The burned out, slow-witted stoner is a popular stereotype, especially when people think of teenagers. Some research has even found evidence to support it, linking marijuana use with poor memory and other cognitive problems. But two new studies of adolescents suggest that if there's any intelligence decline among pot smokers, it's not the weed itself that's causing it.

Both studies followed groups of kids as they moved into adolescence, giving them intelligence tests before and after they hit their teen years.

In one study the researchers tracked more than 3,000 American teens. They found that in pairs where one kid smoked pot and the other didn't, the user's IQ (a numerical score drawn from standardized tests of intelligence) did not decline relative to that of their twin, according to the study published today in the journal Proceedings of the National Academy of Sciences.

In the other study, conducted in the United Kingdom, the IQ decline associated with marijuana use disappeared after the researchers accounted for preexisting differences and other factors. That study was published January 6 in the Journal of Psychopharmacology.

In both investigations, kids who reported smoking marijuana did tend to have lower scores on some intelligence tests. But based on the lack of difference between twins and the role of other characteristics associated with marijuana use (like drinking or using other drugs), the researchers think that marijuana isn't to blame for a declining IQ. Instead, they argue, underlying factors might explain both poor test scores and an interest in weed.

Hazy findings

In the United States, about a quarter of 10th graders and a third of 12th graders report having used cannabis products within the past year. Scientists have been studying the relationship between marijuana and teens' intelligence for years, but the results are mixed. Some studies in rodents have suggested that teens' developing brains might be more vulnerable than those of adults to any effects that cannabis may have on cognition. When scientists scan the brains of human marijuana users and non-users, they do see slight differences in structure and activity, but it's not clear what these differences might mean, if anything.

Researchers have also found some associations between long-term marijuana use and lower intelligence, memory, attention, impulse control and verbal skills. But other studies have failed to find marked long-term effects of marijuana use on cognitive abilities.

"The problem is that most of these studies have been cross sectional—they only measured IQ and assess marijuana use at one point in time," said Joshua Isen, a coauthor of the new study on American teens. "You want to look at how intelligence changes over time."

One 2012 study did track people who started cannabis use as teens until the age of 38. The researchers reported that persistent cannabis use was associated with cognitive decline.

At first glance, that sounds like bad news for stoners. But in fact, it doesn't necessarily contradict the new research, said Madeline Meier of Arizona State University in Tempe, a coauthor of the 2012 paper. Compared with the new study of American teens, "Our 2012 study reported cognitive decline among individuals with a far more serious and far more long-term level of cannabis use."

In the new study, less than half the teens surveyed met the criteria for heavy use (having gotten high more than 30 times in their life), and about a fifth were daily users.

"The message from both of these studies is that short-term, low-level cannabis use appears to be safer than very long-term heavy cannabis use," Meier said. "The problem is that for some teens, short-term low-level cannabis use leads onward to long-term dependence on cannabis when they become adults."

Tracking twins

One advantage the researchers had in the new study was their focus on twins.

"Because we used twin data we were able to account for any factors related to home environment and genes," said Isen, a research psychologist at the University of Minnesota in Minneapolis. "Siblings automatically have the same household income, they have the same race, the same family structure, they're raised in the same neighborhood."

Isen and his team tracked two different pools of twins over time. One group of nearly 2,300 was recruited from Minnesota and tracked over six years. Another group of almost 800 was pulled from Southern California and followed for 10 years. In some cases, neither twin would ever try marijuana, or both would. In others, one twin would experiment while the other did not.

"We actually saw a remarkably similar pattern of results," Isen said. "In both studies there was one particular aspect of intelligence that seemed to be related to adolescent marijuana use."

Over time, the kids who smoked pot had less growth in their vocabularies than those who didn't. The marijuana users' IQ scores dropped compared with those of nonusers by about 4 points for the Southern California kids and 3.4 points for the Midwestern ones. The Californian users also scored lower on tests of general knowledge than their non-using peers.

Other markers of intelligence did not seem to be related to marijuana use. The kids who used marijuana scored similarly to others on tests that involved completing patterns or problem solving.

If marijuana itself was lowering the teens' IQ scores, dedicated potheads would probably have steeper drops than the casual users. But this wasn't the case.

"There was no relationship between how frequently people smoked marijuana and how much their IQ declined," Isen said. "Whether you used marijuana a few times or you'd been a regular pot smoker for a year, it didn't seem that there was any difference."

In the larger, Midwestern group, the kids who went on to smoke marijuana also had lower scores when they were first tested as children, before they'd had a chance to encounter weed.

If marijuana did lower intelligence, you'd also expect to see a difference within pairs in which one twin toked up and the other didn't. But the users didn't have lower scores if compared with their non-using twin. In general, twin pairs where neither kid used marijuana had the highest test scores, pairs that both smoked the lowest. The twin pairs where one kid used marijuana and the other didn't tended to fall in the middle.

"That's what leads us to believe that it's family background characteristics that are responsible for both lower IQ and marijuana use," Isen said.

What these characteristics are isn't clear from this study, although Isen said his team doubts genetics are to blame.

"This is interesting research that speaks to a central issue in the cannabis-IQ debate: The average IQ of teens who smoke cannabis seems to decline relative to non-using peers, but why?" said Ole Rogeberg of the Ragnar Frisch Centre for Economic Research in Oslo, Norway. In 2012, he critiqued the long-term study by Meier and her colleagues.

"Their study simply assumed that cannabis was the only relevant difference between the kids who never used, rarely used, and the kids who went on to use heavily over several years," he said.

Instead, socioeconomic factors could better explain the findings, Rogeberg wrote in his critique. (See Meier's response here.) Isen and his team's suggestion that socioeconomic and environmental reasons are the underlying culprit for stoners' lower scores also fits well with this idea, Rogeberg said.

The whole package

Claire Mokrysz, a coauthor of the new study of British teens, has some ideas what those risk factors might be.

"The evidence suggests that those who start using cannabis from a young age often have less stable backgrounds and more behavioral problems than their non-using peers," Mokrysz, who is getting her PhD at University College London, wrote in The Guardian. "Teenage cannabis use also typically goes hand in hand with other drug use and risky lifestyle choices in general. The poorer cognitive performance of cannabis users may therefore result from other factors associated with cannabis use, rather than cannabis use itself."

Like Isen and company, Mokrysz and her colleagues examined how people scored on tests both as children and teenagers. The 2,235 English teenagers were examined at the ages of 8 and 15, at which point they reported on how much they had used marijuana.

Teens who'd used marijuana fared worse on IQ tests, with even casual users scoring about 2 IQ points lower than non-using peers. But they were also much likelier than non-users to use cigarettes, alcohol and other drugs too.

"When we statistically adjusted for these differences in rates of other substance use, along with other factors including childhood behavioral problems and mental health symptoms, cannabis use no longer predicted lower IQ scores," Mokrysz wrote. "After this adjustment even our heaviest group of cannabis users had predicted IQ scores no different to those who had never tried cannabis."

And when the researchers examined these kids' grades, they saw a similar pattern: the cannabis users had lower scores, but when these same weed-related factors were accounted for, a weed habit alone did not predict worse performance.

Together, the new studies of American and British teens indicate that marijuana itself doesn't dim intelligence, even though users perform poorly on tests that measure it.

"Just preventing [a] child from smoking weed isn't going to have much effect on their IQ," Isen said. "What we really want to know is why that kid is in the position to be smoking pot and drinking and smoking at such a young age."