Neuropsychological Deficits: Fact and Artifact About Marijuana Tests

By Mitch Earleywine, Ph.D
State University of New York at Albany
Chair, NORML board of directors

A new study claims to show small deficits on neuropsychological tests in college students who started smoking marijuana early in life. It might get a lot of press. Prohibitionists love to bang the drum of marijuana-related cognitive deficits, so I’d like NORMLites to know how to make sense of this sort of research. The recurring themes in this literature involve several alternative explanations that never seem to dawn on journalists. These results often arise from artifacts of the study rather than physiological effects of the plant. I’d like to focus on a few: other drug use, dozens of statistical tests, the incentives for performance, and the demands communicated by the experimenters.

The latest paper of this type is actually pretty good. Researchers studied over 30 people aged 18-20 who started using before age 17 (their average starting age was around 15) and who smoked at least 5 days per week for at least a year. They compared them to a comparable bunch of non-users. I hate to see 15-year-olds using anything psychoactive, even caffeine. Spending full days in high school with less than optimal memory functioning is no way to lay the groundwork for a superb life. I admit that I want these same people to grow up and be the next generation of activists, so feel free to call me selfish when I emphasize NORML’s consistent message: THE PLANT IS NOT FOR KIDS WHO LACK MEDICAL NECESSITY.

First, we have to keep other drug use in mind. Unfortunately, the marijuana group in this study got drunk more than 4 times as much in the last six months as the controls. Given what we know about binge drinking and neuropsychological functioning, it’s going to be hard to attribute any differences between these groups to the plant. It’s just as likely that any deficits stem from pounding beers. Studying cannabis users who aren’t so involved with alcohol would help address neuropsychological functioning much better.

In addition, we should always consider the number of measures in any study. Many of these neuropsychological tasks have multiple trials that can be scored multiple ways. The more statistical tests you run, the more likely it is that you’ll find a statistically significant difference by chance. It’s kind of like flipping coins. It’s rare to flip four heads in a row. But if you flip a coin a thousand times, odds are high that somewhere in the list of a thousand results will be four heads in a row. These investigators got 48 different test scores out of the participants. You’d expect at least 2 of them to be significant just by chance. They found differences on 14 different scores, suggesting that something’s going on, but we’re not sure which results are the “real” differences and which ones arose by accident. (That’s why we replicate studies like this.) And, as I mentioned, it might all be because of the booze.

We also have to consider incentives for performance. Most researchers bring participants to the lab for a fixed fee and ask them to crank out a bunch of crazy puzzles and memory assessments. It’s unclear why people would feel compelled to strain their brains. The authors of this study were kind enough to mention some relevant work by my friend (and former student) Dr. Rayna Macher. Dr. Macher showed that cannabis users respond best when you make the effort worth their while. She focused on people who used the plant at least four times per week for a year or more. She read one group some standard instructions for a memory test. The other group got the regular instructions plus an additional sentence: “It is important that you try your very best on these tasks, because this research will be used to support legislation on marijuana policy.”

As you’d guess, this simple sentence fired them up. Compared to cannabis users who didn’t hear that sentence, they performed better on 3 out of 10 measures. (You’d expect less than one difference by chance.) And compared to the non-users, the folks who got the incentive sentence did just as well on all the tests. For those who didn’t hear the incentive sentence, users did less well than non-users on 1 of the 10.

I know that prohibitionists are going to try to call this amotivation. (See my rant on that when you get a chance) I call it putting effort where it pays. But given what we know about how these studies can hamper the reform of marijuana laws, users everywhere should do their best on all tests whenever they get the chance.

Last but not least, we have to consider the demands communicated by the experimenter. Decades of data now support the idea that people often do what others expect them to do, especially if they believe the expectation, too. Another friend and former student, Dr. Alison Looby De Young, showed that these expectations are critical in studies of neuropsychological performance and cannabis. She gave a neuropsychological battery to men who had used cannabis at least three times per week for the last two years. One group of men read instructions that said that cannabis had no impact on their performance on these tests. Another group read instructions that said that cannabis was going to make them perform poorly. You guessed it, those men who heard they were going to flub the tests performed worse on 2 of the 4 tests. (You’d expect less than one difference by chance). As you might imagine, some laboratories communicate their expectations about cannabis and cognitive function subtly or not so subtly. Some participants are bound to behave accordingly. So what looks like a cognitive deficit is just an artifact of the laboratory environment where experimenters stare daggers at cannabis users.

