New Research Questions Marijuana’s Impact in Lowering IQ

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A new study challenges previous work that found a link between marijuana use and lower IQ, but the authors of the original paper stand by their findings.

Last August, Madeline Meier of Duke University and her colleagues published the results of a study assessing the effect of marijuana use on cognition, as measured by IQ. The findings, published in the Proceedings of the National Academy of Sciences (PNAS), found that people who started using cannabis weekly before they turned 18 and continued to use heavily into adulthood lost an average of eight IQ points over that period. That’s enough to move someone with an average IQ of 100 from the 50th percentile of scores down to the 29th percentile.

The study also showed that this level of decline was seen mostly in about 8% of people who started using cannabis early and qualified for a diagnosis of marijuana addiction no fewer than three times between ages 18 and 38. The results suggested that marijuana was having a negative effect on brain development.

But now another analysis published in the same journal calls the IQ findings into question. Although the initial research involved a relatively large number of participants and controlled for factors such as alcohol dependence and schizophrenia that might also affect cognitive development, the new study suggests that the original one did not account for the effects of poverty, which can affect the way IQ changes over time. Using mathematical modeling, the new research found that because education can affect the trajectory of IQ development differently in people of different socioeconomic status, the environment, and not marijuana, may be the source of the poorer cognitive development.

“[Their] statistical models are unable to distinguish between a causal effect of cannabis on IQ-development and a non-causal correlation,” Ole Rogeberg, the author of the study and a research economist at the Ragnar Frisch Centre for Economic Research in Norway, said in an e-mail discussing the work. In the paper itself, he puts it more bluntly, “[The] estimated effect on of adolescent-onset cannabis use on IQ is likely biased and the true effect could be zero. It would be too strong to say that the results have been discredited but fair to say that the methodology is flawed and the causal inference drawn from these results is premature.”

The authors of the original paper disagree. In a statement they released by e-mail responding to the criticism, they write: “Dr. Rogeberg’s…challenge is based on simulations. We used actual data on 1,037 people to carry out the analyses he suggested. While Dr. Rogeberg’s ideas are interesting, they are not supported by our data.” Meier and her colleagues also note that in their investigation, only 23% of the participants were from lower-income families, which made it unlikely that the effect of poverty was strong enough to skew the results for the entire group. They examined both middle and low socioeconomic groups and saw IQ changes linked to marijuana use in both. “By restricting our analysis to only include children from middle-class homes, our findings of IQ decline in adolescent-onset cannabis users remain unaltered, thereby suggesting that the decline in IQ cannot be attributed to socioeconomic factors alone,” they wrote.

Still, Rogeberg claims that poor children tend to start smoking pot at a much younger age than rich or middle-class kids do. Early exposure to marijuana and other recreational drugs is linked to a higher risk of addiction, which the Duke study found connected to IQ loss.

Marijuana may also lead to higher dropout and expulsion rates from school, which may be an indirect way that marijuana is connected to IQ. “As you consider groups with higher and higher exposure to cannabis, these groups will have higher and higher shares of participants from low SES backgrounds,” says Rogeberg, who also notes that other research has not found a connection between teen marijuana use and lowered IQ.

“The issues raised by Rogeberg are those that confound all observational studies: no matter how carefully controlled a study appears to be, there are always other variables that may alter the conclusions once they are uncovered,” says Dr. Nora Volkow, director of the National Institute on Drug Abuse. “In fact, for something like IQ, it would be surprising for one variable to be 100% causal to a particular outcome.”

Neither research group is insisting that is the case; clearly IQ is affected by myriad factors that scientists are only beginning to tally. But whether marijuana is directly affecting brain development — or whether factors like school dropout or expulsion are more influential — is not yet clear.

MORE: Does Marijuana Use by Teens Really Cause a Drop in IQ?

57 comments
JoshRuchty
JoshRuchty

No proposed/analyzed mediator variables? Maybe a person's IQ is a function of their ongoing cognitive "exercise". We know that marijuana reduces motivation, so maybe its the reduction of motivation that reduces people's self-exposure to cognitively stimulating environments, which then detracts from their IQ like muscle atrophy makes a person weaker. 

