Though parents may worry about what their kids are doing after school, a new study suggests that teenagers do not often use their unsupervised afternoons to have sex.

In a study that followed 106 teenage girls for more than two years, researchers found that the teens were more than twice as likely to report having sex in the evening compared with afternoons. And girls were less likely to have sex on school-day afternoons than on weekends.

Anyway, in general, personal and relationship factors were more important than mere opportunity in teenagers’ decisions to have sex, according to findings published in the Journal of Adolescent Health.

It’s been argued that unsupervised after-school hours were largely responsible for the increases in teen sex, pregnancy and sexually transmitted diseases seen in recent decades, according to Dr. J. Dennis Fortenberry, the lead author of the new study.

But this is the first study to follow teenagers over time and look specifically at the correlation between time of day and sexual activity, said Fortenberry, a professor of adolescent medicine at Indiana University School of Medicine in Indianapolis.

The study included girls 14 to 18 years of age who completed periodic questionnaires and kept daily diaries on their relationships, mood, sexual interest and other issues.

Overall, Fortenberry and his colleagues found, less than one-third of the sexual encounters the girls reported happened on weekday afternoons. Unmonitored after-school time appeared not to be a major contributor to teens’ sexual behavior, Fortenberry told Reuters Health.

Instead, girls tended to have sex or not have sex based on the ups and downs of their relationship with their boyfriend, or based on whether they “had a good day or a bad day emotionally,” he said.

Parents’ supervision did matter, the study found. Teens who said their parents monitored them closely were even less likely than their peers to have sex during after-school hours.

However, a parent’s watchful eye did not seem to keep girls from having sex in the evenings.

The findings, according to Fortenberry, point to the complexity of teenagers’ sexual behavior - and suggest that no single action, such as keeping kids in after-school activities, will prevent them from having sex.

Public health efforts that “hit one thing” in order to alter teenagers’ sexual behavior, he said, are unlikely to be enough - nor is a single “birds-and-bees talk” with a parent.

SOURCE: Journal of Adolescent Health, March 2006.