A federal study released Thursday says teen sex has decreased in the past decade, but does not offer a reason why.

“That’s the $100,000 question,” said Bill Albert, chief program officer for the National Campaign to Prevent Teen and Unplanned Pregnancy.

The study, based on interviews with approximately 5,300 young people between the ages of 15 and 24, shows a decrease in sexual contact from 78 percent to about 72 percent in the last 10 years.

Experts speculate it might have to do with the emphasis placed on abstinence, or the concern of sexually transmitted diseases.

Albert described this study as the largest and most reliable of its kind.

“It’s the gold standard,” he said.

Health scientist Anjani Chandra of the Centers for Disease Control and Prevention described the decline in sex as small but significant. She declined to speculate on the reasons. It's difficult to look for a trend earlier than 2002 because previous surveys did not gather as much detail about various types of sex, she added.

However, data over the years on vaginal intercourse among never-married adolescents shows a steady decline since 1988. That seems to be in sync with other CDC studies showing an overall drop in teen pregnancy.

That the trend began in the late 1980s seems to undermine the idea that abstinence-only sex education — heavily emphasized during the 2001-2009 presidency of George W. Bush — is the explanation, Albert said.

But it is possible those messages contributed, he added.

The leading influence on sexual activity among young adults is what parents teach and what peers are doing, experts said. And for whatever reason, smaller proportions are "doing it" than in the past.

Still, the CDC report may be surprising to some parents who see skin and lust in the media and worry that sex is rampant.

"Many parents and adults look at teens and sex and see nothing but a blur of bare midriffs. They think things are terrible and getting worse," Albert said.

The sex study looked at older adults, too. It was based on in-person interviews of about 13,500 men and women ages 15 to 44, conducted in the years 2006 through 2008. The results were compared with those of a similar survey done in 2002.

Participants were offered $40 for sitting for the interview, which usually lasted an hour and included answering very specific questions on a computer about oral sex, anal sex and other sexual activities.

Among other findings:
* More than half of young people who had oral sex said they did that before vaginal intercourse; that pattern was much more common in whites than blacks or Hispanics.

* Among young adults, the proportion who had had vaginal or oral sex declined. But the proportion who had anal sex held steady, at about 21 percent.

* For all ages in the study, women were more than twice as likely to have had sex with a same-gender partner than men were. That was true despite the fact that about the same proportion of male and female survey respondents described themselves as homosexual.

The explanation for that finding seems to be that women are much more willing to describe themselves as bisexual, or to at least acknowledge they find others of their gender attractive.

That may have a lot to do with television shows and other pop culture, which at times seems to celebrate woman-on-woman sexual contact, but not the same kind of behavior among men, said Michael Reece, director of Indiana University's Center for Sexual Health Promotion.

"My guess is women are just more likely to feel that's OK," he added.

There is an assumption that sex between females is more common among more educated women, perhaps experimenting with their sexuality during their college years. But the CDC study found that such behavior was more common among less educated women, Chandra said.