In other words, maybe marijuana causes lower IQ in the same way TV causes obesity. That is, it doesn't at all except through incidental intermediary effects. 



chuckwiz99
chuckwiz99

POT HAS SAME EFFECT AS COMA, STROKE N HEAD INJURY, N  PUPILS DILATE THE SAME: SILLY BEHAVIOUR IS SAME ALSO. AS FAR AS POVERTY, MANY ARE IN IT CAUSE OF POT IN THE FIRST PLACE. CHUCK NORRIS KICKSTART SHOWED GRADES N ATTENDANCE IMPROVED WHEN DRUGS WERE SNUFFED OUT, N WITH LESS EFFORT. NO MATTER HOW SMART ONE IS YOUR BETTER WITHOUT RETARDANT DRUGS N WORST WITH THEM. AS FAR AS CANCER CURES? POT SMOKERS HAD DOUBLE THE CASES OF TESTICLE CANCER THAN NON SMOKERS, N THIS CONNECTS WITH LOWER TESTOSTERONE IN POT SMOKERS STUDY ALSO. DONT HURT LUNGS IS ANOTHER LIE, FOR HACKING IS ALSO A NATURAL WARNING THAT SMOKE INHALATION ISNT GOOD N YOUR TO BREATH ONLY OXYGEN>>>>

JeremyCrochetiere
JeremyCrochetiere

I find this Article rather Factious and based on personal opinion, as I know dozens of people that are regular smokers, and they are Judges, Cops, Lawmakers, School Teachers, Bankers, many Millionaires, I know hundreds of people that smoke and graduated, compared to the few that dropped out, they dropped out for reasons way beyond Cannabis, this article takes out of context the fact that many things cause students to drop out, I know people that would of dropped out of school if it hadn't been for smoking cannabis, it helped them focus, and concentrate on homework and in school, a persons laziness isn't based on smoking cannabis, and everything to do with their character, their values, beliefs, and judgements... I know people that smoke everyday with IQ of 170 or more, so your argument is invalid!!! Research is falsified all the time.


herbalmagick
herbalmagick

I wonder if researchers in the Duke study corrected for how stoned the participants were when they took the IQ test?

Talula
Talula

To the authors of the original study - show us the data. I'm not convinced. And if only 8% of marijuana users had that IQ effect, could they have been a group with a greater number of people in poverty? They say they re-run the numbers looking between the socioeconomic groups - but is there even enough statistical power left to show that once you take the low socioeconomic status variable out?

MalcolmKyle
MalcolmKyle

Meta-analysis study by Amy M. Schreiner

Amy M. Schreiner of the Department of Psychology at the University of Central Florida recently led a study that looked at 33 existing meta-analyses of cognitive impairment experienced by heavy cannabis users. The goal of the study was to determine whether the negative effects of cannabis use, such as memory impairment, attention, and executive functioning issues persist after use has stopped.

Thirteen of her studies included users who had abstained for 25 days, while the others had shorter abstinence periods. This allowed her to review a number of factors contributing to the negative symptoms. For instance, those experiencing withdrawal may have had additional problems with anger, irritability, anxiety, or even depression as a result of stopping their drug use. However, when Schreiner examined all of the variables and reviewed all the studies, she was unable to prove that the residual effects of cannabis caused cognitive impairment.

When Schreiner looked at studies that included heavy cannabis users evaluated during states of extreme intoxication, she found some statistical impairments in memory, learning, and attention. But she was unable to provide evidence of long-lasting impairment. Specifically, the participants demonstrated no significant cognitive deficiencies once the intoxication period ended. Additionally, Schreiner found no symptoms of impairment in the individuals who had abstained for 25 days. In conclusion she said, "These results fail to support the idea that heavy cannabis use may result in long-term, persistent effects on neuropsychological functioning."

Reference: Schreiner, A. M., Dunn, M. E. (2012). residual effects of cannabis use on neurocognitive performance after prolonged abstinence: A meta-analysis. Experimental and Clinical Psychopharmacology. Advance online publication. doi: 10.1037/a0029117

http://www.goodtherapy.org/blog/cannabis-long-term-neurologic-impairment-0710122

MalcolmKyle
MalcolmKyle

Here is a recent Study proving that Marijuana is not linked with Long Term Cognitive Impairment:

* Nearly 2,000 young Australian adults (aged 20-24) were followed for eight years. It was found that marijuana had little long-term effect on learning and memory, and any cognitive damage that did occur as a result of cannabis use was reversible.

