Brain and Body

Largest Ever Twin Study Finds No Link Between Smoking Pot and IQ Decline

January 21, 2016 | Kelly Tatera

Marijuana joint
Photo credit: TheChristOff/Deviant Art (CC by SA 3.0)

Toking up probably doesn’t make you dumber afterall.

Marijuana lovers around the world are probably jumping for joy (or sparking up a joint) and saying “I told you so!” because a new study found little reason to believe that smoking weed has a direct effect on lowering a smoker’s IQ.

In the largest ever longitudinal study of twins, involving over 3,000 adolescents, researchers found that the teens who smoked marijuana regularly lost no more IQ points than their non-using twin siblings over time.

In fact, researchers say that the common perception that marijuana is linked to diminishing a user’s intelligence can be most recently traced to a 2012 study by Duke University scientists. In their research, they conclude that persistent and heavy marijuana use is likely linked to an IQ decline.

SEE ALSO: Trials on This Drug for Marijuana Addiction Took an Unexpected Turn

However, there was a big problem with the study — follow-up research exposed that a number of factors associated with IQ decline were never accounted for, like mental illness, cigarette and alcohol use, and low socioeconomic status.

The lead author of the follow-up study, Ole Rogeberg, said, "Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature."

The new study analyzed the results of two separate twin studies that followed the lives of American adolescent twins for at least a decade. One report, the Minnesota Twin Family Study (MTFS) traced 2,277 teenage twins — testing them once in 1990-1996 at the ages of 9 to 11, and again in 1999–2006. The other report, the Risk Factors for Antisocial Behavior (RFAB) study from southern California, looked at the IQs of 789 twins ages 9 to 10 in 2011, and then measured the IQ levels again on five different occasions throughout the next decade.

The teens filled out confidential surveys about marijuana use and binge drinking, as well as the use of other drugs like opioid painkillers and cocaine. They had their IQs tested and their vocabularies and general knowledge measured also.

Although the findings showed that the twins who smoked pot did worse on the tests of general vocabulary and general knowledge than their non-smoking twin counterparts, the scientists found no link to the amount and frequency of marijuana use, which suggested there was another factor involved.

As reported by the Associated Press, the researchers reasoned that, if smoking weed harmed test scores, the participants who smoked more often would show poorer test scores than those who smoked less. "But that's not what the data revealed. Among users, those who'd smoked more than 30 times or used it daily for more than a six-month stretch didn't do worse."

SEE ALSO: Finally, Pot’s Medicinal Potential Will Be Easier to Research in 2016

Over the course of the study, the twins who smoked marijuana lost about four IQ points, but the twin siblings who abstained from smoking pot experienced a similar pattern of intelligence decline. According to Science, this suggests that other factors than marijuana were at play.

"Our findings lead us to believe that this ‘something else’ is related to something about the shared environment of the twins, which would include home, school, and peers," lead author Nicholas Jackson from the University of Southern California in Los Angeles, told Emily Underwood at Science.

Instead of marijuana leading to an intelligence drop, Jackson and his team believe the opposite effect may be at play — teens who are struggling in school may be more likely to try pot. The researchers write, "[C]hildren who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use."

However, it’s important to note that the study has flaws. The Minnesota and southern California research teams used different surveys about drug use, and the Los Angeles study’s questions were much less thorough than the Minnesota ones. The research also relied on self-reported data, and let’s be honest — teenagers might not always be entirely honest when it comes to drug use. Plus, IQ scores don’t always necessarily accurately represent an individual’s intelligence.

Despite the fact that more evidence is revealing that smoking marijuana does not cause IQ decline, “this does not mean that heavy use in adolescence is problem-free,” Jackson told Science. “We desperately need more research on the effects that marijuana has on the brain.”

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