The findings surprised researchers Peter Bearman, PhD, of Yale University, and Hannah Brückner, PhD, of Columbia University.
Four years ago, the pair reported that pledge-takers -- those who promise not to have sex before marriage -- tended to have sex later, delaying first intercourse by a year to 18 months.
Pledge-takers also tend to marry earlier, have fewer partners, and report more monogamous partners, say Bearman and Brückner.
But when it comes to STDs, there appears to be little difference between the two groups.
"Contrary to expectations, we found no significant differences in STD infection rates between pledgers and nonpledgers, despite the fact that they transition to first sex later, have less cumulative exposure, fewer partners, and lower levels of nonmonogamous partners," write the researchers.
Millions Take Virginity Pledge
An estimated 2 million adolescents had taken a virginity pledge by 1995, say Bearman and Brückner.
That trend showed up in the 1995 National Longitudinal Study of Adolescent Health, a comprehensive in-home survey of more than 20,000 students in grades 7-12.
Nearly 75% of participants provided follow-up data in 2001-2002. At that point, they were 18-24 years old. They answered more questions and provided urine samples to be screened for STDs.
By then, most had had sex.
Still, a "significant minority [of pledge-takers] holds out far into young adulthood," write the researchers.
STD rates were similar among all participants, regardless of whether they had taken a virginity pledge.
That might be partly explained by a few other findings. Condom use was similar among all participants at the most recent interview. But pledge-takers were less likely to have reported using a condom the first time they had sex.
The pledge-takers were also more likely to say they had oral or anal sex, but not vaginal sex. "Amongst those who have only oral sex and/or anal sex, pledgers are overrepresented," say researchers.
Overall, nearly 3% of respondents reported oral sex with one or more partners but no vaginal sex. "Although just over 2% of nonpledgers fell into this group, 13% of consistent pledgers and 5% of inconsistent pledgers do," write Bearman and Brückner. Still, that means most pledge-takers didn't put themselves in those categories.
Similarly less than 1% of nonpledgers report anal but no vaginal sex compared with 1.2% of pledgers.
Condom use was rare among participants for first oral or anal sex -- regardless of their pledge status. Pledge-takers were also less likely to know their STD status or to have seen a doctor about STD symptoms.
Virginity pledges aren't always taken for health reasons. Moral and religious values can also play a role. However, "as a social policy, pledging does not appear effective in stemming STD acquisition among young adults," the researchers conclude.
Their study appears in the Journal of Adolescent Health's March 18 issue.