* Researchers categorized them as light, heavy, former or non-users of cannabis based on their answers to questions about marijuana habits.

* Prior studies have found that drug users do accurately report their consumption levels in surveys like this as long as anonymity is guaranteed and there are no negative consequences for telling the truth.

* Participants took tests of memory and intelligence three times over the eight year period the study. They were also asked about how their marijuana use had changed. When the results were at last tabulated, researchers found that there were large initial differences between the groups, with the current marijuana smokers performing worse on tests that required them to recall lists of words after various periods of time or remember numbers in the reverse order from the one in which they were presented.

* However, when the investigators controlled for factors like education and gender, almost all of these differences disappeared. The lower education levels of the pot smokers — and their greater likelihood of being male — had made it look like marijuana had significantly affected their intelligence. In fact, men simply tend to do worse than women on tests of verbal intelligence, while women generally underperform on math tests. The relative weighting of the tests made the impact of pot look worse than it was.

* Researchers then explored whether quitting cannabis would affect the one difference that remained, which was poorer performance by heavy users on a test that required immediate recall of a list of nouns. They found that heavy users who had quit by the end of the study were no longer distinguishable on this measure from those who had never used.

The authors, who were led by Robert Tait at the Centre for Mental Health Research at Australian National University, conclude:

"Cessation of cannabis use appears to be associated with an improvement in capacity for recall of information that has just been learned. No other measures of cognitive performance were related to cannabis after controlling for confounds."

http://onlinelibrary.wiley.com/doi/10.1111/j.1360-0443.2011.03574.x/abstract

The research was published in the journal Addiction.

MalcolmKyle
MalcolmKyle

An appeal to all Prohibitionists:

Most of us know that individuals who use illegal drugs are going to get high—no matter what, so why do you not prefer they acquire them in stores that check IDs and pay taxes? Gifting the market in narcotics to ruthless criminals, foreign terrorists, and corrupt law enforcement officials is seriously compromising our future.

Why do you wish to continue with a policy that has proven itself to be a poison in the veins of our once so "proud & free" nation? Even if you cannot bear the thought of people using drugs, there is absolutely nothing you, or any government, can do to stop them. We have spent 40 years and trillions of dollars on this dangerous farce; Prohibition will not suddenly and miraculously start showing different results. Do you actually believe you may personally have something to lose If we were to begin basing our drug policy on science & logic instead of ignorance, hate and lies?

Maybe you're a police officer, a prison guard, or a local/national politician. Possibly you're scared of losing employment, overtime pay, the many kickbacks, and those regular fat bribes. But what good will any of that do you once our society has followed Mexico over the dystopian abyss of dismembered bodies, vats of acid, and marauding thugs carrying gold-plated AK-47s with leopard-skinned gunstocks?

Kindly allow us to forgo the next level of your sycophantic prohibition-engendered mayhem!

Prohibition prevents regulation: legalize, regulate, and tax!

yeahitsme
yeahitsme

Whether pot makes your IQ lower or whether a lower IQ leads to pot smoking I really don't care.  The truth is, in my experience, pot smoking is a pretty good indicator that someone isn't razor sharp in the thinking department.  They might be relaxed people with a sweet disposition, but go-getter leader types they are not.

i.am.so.much.more.than.simple
i.am.so.much.more.than.simple

As a teen pot smoker I ca say the development of my IQ was hindered by the fact that I didnt care was a teen and thought there were more important things. As an adult pot smoker I love to learn the History,Discovery and TLC are my favorite channels so Im more than positive my IQ has increased since then. Also like reading this and other things on the web I know Ive added a new wrinkle to my brain. And yes I know IQ is not measured in wrinkles.

michaelshih
michaelshih

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KimWonkwon
KimWonkwon

As a good in approach mind, this kind of research rather would give a rise those who have conservative mind set, solely oriented something affects someone, i would emphasize here a socioeconomic and environment could  be included as environmental factors which would not affect definitive conclusion smoking Marijuana decreases IQ level. Rather this would be far more reasonable why smoking Marijuana gives a high to those who like smoking it at some degree of intensity and then put this result into comparison test according to the race, regular income, and the condition of dwelling, age, population. This kind of study that has long been going on for years has not come down into any conclusive result what is reliable based on clarified research approach. these kindda researches make ppl sick and tired and they would say  afterwards this is waste of money and very much imbecile......