In the end, I’m glad that researchers do this work, but these effects are too small and fleeting to justify prohibition. We already know that cannabis isn’t for healthy kids. People who get heavily involved with the plant early in life might not perform as well as those who never touch cannabis even if investigators control for other drug use, AND use a sensible number of tests, AND provide appropriate incentives, AND communicate a reasonable expectation.

But how many people should go to jail for that?

If you said, “None,” you’ve done an excellent job on an important cognitive test.

45 thoughts

  1. Dear Norml, it continues to amaze me that marijuana users are called, “lazy” when really we just don’t volunteer for stupid behavior as often as people that don’t partake. I just don’t get it. Hey dyslexics, it is called being effective and efficient but if you think backward, it is just lazy. My message to the prohibitionists: FUCK OFF!!!!!!!!!!

  2. If only the Drug Warriors used the same common sense and human compassion as was used in this article…

    Oh, that’s right… Drug Warriors make money off of exploiting the People that choose to consume what they have deemed “illegal.”

    Are we awake from our Constitutional slumber yet, folks??

  3. The way Obama avoids the issue on cannabis id think he was suffering from Neurological Deficits, not Neuropsychological Deficits…As most of us know he smoked a ton of pot in high-school/collage.

  4. “It is important that you try your very best on these tasks, because this research will be used to support legislation on marijuana policy.”

    This would have pumped me up for the test.

  5. Incidentally, I saw Attorney General Holder at one of those Congressional hearings you see on CSPAN. One Representative, Mr. Cohen I believe, beseeched Holder to re-examine Cannabis’ placement in Schedule 1, pointing out the Holder had the authority to have the issue settled based on the science.

    Holder flatly refused! One thing about the Obama administration – They are NOT shy about wielding power, so it seems they are dead set against giving the social conservatives any ammo for use in the next election.

    It appears that for political reasons, it will take an act of Congress to re-schedule cannabis.

  6. When will it end!! I am so tired of prohibitionists trying to stir up a fight. To say that marijuana make link to neurological deficits is absurd. I and many other started using marijuana at a young age and haven’t seen any negative effects. I currently program machines and robotics and continue to learn and see no affects.

  7. I read the whole study, this was one of the weird case study because they use so many other studies as facts, and with a study of under 40 people including the controls I would like to see them list their findings on each one not just the average. Most of the time you would see some not effected and some effected if it was broken down properly. I wouldn’t trust this study. Look who funded it, and look at the article its not like most other medical studies

  8. If they do not test on Marijuana users only, they can not assume the results have everything to do with the “abnormalities”. As a person with a bizare sugar issue NO ONE WILL ADDRESS, there are to many variables to state that Marijuana was the only reason for brain loss,or anything else for that matter.

  9. I find it funny that these “studies” come out when so many states are talking legalization and or at least decriminalization and medicinal?! Funny how that works? Let’s stand back and look at who is affected by legalization……… Big pharm? Oil and gas companies? paper companies? cotton companies? And all related companies to each field? A HUGE lobbying fund!
    Then you ask yourself, how did they test a schedule 1 drug, when people have been trying to test the very same drug, to no avail, on our PTSD Veterans?!
    I smell bs………..

  10. “Unfortunately, the marijuana group in this study got drunk more than 4 times as much in the last six months as the controls.”

    Seriously, every single time I see anything that seems to legitimately show a danger of marijuana, there is alcohol involved.


  11. Hmmm dont binge drink and your brain will not get hurt. Check. Looks like a study on binge drinking then cannabis effect

  12. Remember folks we are in an election year. Do your research and find out who is for and who is against prohibition. Look at what their votes were on previous legislation and vote out of office the ones that stand by this ludicrous policy of prohibition. USE YOUR VOTE!! Don’t sit idly by and say you can’t make a difference. Every vote matters even on the city level. Look at Michigan! Take action folks! Ending this insanity is close if we continue to scream in one voice “END PROHIBITION!!”

  13. Hmm so don’t binge drink and you won’t hurt your brain. Check! This sound like a study of the effects of alcohol not cannabis

  14. Plus, can we also recognize that the issue is not necessarily whether pot may have some negatives, but that it is my right to choose to use it and not have to face criminal penalties for that choice, given that equally or more harmful substances such as alcohol and tobacco are legal?

  15. This is hilarious, I graduated with a 3.7 GPA, missed one question on the ACT, and have an IQ of almost 200 as of less than three weeks ago, I smoked pot almost religiously since I was 12; as in 365 days a year from age 12 to age 19, I still smoke at least 3 times a week, I’ve been drunk so often I don’t really remember much of my freshman and sophomore year in high school, I’m 22 now and not once has my use of cannibis effected any part of my mental capacity to learn and remember, or function normally, apart from the smell of pot on my clothes, you would never know I smoke, marijuana in no way shape or form is detrimental to anyone unless you get a placebo effect, where what you think it’s going to do, it does, personally I believe pot is less harmful than either tobacco or alcohol, so why is it illegal in 48 states? LEGALIZE IT!

