A large study of twins suggests family-related factors may contribute to both pot smoking and a drop in IQ.

Is marijuana bad for your brain? A 2012 study that found persistent, dependent use before the age of 18 can cause lasting cognitive damage raised considerable concern—understandably, given the drug's popularity among American adolescents.

A new, large-scale study comes to a very different conclusion. Using several measures, it found young pot smokers did, on average, experience greater cognitive declines than non-smokers by late adolescence.

But studying sets of twins, it found youngsters who used marijuana "did not exhibit consistently greater deficits" than their same-age, non-smoking siblings.

"We find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline," the researchers write in the Proceedings of the National Academy of Sciences. The problem, they suggest, is more likely "attributable to familial factors that underlie both marijuana initiation and low intellectual attainment."

In other words, a crappy home environment is unlikely to give kids the support and stimulation needed to build up their brain power—and this remains true whether or not they respond to their difficult situation by turning to pot.

"Marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings."

The study, led by psychologists Nicholas Jackson of the University of Southern California and Joshua Isen of the University of Minnesota, used data from two longitudinal studies of adolescent twins: one from Southern California, which featured 789 kids (predominantly Hispanic), and one from Minnesota, with 2,277 kids (predominantly white).

The California kids were nine to 10 years old when they enrolled in the study, and were assessed five times over the next 10 years. The Minnesota sample featured 11-year-old twins recruited between 1990 and 2006, who were assessed at three-year intervals.

Members of both groups were given a variety of intelligence tests when they joined the study, and again at age 19-20 (California) or 17-19 (Minnesota). They were also periodically asked if they smoked marijuana and, if so, how frequently.

While test results found pot smokers "showed greater decline than non-users," the researchers conclude this does not appear to be due to "a neurotoxic effect of marijuana."

If marijuana use really hurt the brain, the researchers note, IQ scores would decrease more among heavy users than casual ones. That was not the case. "Furthermore," they write, "marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings."

So why do pot smokers typically fall behind their peers? The researchers believe the cause is likely "risk factors that unfold in early adolescence, during middle school." The probable issue, they write, is not a family's socioeconomic status, but rather "familial-cultural deficits such as 'less parental monitoring' and 'less emphasis on scholarship'" in the home.

The researchers concede they cannot definitively say pot smoking is harmless. They note that the aforementioned 2012 study, which concluded it damages intelligence, attention, and memory, tracked its participants (1,000 New Zealand residents) all the way to age 38.

Nevertheless, this evidence points in a different direction, suggesting marijuana use is a coping mechanism used by some kids who grow up in less-than-nurturing environments. It suggests that, while adolescent pot smoking may not be a huge problem in itself, it's sometimes a sign of deeper issues in the home.

Findings is a daily column by Pacific Standard staff writer Tom Jacobs, who scours the psychological-research journals to discover new insights into human behavior, ranging from the origins of our political beliefs to the cultivation of creativity.