Shimmanni Park
Shimmanni Park

That shouldn't slow down the speed of the legalization of the grass.

Lasse Hanman
Lasse Hanman

Im very disappointed on Time magazine, about this marijuana ad.. government cant fight against it so they want to legalize it so they can tax it. And this is huge amount of money. I sincerely believ that marijuana does not make any of us smarter but it does makes us more stupid and lazy. Sorry if u dont agree with me.

Nino Andreotti
Nino Andreotti

Dr. Duncan Clark, a researcher at the University of Pittsburgh, said that the main lesson he sees in the overall study results is that to preserve one's IQ, it's best to avoid marijuana. The drug is especially harmful to the developing brain.

Aftab Anwar Khattak
Aftab Anwar Khattak

The overall misconceptions about marijuana in the society make the satiation worse and it is fact if every person who knows that you're a marijuana smoker think that you have a low IQ, then you might worry about your IQ

Aftab Anwar Khattak
Aftab Anwar Khattak

Marijuana affect memory only when you are addicted and you can't find or can't afford.otherwise a very good thing specially if you are conducting research or other concentration required tasks.One of best surgeons in Islamabad smoke a cigarette of marijuana before critical bypass operations and he is famous for his successful operations.

hempista
hempista

Carl Sagan used cannabis daily.  :)    

Christopher Smith
Christopher Smith

I quit drinking Alcohol 7 years ago !!! I quit cigarette smoking 3 years ago !!! I smoke Weed instead !!! I have a Extremely High IQ !!!

Lok Ting Lui
Lok Ting Lui

Being poor makes you stupid? Conservatives don't agree.

Jodie Anne Gastel
Jodie Anne Gastel

It is absolutely preposterous that this is still an issue in this day and age. Almost makes me want to get into politics to fix it. Almost.

David Houghton
David Houghton

We don't judge alcohol drinkers like we judge pot smokers... We have been conditioned to think that alcohol is refined and adult and that pot is dirty and juvenile. However, alcohol is more dangerous than pot in every measurable way. And no... I don't smoke.

Yvette Marie Gaudreau
Yvette Marie Gaudreau

ones from poor socioeconomic lifestyles tend to smoke more of it than from a higher socioeconomic class. less supervision and dropping out of school certainly has more to it than smoking marijuana. Marijuana is not addictive. I've smoked when I was a teenager and also as an adult and can go long periods and have w/out it and feel no side effects, withdraw symptoms etc. My IQ is no lower than it was before and memory problems due to surgery.

Philip Dias
Philip Dias

prolong use causes the same type of psychosis as alcohol abuse, kills off brain matter, and iq drops

Ross Krothe
Ross Krothe

Makes me smarter... I don't listen do research...

David Lautz
David Lautz

So pot doesnt make you stupid, being poor does. Theres your cliff notes.

Collin De Bellotte
Collin De Bellotte

@My-Huong, if you only knew all the geniuses that smoked marijuana.

Dax Energi
Dax Energi

whoa, dude. Cliff Notes, please. Reading is tooo hard.

James Tobin
James Tobin

My IQ is incredibly high thank you very much, My-Huong Trinh

Mike Mangone
Mike Mangone

End the "War On Drugs"...Waste of time and resources.

Lyle Ketchum
Lyle Ketchum

I still rather smoke pot than drink any day.

Cohen Chinta
Cohen Chinta

I believe the word you're looking for is crack.

nocroman
nocroman

Oh God !!!  Another study reporting something that we marijuana supporters do NOT advocate. Use of marijuana before the age of 21. We want marijuana legalized for ADULTS, not Minors!!!!!!

