In that setting, teenagers often say they “fool around,” and in a reversal of the old pattern, such an encounter may or may not lead to regular dating. The shift began around the late 1960s, said Dr. Bogle, who explored the trend in her book “Hooking Up: Sex, Dating and Relationships on Campus” (N.Y.U. Press, 2008).

The latest rise in teenage pregnancy rates is cause for concern. But it very likely reflects changing patterns in contraceptive use rather than a major change in sexual behavior. The reality is that the rate of teenage childbearing has fallen steeply since the late 1950s. The declines aren’t explained by the increasing availability of abortions: teenage abortion rates have also dropped.

“There is a group of kids who engage in sexual behavior, but it’s not really significantly different than previous generations,” said Maria Kefalas, an associate professor of sociology at St. Joseph’s University in Philadelphia and co-author of “Promises I Can Keep: Why Poor Women Put Motherhood Before Marriage” (University of California Press, 2005). “This creeping up of teen pregnancy is not because so many more kids are having sex, but most likely because more kids aren’t using contraception.”

As for that supposed epidemic of oral sex, especially among younger teenagers: national statistics on the behavior have only recently been collected, and they are not as alarming as some reports would have you believe. About 16 percent of teenagers say they have had oral sex but haven’t yet had intercourse. Researchers say children’s more relaxed attitude about oral sex probably reflects a similar change among adults since the 1950s. In addition, some teenagers may view oral sex as “safer,” since unplanned pregnancy is not an issue.

Health researchers say parents who fret about teenage sex often fail to focus on the important lessons they can learn from the kids who aren’t having sex. Teenagers with more parental supervision, who come from two-parent households and who are doing well in school are more likely to delay sex until their late teens or beyond.

“For teens, sex requires time and lack of supervision,” Dr. Kefalas said. “What’s really important for us to pay attention to, as researchers and as parents, are the characteristics of the kids who become pregnant and those who get sexually transmitted diseases.