A growing proportion of teens are abstaining from drugs and alcohol, according to a new study.

One-quarter of high school seniors said they didn’t use drugs or alcohol in a nationwide survey conducted in 2014. In contrast, only 5 percent of high school seniors said they were abstinent in 1976, HealthDay reports.

Lead author Dr. Sharon Levy, who directs the adolescent substance use and addiction program at Boston Children’s Hospital, reports in Pediatrics that 60 percent of students in 8th grade were abstinent in 2014, up from 25 percent in 1991. Among 10th graders, 40 percent said they didn’t use drugs or alcohol in 2014, up from 10 percent in 1991.

Dr. Levy said the increase in abstinence may be due to public health efforts that encourage teens to stay away from drugs and alcohol because they are dangerous and unhealthy, rather than immoral or forbidden.