, found that, after controlling for environmental factors, cannabis use did not lead to a drop in IQ, and that other factors were more significant than cannabis use. This is good news to the

This study is the first of its kind for a couple of reasons, the first, is that the researchers actually controlled for environmental factors; previous studies have failed to adequately control for confounding variables such as alcohol use. The second reason this study is unique is because it is the first to look at both fraternal and identical twins, measuring how their IQ changed over a decade. Twins are ideal for research because they share known amounts of DNA, and they also commonly share the same environment (home, schools, parental upbringing, etc).

The current study is actually a pairing of two longitudinal studies, done by different universities, the University of California Los Angeles (UCLA) and the University of Minnesota (UM). UCLA researchers conducted the Risk Factors for Antisocial Behavior (RFAB) study, which measured the IQ of 789 twins, first when they were 9-10 years old, then they tested them again five times over the next decade. The second study, the Minnesota Twin Family Study (MTFS) by UM, looked at 2,277 twins and tested them once when they were between 9 and 11 years old, then again a decade later. Researchers asked about cannabis use, among other things (including binge drinking and other drug use), and they were able to successfully determine that cannabis users were not any less intelligent than their non-using peers; all participants saw a decline in IQ that was consistent with age. Even the heaviest users, who used the substance daily for over six months, did not see any measurably worse drop in intelligence.

The study’s lead author, statistician Nicholas Jackson of UCLA, compared previous research on cannabis and IQ decline to being “a classic chicken-egg scenario,” where it was impossible to tell whether cannabis use or the drop in IQ came first. Dr. Jackson’s study is the first to provide needed data to see that it isn’t a matter of what comes first with cannabis and IQ decline, because cannabis use does not contribute to a drop in IQ in any meaningful way. A postdoctoral research associate at UM, Joshua Isen, added that their findings do not “mean that marijuana use itself is harmless,” their findings only mean that using cannabis doesn’t lower your IQ.

A Cohort Study with Similar Findings

Dr. Valerie Curran, a psychopharmacologist at the University College London, released a cohort study earlier this month which she says reached “broadly the same conclusions.” Dr. Curran’s research was published in the Journal of Psychopharmacology and looked at more than 2,000 British teenagers, but did not specifically observe twins. Dr. Curran commented on the findings by the ULCA/UM researchers saying, “ This is a very well-conducted study … and a welcome addition to the literature.” Claire Mokryz, a PhD student who works in Dr. Curran’s lab added that these findings are a “clear indication that cannabis is unlikely to be the cause of any IQ decline.”

Potential Flaws

No study is perfect and a few limitations have been identified in the current study. The major limitation is that the study relies on self-report data for cannabis use, and self-report data can be inaccurate, but there are several techniques that can be used to compensate for this, which presumably were used in the present study.

Another flaw identified in the study was a lack of detail regarding the frequency and amount of cannabis teens used, this may have to do with both groups of researchers using different surveys. George Patton, who was a co-author of Duke University’s highly contested 2012 study on cannabis, has been a vocal critic of the new findings, since they undermine his findings that cannabis use does lower IQ. In Patton’s view, “this paper does not do enough to dismiss the concerns from [our] Dunedin study about the effects of early heavy cannabis use.”

Dr. Scott Krakower, who was not involved with either the Dunedin study or the present one, attempted to discredit Dr. Jackson’s findings but instead touched on an interesting finding that has gone unreported on, namely that even though marijuana users in the study used more alcohol and other drugs, they did not see their IQ drop anymore than people using less drugs and alcohol. This is fascinating because previous studies have shown a well-documented link between increased alcohol use and lower intelligence.

The Contested Dunedin Cohort Study

The Dunedin study, which George Patton was a co-author of, was a 2012 study done by Duke University which examined a cohort of only 1,037 people, and found that cannabis use lowered intelligence. This study had a sample size roughly half that of Dr. Curran’s cohort study which was just released. The Dunedin study also failed to control for numerous confounding environmental variables. The Dunedin study was so poorly constructed that a rebuttal study was published in the same journal six months later, which found that the Dunedin researchers failed to control for mental illness, family life, school drop out rates, socioeconomic status, and even didn’t control for alcohol, tobacco, and other drug use. In short, while the Dunedin study did an impressive job showing a correlation when they did not control for any confounding variables, once those variables were controlled the correlation vanished.

NIDA Weighs In

A finding of this magnitude is not to go unnoticed, and even the head of the National Institute On Drug Abuse, Dr. Nora Volkow, has weighed in on the debate. While Dr. Volkow pointed out the imitations of the study, she also recognized the gravity of the findings and called for further research. She noted that the Federal Government is already working on a longitudinal study to follow 10,000 children over time to better understand the impacts of alcohol, cannabis, and other drug use.

Alcohol Shown to Be Worse?

Dr. Curran’s research team has also commented that alcohol use on its own led to a lower IQ store, supporting previous studies members of the team have authored on the topic. A 2014 study by PhD student Claire Mokryz looked at the effects of cannabis and alcohol on the brain and found that alcohol use was “strongly associated with IQ decline,” and that “no other factors were found to be predictive of IQ change.” These findings seem to have been proven wrong by the researcher’s current study, which did not see lower IQ rates among heavier drinkers, as would be expected. Clearly, we do not fully understand either the effects of alcohol or cannabis on the human brain, but every study done brings us a little bit closer to knowing how we tick.