  16. I know this is off subject but would Norml please post on this website, or blog your current address. I sent a money order for 36 dollars to your Washington DC. 1600 K street address but got a return to sender, can not deliver as addressed. You want 36 bucks don’t you?

    [Editor’s note: NORML’s address is listed on the bottom of the homepage: 1100 H St., NW, Suite 830, Washington, DC, 20005]

  17. Really, I want to about studies on the fucking morons that say burning up more gasoline faster than ever before is an example of “efficiency”. I’ve had a beef with these people my whole and always wondered why they didn’t know that using less fuel and getting further without making excessive pollution is what “efficiency” means???????????????? No, not these morons, no matter how many ways you try to explain that burning up more gas is less efficient but they just don’t get it. And we have so many dummies, we actually have a SUV loophole so they pay less taxes so they can afford to burn extra gasoline–for efficiency. The dumb fucks that came up with this plan and sold it to our government were not marijuana users and it shows.

    And then to help offset the cost of this dumb fuckery, they want to raise taxes on cars and other vehicles that burn less gasoline to make it fairer. Fairer for who? The people that already paid more (and thus more taxes too) to burn less gas??? Fairer to the SUV drivers who beat up our roads because they enjoy driving behemoths? How is that fair?? They are the ones tearing up the roads!! If you want to drive over-sized vehicles that burn extra gasoline you can fucking pay for it–that is exactly what is fair. But not to these cracked out morons, they want to help encourage more damage to our roads and environment. Can someone study these kinds of “Neuropsychological Deficits”???? This dumb shit costs society a lot more than the problems all drugs legal and illegal combined cause.

  18. I’m no prohibitionist. I smoked ample weed myself in my 20s, and have a son who started smoking weed in high school (sooner than I knew about, naturally), soon developed a dependency relationship with it, had a psychotic break at age 19, was diagnosed as paranoid schizophrenic, became addicted to crack and heroin, had his diagnosis revised to schizo-affective, and fell deeper and deeper into addiction.The question of how THC affects the brain is very complex and we are by no means at the point, as a culture, where we have all the answers. To me, it is simple common sense to proceed with caution and stay informed of the most current science. We know that there are many beneficial therapeutic uses for cannabis. We know that “moderate” use does not seem to have harmful effects as serious as does alcohol, BUT to make this a black and white issue is naive and unworthy of the intellectual capacity of an educated citizenry. We know better than to reject it out of blind fear (“reefer madness”), not well enough to give it an unconditional scientific stamp of approval, just enough to be cautious and use moderately, if at all, while science continues to explore and examine the effects of habitual use.

  19. If the EVIL HERB were proven as dangerous as alcohol and tobacco have, it would legal! Now I understand ???????

  20. Drew, the sample pool is small because that’s just how research studies work at first. There is no point in going all the way for a first-run study. If the results are good, they may warrant larger, more conclusive polling demographics.

    First studies more or less lend credence to a correlation. Which leads to longer, more complex studies that test actual causation.

  21. Anything our President puts his name on is shot down, the cause is better off waiting until a republican puts their name on it.

  22. From the abstract: “Comorbid use of alcohol, which was heavier in marijuana users, was unexpectedly found to be associated with better performance in some of these areas.”

    Could someone translate that into English for me please?

  23. There have been many classic studies done, where the researchers intentionally add a verbal detail, or one of a different sort, and the subjects perform differently, based on what is said. If people are trying to make puts on a golf short game, they score about twice as badly if they’re handed the ball and told “this is the ball everyone’s been using,” vs. “this seems to be the lucky ball.” Or if they say “here’s the test, I’ll keep my fingers crossed,” vs. “here it is, it’ll be a piece of cake for you.”

    Those studies are crap, inserted with built-in bias. They’re not scientific except in understanding how people obey the subtle messages they’re GIVEN on purpose.

    Like drug testing, it’s biased nonsense that deserves only enough attention to discredit the researchers responsible, for life. No more credibility for certain researcher/sycophants who are intellectually dishonest. No soup for you anymore, despite betting on a prohibition to last forever, and they can pretend to be the “good guys” when they’re just liars, even when poor studies based on lies and grant proposals that satisfy what NIDA’s sole mission is, to support lousy studies that demonstrate (speciously) the dangers of Nixon’s ‘controlled substances.” They do this knowing they are the antithesis of science, and unbiased research. Taxes are paid to keep liars in their positions, to lie.

