
 
 Elan Nelson, a spokeswoman for a marijuana retail and growing facility, answers questions amid marijuana under cultivation in northeast Denver. (AP/David Zalubowski) 
 
 You might have heard that smoking marijuana makes you stupid. 
 
 If you grew up in the '80s or '90s, that was more or less the take-home message of countless anti-drug PSAs. In more recent years, it's a message we've heard — albeit in more nuanced form — from Republican candidates on the campaign trail and from marijuana opponents at the state-level. 
 
 The contemporary version of argument can be traced to a 2012 Duke University study, which found that persistent, heavy marijuana use through adolescence and young adulthood was associated with declines in IQ. 
 
 As of Feb. 26, 2015 marijuana was made legal in D.C.—sort of. Here are the ins and outs of the complex pot law. (Gillian Brockell/The Washington Post) 
 
 Other researchers have since criticized that study's methods. A follow-up study in the same journal found that the original research failed to account for a number of confounding factors that could also affect cognitive development, such as cigarette and alcohol use, mental illness and socioeconomic status. 
 
 Two new reports this month tackle the relationship between marijuana use and intelligence from two very different angles: One examines the life trajectories of 2,235 British teenagers between ages 8 and 16, and the other looks at the differences between American identical twin pairs in which one twin uses marijuana and the other does not. 
 
 Despite vastly different methods, the studies reach the same conclusion: They found no evidence that adolescent marijuana use leads to a decline in intelligence. 
 
 [These are the states that could legalize pot next] 
 
 I wrote about the study of British teenagers before, when it was still a working paper. It has been peer-reviewed and accepted for publication, and its findings still stand: After adjusting for a range of confounding factors, such as maternal health, mental health and other substance use, the researchers found that "cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance. These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment." 
 
 They did find, though, a distinct relationship between cigarette use and poor educational performance, which is in line with what other research has found. The researchers did not find a robust link between cigarette use and IQ. 
 
 The authors of this study stress that their results don't necessarily invalidate the findings of the 2012 Duke University paper. That paper focused on persistent heavy use over a long period of time, while this study looked only at low to moderate levels of adolescent use. "While persistent cannabis dependence may be linked to declining IQ across a person’s lifetime," the authors write, "teenage cannabis use alone does not appear to predict worse IQ outcomes in adolescents." 
 
 But the researchers in the study of American twins tackle the Duke University findings head-on. Examining the life trajectories of twin pairs in which one uses marijuana while the other doesn't, they found that those who used marijuana didn't experience consistently greater cognitive deficits than the others. 
 
 Identical twin comparisons are a powerful tool for this kind of analysis, because their genetic makeup is nearly identical and their early home environment is consistent. This automatically controls for a lot of the confounding factors that can make sussing out causality difficult. 
 
 The twin data "fails to support the implication by Meier et. al. [the authors of the Duke study] that marijuana exposure in adolescence causes neurocognitive decline," the study concludes. The numbers suggest, on the contrary, that "children who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use." In other words, rather than marijuana making kids less intelligent, it may be that kids who are not as smart or who perform poorly in school are more inclined to try marijuana at some point in their lives. 
 
 Also, if marijuana use were responsible for cognitive decline, you might expect to find that the more marijuana a person smokes, the less intelligent they become. But this paper found that heavier marijuana use was not associated with greater decreases in IQ. 
 
 None of this is to say, though, that you can smoke all the weed you want and not have to worry about negative outcomes. There are any number of negative physical and mental health outcomes linked to marijuana use — especially heavy use. Some research suggests that heavy marijuana use may increase the risk of psychosis or suicide. These risks are further compounded among people who start using marijuana early in their lives. And people who use heavily or start at an early age are at a high risk for cannabis-use disorder, a form of drug dependency. 
 
 Marijuana is a drug. And just like any other drug — alcohol, nicotine, caffeine — there are risks and benefits associated with use. But exaggerating the extent of those risks and benefits won't help create smarter policies. For proof of this, simply review the history of the drug war. 
 
 More from Wonkblog: 
 
 Legal weed having little effect on teen marijuana use, federal data show 
 
 Why Bernie Sanders' marijuana proposal would be a big deal 
 
 These 'marijuana goggles' are supposed to make you feel stoned 

 Roughly half of Americans use marijuana at some point in their lives, and many start as teenagers. Although some studies suggest the drug could harm the maturing adolescent brain, the true risk is controversial. Now, in the first study of its kind, scientists have analyzed long-term marijuana use in teens, comparing IQ changes in twin siblings who either used or abstained from marijuana for 10 years. After taking environmental factors into account, the scientists found no measurable link between marijuana use and lower IQ. 
 
