Ever wonder what happens over time if one twin smokes pot and the other doesn’t?

Josh Isen did, and his analysis of the Minnesota Twin Family Study — along with data from a similar California study of twins as they age — revealed intriguing findings about marijuana and whether it retards cognitive development.

“The public ought to know if, in fact, marijuana use can have these kinds of negative outcomes on a person’s IQ,” said Isen, a former postgraduate researcher at the University of Minnesota.

Twins in the two data sets conducted cognitive tests and provided information about drug use and other behaviors in early adolescence and then again in their late teen years.

Comparing all of the marijuana smokers as a group with all of the nonsmokers showed that the smokers performed poorer by their teen years on measures of vocabulary and knowledge retention.

However, when comparing individual twins who smoked with their siblings who didn’t, the analysis showed no difference in cognitive gains. If marijuana on its own hurt IQ, Isen said, then there should have been differences between the smoking and nonsmoking siblings who were raised in the same households.

The results, published in the Proceedings of the National Academy of Science, don’t give marijuana a free pass, but do suggest other problems in the lives of smokers are equally or more predictive of cognitive problems. It was significant, Isen noted, when comparing the two groups in aggregate that the marijuana users had lower IQs in adolescence — before they started smoking.

Marijuana use might be a warning sign rather than a culprit all by itself.

“If a kid is smoking pot at age 14-15, it’s a red flag,” Isen said. “It suggests the child is on the trajectory for poor intellectual growth in adolescence. Obviously, we’d want to prevent that kid from smoking pot, but [the study findings suggest] that’s not going to change the trajectory that the kid is on.

“If the kid is cutting class or in a bad peer group, that is just as problematic. It’s more like the kid is on a delinquent track.”