The study also looked at the use of contraception and found that older children were more likely to use it, including the 15- and 16-year-olds who can't get the Plan B contraceptive over the counter.
"The people who are most affected by that policy are the ones most likely to be having sex: 15- to 16-year-olds are far more likely than the youngest adolescents, the 10- to 11-year-olds," Finer said.
Amy Schalet, an assistant professor of sociology at the University of Massachusetts Amherst who studies teen sexuality, praised the study and said it shows that the "issues at stake" for younger kids are different than for older ones. "It is not common for those in their early teenage years to choose to have sex," she pointed out.
As for access to contraception, rates of teen pregnancy are high in the United States "in part because U.S. teens are less likely to consistently use the most reliable forms of contraception," she noted.
"One reason for that is that young people find it difficult to access contraception options, like the pill, which require them to talk to adults," Schalet said.
David Finkelhor, director of the University of New Hampshire's Crimes Against Children Research Center, has a more critical view of the study. He said it doesn't use the best strategies to study the sexual activities of teens because it relies too much on long-term remembrances of childhood sexuality.
Also, he said, "understanding the mix of coercive and consensual sex among teens is very important. To do this, you need a lot more details about the circumstances, such as alcohol usage."
The study is published in the May issue of the journal Pediatrics.
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