A new study of risky teen behavior has shown a drop in the number of American teenagers having sex, according to NBC News. Only 41% of high school students said that they’d ever had sex in 2015, a 6% drop from 47% in 2013, a number that’s held relatively steady for most of the past decade. The survey, which polled 16,000 high school students, is conducted by the Centers for Disease Control (CDC) every two years and also measures drug use and other “risky” behaviors. Researchers say that they don’t know why the drop happened, but they plan to examine the following survey in 2017 to make sure it’s not just a statistical one-off. Though the number of teens having sex had been more or less steady (at 47%) before this year’s decline, rates of teen pregnancy and abortion have dropped over the past decade or so. In 2014, the teen birth rate was at 24.2 per 1,000 for teen girls ages 15 to 19, a huge drop from 41.5 per 1,000 in 2007. Though the CDC says the reasons behind the drop are unclear, it may be attributable to better education about and use of birth control. However, despite the drop, the United States still has one of the highest rates of teen pregnancy in the developed world, according to the CDC. The organization says that reducing the U.S. teen pregnancy rate is one of its top priorities.