As many as one in eight teens in the United States may take a virginity pledge at some point, vowing to wait until they're married before having sex. But do such pledges work? Are pledge takers more likely than other teens to delay sexual activity?

A new study looked at the sexual behavior of hundreds of young people, some of whom took virginity pledges.

A new study suggests that the answer is no. While teens who take virginity pledges do delay sexual activity until an average age of 21 (compared to about age 17 for the average American teen), the reason for the delay is more likely due to pledge takers' religious background and conservative views -- not the pledge itself.

According to a study published Monday in the journal Pediatrics, pledge takers are as likely to have sex before marriage as other teens who are also religious, but don't take the pledge. However, pledge takers are less likely than other religious or conservative teens to use condoms or birth control when they do start having sex.

In the new study, Janet Rosenbaum, Ph.D., of Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland, analyzed the large chunk of data used in all the studies that have looked at virginity pledges: the National Longitudinal Study of Adolescent Health. In this survey, middle and high school students were asked about their sexual behaviors and opinions starting in 1995-96.

In the analysis, Rosenbaum compared 289 young adults who took virginity pledges in their teens with 645 young people who did not take such a pledge. The researcher was careful to only compare teens who had similar views on religion, birth control and sex in general, regardless of whether or not they took a pledge. Health.com: What should I do if the condom breaks?

Five years after the initial survey the study subjects were aged 20 to 23. Eighty-two percent of pledge takers denied (or forgot) they had ever taken such a vow. Overall pledge takers were no different from non-pledge takers in terms of their premarital sex, anal and oral sexual practices, and their probability of having a sexually transmitted disease.

Both groups lost their virginity at an average age of 21, had about three lifetime partners, and had similar rates of STDs. "And the majority were having premarital sex, over 50 percent," says Rosenbaum. Overall, roughly 75 percent of pledgers and non-pledgers were sexually active, and about one in five was married. Health.com: Who's most at risk for STDs?

Health Library MayoClinic.com: Talking to your teen about sex

Unmarried pledgers, however, were less likely than non-pledgers to use birth control (64 percent of pledge takers and 70 percent of non-pledge takers said they used it most of the time) or condoms (42 percent of pledge takers and 54 percent of non-pledge takers said they used them most of the time).

"There's been some speculation about whether teenagers were substituting oral or anal sex for vaginal sex and I found that wasn't so," says Rosenbaum. "But I did uphold a previous finding that they are less likely to use birth control and drastically less likely in fact to use condoms -- it's a ten percentage point difference."

Rosenbaum is concerned that abstinence-only sex education programs that promote virginity pledges may also promote a negative view of condoms and birth control. The result may be teens and young adults who are less likely than their peers to protect themselves from sexually transmitted diseases and unplanned pregnancies. Health.com: Sex and teens: Test your knowledge

Federal funds for abstinence only education programs have increased from $73 million in 2001 to $204 million in 2008. About 25 states apply for such funds each year to educate teens, says Rosenbaum. Sometimes programs are measured by how many teens take virginity pledges, not whether the teens stick to them, avoid sexually transmitted diseases or unplanned pregnancies, says Rosenbaum.

"Studies find that kids in abstinence-only programs have negative, biased views about whether condoms work," she says. Since such programs promote abstinence only they tend to give only the disadvantages of birth control, she says. Teens learn condoms don't protect you completely from human papillomavirus (HPV) and herpes, which is true, but they may not realize that they protect against all the "fluid-based STDs," she says. "People end up thinking you may as well not bother using birth control or condoms."

Virginity pledges, along with a six-hour curriculum, were first introduced in 1993 by an evangelical Christian group, and a 1995 survey suggested that 13 percent of teens had taken such a pledge (current survey data are lacking, says Rosenbaum.)

"Virginity pledgers are very different than most U.S. teens -- they are obviously more conservative, they have more negative views about sexuality and birth control and so, even if they didn't take a pledge, these would be teenagers who would be very likely to abstain anyhow," says Rosenbaum. About 40 percent of the study subjects were born-again Christians, she notes.

The new study does not suggest that virginity pledges are harmful, says Andrew Goldstein, M.D., an obstetrician and gynecologist at Johns Hopkins School of Medicine, because they were not associated with an increase in STDs or unplanned pregnancies. However, they do seem to be "useless," says Goldstein, who was not involved in the study.

Promoting the pledges gives a "false sense of security and energy could be better spent in education," he says. "It is time to stop spending money on these useless programs and funnel it into safer-sex counseling." Health.com: Six things your teen needs to know about sex

When it comes to advice for the parents of teens, Rosenbaum notes that just about every organization, from Focus on the Family to Planned Parenthood, offers a similar message.

"Parents should talk to their kids about their sex. It should not be single conversation, it should be a continued conversation at the moments that are teachable moments," she says. "Parents tend to hope that schools will take care of it -- they can't, obviously."

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