The number of high-school-aged teens who are having sex dropped markedly over a decade, a trend that includes substantial declines among younger students, African-Americans and Hispanics, according to a government report released Thursday.

The survey by the Centers for Disease Control and Prevention showed especially steep declines in the past two years. It adds to evidence about ongoing progress in reducing risky behavior by teenagers, who are becoming pregnant, smoking cigarettes, drinking alcohol and using marijuana at lower rates than younger people before them, according to public health surveys.

“Early initiation of sexual activity is associated with having more sexual partners, not using condoms, sexually transmitted infection and pregnancy during adolescence,” the report noted. It called the falling rate of sexual activity among 9th- and 10th-graders “especially encouraging.”

The researchers said they could not attribute the trend “directly to any specific intervention,” but experts have previously cited a number of factors, most importantly access in school and online to straightforward information about sex and contraception.

Laura Lindberg, principal research scientist at the Guttmacher Institute, a nonprofit organization that studies reproductive rights and health, noted that the vast majority of the decline occurred between 2013 and 2015.

“So we need to see if this is a short term blip or this is something that is going to continue,” Lindberg said. “The drops are very large in 2015, and that raises questions of survey value.”

The results differ from another national survey conducted in teenagers’ homes, which shows little change in sexual activity. The National Youth Risk Behavior Surveys cited in Thursday’s report are conducted in school.

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