Off Topic: Critical Thinking about Marijuana Use

I don’t smoke pot, so I don’t have a significant personal stake in questions about the risks and dangers of marijuana use.  I am glad that my state recently legalized marijuana, but I think it should be legal whether or not it has significant health and safety risks, just like smoking tobacco and drinking alcoholic beverages is legal even though these activities clearly have significant health and saftey risks.

Ever since I was a young person I have noticed that many people have a strong prejudice towards belief that marijuana use has serious health and saftey risks.  But the evidence never seems to support the desire of many to find some serious risk or danger with the use of this substance.

I recently read an article in the Seattle Times with this title:

Heavy pot use can damage short memory

The article summarized a recently published study on marijuana use.  I immediately thought to myself, “I bet that they found a fairly small degree of short-term memory degradation correlated with heavy marijuana use and no other significant impacts, and are sounding the alarm here, when there is very little to be concerned about, as has been the case with dozens or hundreds of other studies like this over the past several decades.”  When I read the article, I found that my initial hunch was spot on.

The real implication of the study is that even very heavy use of marijuana has very little impact on cognitive functioning.  The study basically shows that marijuana is an extremely safe and low-risk drug, at least in terms of impacts to one’s mental abilities.

1. “the subjects took a battery of tests designed to assess cognitive ablilities: memory, focus, ability to make quick decisions, etc.”

Lots of mental abilities were tested, so one would expect that it would be somewhat likely that ONE of the various tests would show a negative impact correlated with heavy marijuana use, even if there was no actual causal connection.  The more tests you run, the greater the chance of coming up with a false correlation, a correlation that happens by chance rather than because of a causal connection between the drug and the ability being tested.

Although several different sorts of cognitive abilities were tested, only ONE showed a significant correlation between heavy marijuana use and lower scores on that cognitive ability test:

2. “…other cognitive abilities researchers tested–focus and processing speed–did not seem to be significantly impacted by heavy marijuana use.”  “…you can smoke weed literally every day for five years, and not have it impact your problem-solving abilities or your ability to focus at all.”

The researchers “examined data on the marijuana habits of nearly 3,400 Americans over a 25-year period.”

Here is where they discovered a correlation between heavy marijuana use and lower scores on a cognitive ability test:

The subjects were told “a list of 15 words” and asked to memorize the words.  25 minutes later, the subjects were asked to “recall all the words to the best of their ability.”  The people who never smoked marijuana or only smoked it occasionally remembered 9 words on average.  But people who “smoked pot every single day over a period of five years” could remember only 8.5 words on average.  Holy Shit!  If you have to quickly memorize lists of a dozen or more words, then you are at risk of remembering one less word than other people who don’t smoke if you smoke pot every single day for five years.

The effect appears to increase as the number of years of heavy marijuana use increases, so it is estimated that if a group of people smoke pot every day for 25 years, these people would remember on average “2.5 fewer words” than a group of people who “had smoked occasionally or not at all over the same period”.  So, smoking pot every day for 25 years would likely result in being able to quickly memorize a couple less words out of a list of 15 words than one could have done otherwise.  One might only recall six or seven of the 15 words instead of nine of them.

So, this study of the effects of marijuana use on cognitive abilities showed that daily use of marijuana for five years has practically no significant impacts on any cognitive abilities, and that in order to get even a small impact on short-term memory, one would have to smoke pot every day for 25 years.

The portion of the population that smokes pot every day for several years is small.  In the study, about 8% of the subjects had more than five “marijuana-years of exposure”.  Five “marijuana-years” could be either from smoking pot every day for five years, or from smoking pot every other day for ten years, or smoking pot once every three days for fifteen years, and so on.

The article does not mention whether anyone in the study had more than ten marijuana-years of exposure, but that would probably be only a fraction of the 8% who had more than five marijuana-years of exposure.  And those who have had more than 20 marijuana-years of exposure would be only a fraction of that fraction.  So, the small impact of being able to memorize a couple fewer words than others who don’t smoke marijuana or don’t smoke frequently, is only a risk for probably less than 2% of the population.

The lesson to be learned:  

When there is a widespread prejudice against something (like marijuana use), be suspicious of headlines and summaries of reports of research that has supposedly discovered correlations of that thing with some risk or danger or other undesireable outcomes.

Read the details.  The devil is in the details of such research findings.  People and reporters are looking for evidence that supports their prejudices, and they are not so good at looking for evidence that goes contrary to their prejudices.