Just under 40% of U.S. high school students surveyed last year reported ever having had sexual intercourse, according to a new Centers for Disease Control and Prevention survey of risk behaviors among young people.

That represents the lowest rate since the agency began conducting the survey in 1991, as well a significant long-term decrease since then, from 54.1% to 39.5% in 2017. (The 2017 figures represent just a slight decrease from the last survey’s findings of 41.2%, in 2015.)

There were also declines in the percentage of students who reported having had four or more sexual partners, and in those who reported being currently sexually active.

The share of teenagers reporting current sexual activity has also decreased overall since 1991, from 37.5% to 28.7%, decreasing until 2013 and then more rapidly between 2013 and 2017.

The CDC’s findings are just the latest showing a drop in sexual activity among young people, with teens having sex later and the teen birth rate hitting new lows.

Teenagers “are taking longer to grow up — they are less likely to do things that adults do and children don’t,” including working, getting a driver’s license, dating and drinking alcohol, said Jean Twenge, a psychology professor and author of the book “iGen,” about young people today.

But smartphones also play a role, she said, as “teens started going out less and staying home more, likely because they communicated with their friends online and via text.”

Another CDC survey released early this year, which came to similar conclusions about teen sexual activity, described it as encouraging because it puts students less at risk for things like unintended pregnancy and sexually transmitted diseases.

Read: Why American teenagers are not interested in adult activities like sex, drinking — or working

Yet the agency’s most recent survey highlighted some concerning discoveries. A smaller percentage of students reported using a condom during their most recent sexual encounter — representing a decline from 2005 — and the percentage of individuals who did not use any method to prevent pregnancy hasn’t budged since 2007.

The Rebellious Youth Is Getting A Lot Less Rebellious

Concerns about unintended pregnancies and STDs are the very reason that the health agency tracks students’ sexual activity in the first place.

They are among the leading causes of death, disease and social problems for U.S. youth and adults, according to the agency, and are more common among certain populations. For example, HIV virus infection, STDs and pregnancy are more frequent among gay, lesbian and bisexual youth, as well as those who are unsure about their sexual identity.

The CDC notes that it has worked with schools in the last few years on prevention programs aimed at the HIV virus, sexually-transmitted diseases and pregnancy, giving funding and assistance to education agencies in 18 states and the District of Columbia, as well as 17 large urban school districts.

See: This is why Dutch kids are much happier than American children

The 2017 survey results come out of the Youth Risk Behavior Surveillance System, which tracks various health behaviors among young people and young adults, including violence, use of tobacco, alcohol and other drugs, dietary behaviors and physical inactivity. The system includes a Youth Risk Behavior survey, which is administered every two years to high-school students; the latest one included nearly 15,000 questionnaires completed in 144 schools.

Because the survey was school-based, high school dropouts would not have been included, the agency noted. The survey also didn’t define sexual contact, so “students likely considered a range of sexual activities when responding to this question, possibly including involuntary activities,” the it pointed out.