Cultural attitudes in this country toward recreational and medical marijuana use are swiftly changing: Earlier this month, for instance, New York City’s first medical marijuana dispensary opened its doors. The latest research suggests that half of all Americans will use marijuana at least once, and just over one-third of high-school seniors have used the drug in the past year. And yet those PSAs of the ’90s are hard to forget, and many — scientists included — are still unsure about the long-term cognitive effects of marijuana, especially in teens.
Now, by following two groups of twins from late childhood through late adolescence and early adulthood, an international team of scientists showed that teens who had used pot did indeed have lower scores on cognitive tests — but, crucially, those differences were likely caused by other genetic and social factors within the family rather than marijuana itself.
Twin studies help researchers to understand the various genetic and environmental factors that contribute to a specific outcome, such as marijuana use, since monozygotic (identical) twins share 100 percent of their DNA, whereas dizygotic (fraternal) twins share 50 percent, just like normal siblings. However, the environmental conditions for these twins are considered to be broadly similar. By determining the effect of one twin’s marijuana use on the likelihood that the other will also light up, scientists can begin to parse out how much biological and social factors contribute.
That’s what a group of researchers from the U.S. and Sweden did, using two different groups of twins, the Risk Factors for Antisocial Behavior study from Southern California and the Minnesota Twin Family Study, that together resulted in 3,066 participants. The scientists asked about marijuana use at several different time points, and assessed several different types of intelligence and cognition. As had been reported in several previous studies, the new study published this week in PNAS found that marijuana-using teens had lower scores on cognitive and intelligence tests, and also showed declines on crystallized intelligence, or the ability to use previously acquired knowledge and skills.
However, the teens who used marijuana the most didn’t show greater declines in cognition and intelligence than teens who only lit up occasionally, nor did these teens show greater declines than their twins who didn’t use. The authors conclude that these results show that marijuana use itself doesn’t seem to cause decreased intelligence and cognition, but rather that familial factors lead to both of these outcomes. Whether or not you’re allergic to the stuff is a whole other story.