    The NSF should single out these people, tarnish their reputations as they deserve, intellectually dishonest, self-interested parasites who should be barred from the profession as researchers because their bias is obvious and shameful.

  24. Also, these subjects were self-proclaimed as cannabis users, this was not some long-term, longitudinal study, which would be the bare minimum for studying long-term effects, which this bogus research claims.

    It is possible that some people who use cannabis test differently than they would if they had not used for a couple of weeks. But saying that a small group of subjects tested worse than the “control” who could be loaded up on pharma amphetamines.

    This is just like the claims that cannabis has a detrimental effect on those ‘susceptible’ to schizophrenia, when those people, like those with PTSD and glaucoma, use cannabis BECAUSE they discovered it helps ease their pain, and their condition. The cannabis didn’t cause their illness, they use cannabis because it helps them.

    They used to asphyxiate rhesus monkeys with concentrated smoke under gas masks, which obviously leads to a deficiency in uptake of oxygen. They basically forced these animal subjects into a state of anoxia, not enough oxygen to the brain, for prolonged periods. Then they did dissections on autopsy to “prove” what they knew…strangling/choking/asphyxiating breathing creatures deprives their brains of needed oxygen, killing brain cells, reducing whole brain volume.

    It was monkey-torturing a-holes who ran the study for R. Raygun to claim (laughably) that “marihuana kills brain cells” and could possibly be “the worst drug known” to humans.

    And that was the pos who sold us a sham arms-race, creating hundreds of thousands of ICBMs that were meant to destroy, not last intact in “storage.” Hmm. life-long GE employee selling an arms race to people who easily bought into the fallacious appeals to fear, appeals to militant authority that made no sense whatsoever, “if we die they have to all die too” as if that mattered.

    Taxpayers ended up with the burden of useless WMD, and will be stuck with the bill to clean up the radiation leakage. Taxpayers end up paying the bill too, for a fraudulent prohibition that pretends to be concerned, but has to lie and bombard people with myths and stereotypes.

    Cannabis does not change people into raving lunatics, any more than alcohol “causes” domestic violence, even though 50% at least, of murders are committed under the influence of alcohol.

    The stereotype of cannabis users does not hold any weight. But we do know a large number of violent crimes are committed under the influence of alcohol, most domestic violence occurs on Super Bowl Sunday, by drunks who had their team lose, or even win.

    Even if the false stereotype of being “lazy and unmotivated” is better than “wife-beating, violent waste of space.” Oh wait, we live in a Disneyland fantasy made up by propagandist lies, it’s far worse to use cannabis freely and get the munchies and supplement the endocannabinoids of which they are deficient, a real health problem ignored by free-rider lazy pieces of crap who indeed lack motivation, or conscience.

  25. And ironically, cannabis could have helped mitigate the symptoms of Alzheimer’s disease in that particular individual. Cannabis has been demonstrated numerous times to serve as a neuro-protective, protecting nerve cells in the central and peripheral nervous systems against damage, which is the opposite of “killing brain cells.”

    i recommend “Cannabis: Gateway to Health, How cannabis protects against cancer and Alzheimers,” very good book loaded with information.

    Also, “What if Cannabis Cured Cancer?” DVD, in which many esteemed researchers (who have not been discredited of lying just to keep a govt agency position) describe how cannabis kills abnormal growth and/or cancer cells, visible under microscope.

    In that video, is presented, the studies that show that people who use cannabis (smoked/burned) have less incidence of cancer than tobacco smoker, have less incidence of cancer than those who smoke nothing at all.


    If the “best and brightest” we have (being as facetious and ridiculous as possible) are running the show for us, we’ve been fooled into believing it. Why? Because the people occupying the seats of authority are spineless if not thieving degenerates who are watching the time clock for when the paychecks for crimes against humanity will stop being direct deposited.

    Learn a new skill that does not put you on par with real criminals, like the organized crime directly caused by prohibition. That’s a career tip.

  26. I agree that a study on the use of cannabis should be conducted on individuals that don’t drink alcohol of any kind. I enjoy the occasional drink (once every 3 months or so), but since being a daily user for the last 3 years, I would rather have cannabis than alcohol. It really helps my stress levels, depression, and appetite when I don’t eat normally anyway. I believe you have a more open mind to think about everything differently and sensibly. People will find a way to have it, regardless of it being illegal. It makes me a better parent. I was abused as a child and it’s helps me not become the parent that disciplined me aggressively. Without it, I’m uptight and over-worried about anything under the sun. Does it make me a bad person/parent? Absolutely not! Most times, my only way out of my depression is to smoke, rather than take the anti-depressants. All the other hardcore drugs out there were created because cannabis was made illegal. I’ve never touched anything but cannabis and don’t intend to and I’m in my mid 30’s. Side effects? Happiness, tolerable appetite, and motivation to lead a more productive life in light of all the stresses that life deals. Plus now the benefits of curing cancer and eliminating seizures? Legalize!

