By Karen Weintraub

Special for USA TODAY

American teenagers are the best behaved they’ve ever been — drinking and smoking less and doing fewer drugs than their predecessors in more than 40 years of tracking.

Even use of marijuana is down among 8th- and 10th-graders, though it’s flat among high school seniors, according to the annual Monitoring the Future survey of American teens.

“The question is: Why is all this happening?” asked Lloyd Johnston, who has led the survey since it was begun in 1975. “Even though we have some hypotheses, I don’t know that we necessarily have the right ones.”

He and other experts believe that a decline in smoking may be largely responsible for the broader decline. For young teens, smoking is a gateway to other illicit activities, and by cutting smoking rates, fewer adolescents are moving on to alcohol and drugs, said Johnston of University of Michigan’s Institute for Social Research.

In 1991, nearly 11% of high school seniors smoked a half pack of cigarettes or more a day. This year, only 1.8% said they smoke that much, and 10.5% reported any smoking in the last month. Even e-cigarette use fell among high school seniors, from 16% last year to 12% this year.

Alcohol use is also at its lowest level ever: 37.3% of 12th-graders said they have been drunk at least once, down from a high of 53.2% in 2001.

“That is gigantic good fortune, and really I don’t think we as a field or society more generally have spent as much time as we should have celebrating and reflecting on why today’s kids are so great in this regard,” said Jonathan Caulkins, a drug policy researcher at Carnegie Mellon University.

Nora Volkow, director of the National Institute on Drug Abuse, suggested that social media and video games might have helped, too, keeping kids busy at home and away from peer pressure to drink or use drugs.

“There may be a protective effect brought about by the fact that they don’t have so many occasions to get together where the use of drugs would be facilitated,” she said, adding that she doesn’t yet have hard data to support this idea. “It’s wonderful to see, but understanding it will be very important because then we can try to emulate it, be proactive and try to sustain it.”

The percentage of 8th-graders who reported using marijuana in the past month fell from 6.5% in 2015 to 5.4% this year. Among high school seniors, 22.5% used the drug within the past month and 6% used it daily, essentially unchanged from last year, the survey showed.

States where medical marijuana is legal again had higher rates of daily and monthly teenage use, though it is not rising as Volkow had feared it would.

Other research, she said, makes her anxious about 18- to 24–year-olds, whose marijuana use is increasing. Although these young people are considered adults, their brain development isn’t yet complete, she notes

Research, which was stalled for years by the drug’s illegal status, has yet to catch up to usage, she said, so it’s not yet clear how marijuana use affects the developing brain. Regular marijuana use leads to impairments in memory and learning, but it’s not known how long those effects continue. “Is it reversible or long-lasting? We don’t know,” she said.

But at least for now, younger teens are getting the message that they’re better off staying away from marijuana, she said.

They also have managed to largely avoid the opioid epidemic that is devastating young adult communities. Among 12th-graders, use of prescription opioid pain relievers has dropped significantly. Vicodin use, for example, fell from nearly 10% a decade ago to 2.9% this year.

Use of illicit drugs other than marijuana hit historic lows in the 42nd annual survey, which included 45,473 8th-, 10th- and 12th-graders from 372 public and private schools nationwide.

“On the whole, ‘the kids are all right’ over the last couple of decades,” Caulkins said. “Anecdotally in my life, I’d say that relationships between today’s teens and their parents are also better than in past generations.”