I believe that those advocating againts marijuana use are specifically bringing in Information on minors just to cloud the issue. NO ONE has ever ask that minors be allowed to smoke marijuana UNLESS they have a seveer debilitating disease.

Recreational marijuana would be made legal for ADULTS over the AGE of 21 as is alcohol. To grow, use in any form, pocess seeds, plants and any other part on the marijuana plant.

I for one do not want marijuan to be sold for profit. nor do I want it taxed, making the government the new drug lords. Nor do I want the government telling me how much I can grow, as I have room to grow marijuana and some friends and family do not, And I would grow for those that due to illness cannot grow for themselves at no cost to them. It just means I would be growing a different kind of weed than the kinds that currently grow on my property. like ragweed, dandy lions, and the ones that sting when touched.

ALCOHOL is not for children, MARIJUANA is NOT for children! Tobacco is NOT for children.

NO ONE wnats to pass a law to give children Marijuana. So STOP trying to make it seem like this is part of the legalizing issue.

OsirisSgk
OsirisSgk

@chuckwiz99 Hard to take you serious when you cant figure out the function of cap locks. 

MalcolmKyle
MalcolmKyle

@yeahitsme your statement couldn't be further from the truth.

MARIJUANA IS USED BY THE MAJORITY OF TOP SPORTS PEOPLE EVERYWHERE:

"Everybody thinks that if you did this random testing you’d catch so many guys on PEDs. No, you’d catch more of the guys on marijuana. So [we've got] 475 guys under contract and 400 of them would be out with marijuana [suspensions]."

—Dana White, president of the UFC.

"At least a good 50 [US] Olympic athletes use marijuana regularly before they stop in time for testing."

—Stephany Lee

"I just let him know that most of the players in the league use marijuana and I have and do partake in smoking weed in the offseason"

—Josh Howard, forward for the Dallas Mavericks. Howard admitted to smoking marijuana on Michel Irvin's ESPN show.

"You got guys out there playing high every night. You got 60% of your league on marijuana. What can you do?"

—Charles Oakley (Chicago Bulls, New York Knicks, Toronto Raptors, Washington Wizards and Houston Rockets)

"I personally know boxers, body builders, cyclists, runners and athletes from all walks of life that train and compete with the assistance of marijuana,"

—WWE wrestler Rob Van Dam

* Even many of the best cricket players of all time, like Phil Tufnell and Sir Ian Botham, have admitted to regularly using marijuana to deal with stress and muscle aches. In 2001, half of South Africa's cricket team was caught smoking marijuana with the team physiotherapist. They were celebrating a championship victory in the Caribbean.

Here are just a few of the many highly motivated athletes whose drug of choice is cannabis/marijuana:

* Usain Bolt, the 2008 World Record holder of the 100 and 200 meter sprint.

* Michael Phelps, the most decorated swimmer ever with 14 Olympic gold medals.

* Tim Linecum, the National League baseball’s Cy Young Award winner for 2009.

* Santonio Holmes, the Super Bowl XLII’s MVP.

* Mark Stepnoski, two-time Super Bowl champion. "I'd rather smoke than take painkillers."

* Randy Moss, NFL single season touchdown reception record (23, set in 2007), and the NFL single-season touchdown reception record for a rookie (17, in 1998). Moss has founded, and financed many charitable endeavors including the the Links for Learning foundation, formed in 2008.

* Kareem Abdul-Jabbar, the NBA's all-time leader in points scored (38,387), games played, minutes played, field goals made, field goal attempts, blocked shots and defensive rebounds. During his career with the NBA's Milwaukee Bucks and Los Angeles Lakers from 1969 to 1989, Abdul-Jabbar won six NBA championships and a record six regular season MVP Awards. He has a prescription to smoke marijuana in California, which he says he uses to control nausea and migraine headaches. He has been arrested twice for marijuana possession.

BobWilliamKnight
BobWilliamKnight

@Philip Dias wrong pal!none of the above. cite your sources to back up your silly remarks or scram back to wherever trolls come from

mtthwschllng
mtthwschllng

@MalcolmKyle @yeahitsme 


Also the dumbest kids at my old high school smoked cannabis.  Most are now in prison or working full time at a fast food restaurant.   What's your point?