  27. This is the only way the DEA will allow cannabis to be looked at. They don’t allow positive results only negative.

  28. How do I get in on these tests? I’ve smoked tobacco since 13 and weed since 14 and am still alive and mostly normal. I held decent jobs for 28 years before I became disabled due to back and neck pain. Now I couldn’t work if I had to because of my “medicine” I took Lortab for 2 years and I don’t know why every one wants them so badly- my last bottle of 90 pills lasted me two years- so far.

  29. Preconditions to neurological issues could be exacerbated by cannabis consumption whether a child or an adult. The truth is buried beneath tons of personal and professional preconditions left behind from a corrupt judicial system allowing for confiscation of personal property without due process.

  30. Even if using marijuana causes a slight mental deficit, that deficit is slight compared to thousands of legal substances; i.e. the ones produced in pharmaceutical labs. I first tried it at age 15 (admittedly very young) and still use it at age 58. I served 4 years in the USMC as an aircraft avionics tech, got an honorable discharge, went on to college and graduated. Since then I have worked as an electronics tech and computer programmer/engineer. What else do these prohibitionists want of us citizens?

    All the while, I’ve been using marijuana! If there is any negative measurable effect, it is slight and should not be considered a reason to punish anyone for using it. Obviously to anyone with an IQ over 50 getting busted and locked up will have a vastly more negative effect on anyone’s life than simply using marijuana.

    I say again, as I’ve said in previous comments, that the true reason that marijuana has been illegal all these years are because of: lies, ignorance, greed, and racism.

  31. Maybe some gov. agencies do not kwow “50%” of pharmaceuticals are discovered “in Herbs”; such as Digoxin is The common Foxglove Herb…
    Marijuan is a Medicinal Herb; like “Hawthorne”
    Food Grade Hemp is a Nutritional Supplement.”

  32. Its simple look …..if you do not wish to partake in marijuana consumption,don’t purchase it ,but legalize it then everyone has a choice instead of just a hand full of stereotypical people who obviously wish to bully everyone else around,stop cheating the medical,finincial & leisure world,pharmaceutical companies and pharmacies are the biggest drug dealer ever,every thing they sell is worse by far than marijuana ,wakeup America!!!

  33. We need the revenue,all states,especially alabamians catchup with the Times alabamians and u other states and your idiot’ like Alabama’ governor Bentley,who need to take a powder puff break,he’s such a puss

  34. Bentley must have not got much attention growing up so he tries to hog the spotlight and override the huge majority of people’s vote,or maby it’s a ploy so he can get rich ,someone,he hopes will ,pay him to,disappear,ha…ha..ha

  35. How can the majority of the people’ vote be ignored,do we not matter we the tax payer’ ….we the people govern….our constitution is founded on th@ principle,do we not matter,is this the message our government is sending,it is if our huge majority of votes to legalize marijuana is overturned by a few or by one ,fuck th@ ,time to restructure when government’ ignore the people, your better off dead if your silenced people,grow some balls,stand the fuckup,else your already dead ,what are we leaving for our children if we let the government take our say & what kind of roll model’s do they have to lookup to…get ur head out of the sand ,so your future generation’ to have a figure head to lookup to& set examples for their children to learn by

  36. I want marijuana legalized but it seems like this site will fight anything studied that portrays marijuana as negative. I used marijuana excessively and had a few psychotic breakdowns. I can still feel lingering effects occasionally. I definitely overdid it but marijuana is by no means completely safe. It’s still much better than alcohol and nicotine but sometimes this site exaggerates as much as the idiots on the other side.

  37. 30 people studied; big deal. I don’t see how this test can be accurate.
    When I first heard of this, I assumed people were going under brain scans (MRI).
    Doing puzzles, and memory tests is not enough proof what so ever.
    Bill Maher (HBO) will have a field day with this.

  38. I’ve been smoking pot on average 3 times a day for over 40 years with a few breaks here and there. I just finished the spring semester at Bismarck State College taking 19 credits towards a degree for computer support specialist and maintained a 4.0 average so society’s idea that using pot makes you stupid is total bunk.

Leave a Reply