 “This is a very well-conducted study … and a welcome addition to the literature,” says Valerie Curran, a psychopharmacologist at the University College London. She and her colleagues reached “broadly the same conclusions” in a separate, nontwin study of more than 2000 British teenagers, published earlier this month in the Journal of Psychopharmacology, she says. But, warning that the study has important limitations, George Patton, a psychiatric epidemiologist at the University of Melbourne in Australia, adds that it in no way proves that marijuana—particularly heavy, or chronic use—is safe for teenagers. 
 
 Most studies that linked marijuana to cognitive deficits, such as memory loss and low IQ, looked at a single “snapshot” in time, says statistician Nicholas Jackson of the University of Southern California in Los Angeles, lead author of the new work. That makes it impossible to tell which came first: drug use or poor cognitive performance. “It's a classic chicken-egg scenario,” he says. 
 
 To better probe whether marijuana erodes IQ or inflicts harm in other ways, scientists have started following large groups of teenage drug users over time. The first study to do so, in Dunedin, New Zealand, in 2012 reported significant declines in IQ between ages 13 and 38 in heavy users compared with those who used marijuana occasionally before age 18 or not at all. The paper “had a major effect on thinking about the risks of early heavy exposure to cannabis,” says Patton, a co-author. Critics, however, pointed out that the study failed to rule out other potential explanations for the decline in IQ, such as a teen’s family environment or whether they dropped out of school. 
 
 One “powerful” way to address such concerns is to study identical twins, who share genes and upbringing, Jackson says. In the new study, he and his colleagues looked at 789 pairs of adolescent twins from two ongoing studies—one from the Los Angeles, California, area and the other from Minnesota—who enrolled between the ages of 9 and 11. Over the course of 10 years, the team administered five intelligence tests and confidential surveys about marijuana use. They also asked about other drug use such as opioid painkillers, cocaine, and binge drinking. 
 
 Marijuana users lost about four IQ points over the course of the study. But their abstinent twin siblings showed a similar pattern of decline, suggesting that the loss of mental sharpness was due to something other than pot, Jackson says. “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,” he says. 
 
 In the new study, teens who reported daily marijuana use for 6 months or longer did not show any difference in how much their IQ changed, compared with teens who had tried pot fewer than 30 times. This is a “clear indication that cannabis is unlikely to be the cause of any IQ decline,” says Claire Mokryz, a Ph.D. student in Curran’s lab. 
 
 But others say the new study has flaws—most importantly, a lack of detail about how often and in what quantity the teens used marijuana. The Minnesota and Los Angeles groups used different surveys about drug use. The Los Angeles group’s questions were far less thorough, Patton says. In surveys administered to that group, for example, participants were asked, “Have you ever tried marijuana?” If a 13-year-old respondent answered “yes” after taking just one puff, they could be considered a drug user for every subsequent measurement. “My sense is that this paper does not do enough to dismiss the concerns from [our] Dunedin study about the effects of early heavy cannabis use” in teenagers, Patton says. 
 
 Sarah Feldstein Ewing, a psychiatrist at Oregon Health & Science University in Portland, agrees. “While it is possible that the findings are absolutely accurate,” she says, the study represents a “missed opportunity to get a truly fine-grained analysis” of the contribution of cannabis and other substances to IQ. 
 
 Although there is “emerging evidence” that marijuana does not erode IQ, “this does not mean that heavy use in adolescence is problem-free,” Jackson says. Other aspects of daily functioning could be affected, he says, adding, “we desperately need more research on the effects that marijuana has on the brain.” 
 
 The best way to study marijuana’s cognitive effects would be to administer the drug to individuals and see how duration, frequency, and dose affect the brain, Jackson says. “Unfortunately these types of studies are nearly impossible due to federal restrictions,” he says. For now, he says, "I'm mostly concerned about what's going on in the child's environment, that a 14-year-old is seeking refuge in drugs." 

 Marijuana is used more than any other recreational drug, with recent trends toward greater social and legal acceptance in some regions. Concerns remain, however, about a possible causal relationship suggested in scientific studies between marijuana use and decline in IQ, particularly among adolescent users . Other analyses have found no or inconclusive evidence for a link between IQ decline and marijuana use. However, most prior research, regardless of conclusion, has suffered from small participant cohorts, absence of pre-use data, potential influence of confounding effects, or some combination of these factors. 
 
