Teenagers in the U.S. are apparently better behaved than they’ve been in 40 years. According to a new Monitoring the Future survey, young people are drinking and smoking less, as well as doing fewer drugs, with marijuana use dropping among 8th-graders through 10th-graders. “The question is: Why is all this happening?” asked University of Michigan researcher Lloyd Johnston, who has led the study since 1975. “Even though we have some hypotheses, I don’t know that we necessarily have the right ones.” Johnson said smoking is often a gateway drug for teens, and that a lower smoking rate could be partially responsible for the rest of the decline. In 2001, 53 percent of 12th-graders said they’d been drunk at least once. That number has dropped to 37.3 percent. This year, only 10.5 percent of high-school seniors said they’d smoked at all in the last month.