is bogus, part of their reliable "hear no evil, see no evil" stratagem:
"The Harvard report is wrong," said Dr. Janice Crouse of Concerned Women for America. "I know numerous couples who have saved sex for their wedding night. Research is clear –– that it is the best recipe for marital happiness and well-being. Abstinence-until-marriage is a beautiful promise that should be encouraged and promoted."
In other words: Screw data ... my anecdotal research is much more trustworthy.