 A new study led by scientists from UCLA and the University of Minnesota attempts to examine the link between marijuana use and IQ in a larger context. Reporting in PNAS, the authors analyze results from two longitudinal studies of twins, with data from more than three thousand individuals from Southern California and Minnesota. Participants were tested in six cognitive areas at two time points: between 9-12 years of age, with follow-up between ages 17-20. Study subjects self-reported marijuana use and frequency, and additional information, including genetic data, family background, socioeconomic status and use of other substances was collected at each encounter. 
 
 The authors focused on three criteria they proposed as measures for evidence of a direct causal relationship between marijuana use and cognitive decline. First, if marijuana use causes IQ decline, as opposed to merely being associated, then poor cognition scores should only be evident after use begins, and not before. Second, with a causal link, a dose-response relationship – that is, higher decline with heavier marijuana use – would be expected. And finally, if the relationship is causal, then the association of marijuana use and IQ decline should remain, even after genetic and social factors are taken into consideration. 
 
 Among the six IQ-related areas tested, the authors found decreases in ability among marijuana users compared to non-users in two modules – Vocabulary and Information – associated with “crystallized intelligence”, or the ability to use learned knowledge. Other tests, measuring abstract reasoning and problem solving associated with “fluid intelligence” showed no significant differences between marijuana users and non-users. The authors noted, however, that in one of the two studies, the baseline IQ scores of eventual users were already significantly lower in the affected areas. Here, marijuana use does not precede cognitive decline, and they point out prior evidence that suggests other factors such as behavioral disinhibition and conduct disorder that may predispose individuals to both lower IQ and substance use. 
 
 The study also found no relationship between heavier or more frequent marijuana use and the magnitude of IQ decline. The authors note that their study, unlike some previous research, did not ask participants about their current use, but only the greatest use since the initial testing. So while the results cannot rule out a causal relationship between near-term use and cognitive ability, the findings do agree with previous studies finding no relationship between prior heavy marijuana use and long-term cognitive impairment. 
 
 The findings agree with previous studies finding no relationship between prior heavy marijuana use and long-term cognitive impairment. 
 
 Finally, the authors examined the effects of outside factors associated with IQ decline. They found the decrease in Vocabulary scores was reduced in one study and “completely eliminated” in the other when adjusted for participants who self-reported binge drinking and use of other drugs. The authors also focused on twins where one sibling used marijuana and the other didn’t, assuming similar genetic, socioeconomic and environmental factors for each member of the pair. These analyses, performed on more than 200 twin pairs, found no significant difference between users and non-users. 
 
 The authors conclude that taken together, the results provide “little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline”. While decline in some cognitive areas was detected, differences were also seen at baseline, no dose response was evident and relevant sibling pairs showed no significant changes. It’s possible that confounding factors and/or upstream causes of both marijuana use and cognitive decline are the true causative drivers of these behaviors. 
 
 Charlie Hatton | Meta Staff Writer
– Smoking dope doesn't actually make you a dope. That's the takeaway from a new study on hundreds of pairs of twins—one of whom smoked pot as an adolescent and continued for over 10 years while the other abstained. Though a 2012 study linked marijuana use with a decline in IQ, critics say it failed to account for certain factors. Hoping to avoid such complaints, scientists at UCLA and the University of Minnesota reviewed data from 789 pairs of twins. They found marijuana users lost about four IQ points over time, but their twins showed a similar decline, reports Science. When it came to an apparent decrease in a marijuana user's vocabulary and information ability, the alteration was reduced or "completely eliminated" when researchers adjusted for a participant's reported binge drinking and use of other drugs including tobacco, which may affect cognitive development, per Meta. Researchers also found no link between heavier or more frequent marijuana use and greater IQ decline. This fails to back up the notion that "marijuana exposure in adolescence causes neurocognitive decline," researchers say, per the Washington Post. Instead, it suggests "children who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use." A British researcher says she reached "broadly the same conclusions" in a recent study of 2,235 adolescent Brits. "Cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance," researchers noted in that study, concluding that modest cannabis use "is not by itself causally related to cognitive impairment." Critics say the American study did not properly question adolescents on their marijuana use. (This study revisited teen pot smokers in their 30s.)