Teenagers are drinking less and smoking fewer cigarettes than in years past. The trouble is, they're also getting high in record numbers. In a University of Michigan study, half of the surveyed high school seniors admitted to experimenting with marijuana, with about one in four saying they had smoked it in the last month — the highest rate in 30 years. And eighth-graders didn't fare much better, with one in five admitting they had used pot recently. What's going on? Here, four theories on marijuana's "historic" resurgence: 

1. The perceived risk is down
One theory for why more teenagers are taking to weed is that the "perceived risk" of using the drug is down, says the survey's principal investigator, Lloyd Johnson. Fewer kids appear to be thinking that potential side effects, such as slower reaction times and memory loss, are really going to hurt them. That weakens the unwritten social rules against pot's use, leaving kids less afraid to experiment. 

2. Parents are to blame
Think about it this way: In 1978, 40 percent of 12th-graders admitted to smoking pot. Now most of them are 50 years old with kids of their own. "Would those parents who smoked marijuana in high school be very upset if they found out their kids were doing it?" asks Janice D'Arcy at The Washington Post. Probably not. It's "math," really.

3. Medicinal use is up
Medical marijuana is legal in 16 states, including Colorado, and in the District of Columbia. The rise in teenage marijuana use "pretty clearly" corresponds with the increase in medical marijuana use, Dr. Christian Thurstone, medical director of a substance abuse center in Denver, tells The New York Times. If caregivers are saying that weed is okay to use legally, how bad can it be?

4. It might explain the decrease in teen pregnancies
"Could the spike in marijuana use explain some of the decreases in other teen behavior we've seen recently?" asks Erin Gloria Ryan at Jezebel. The teen pregnancy rate has dropped 9 percent in the past year alone. If more teens are stoners, it could either be the "lowered sperm counts as a result of THC" or pot-induced "laziness." "Should we take our pants off? Nah, let's keep watching Cartoon Network.

Sources: Christian Science MonitorJezebelNY Times, Wash. Post