Dumb people smoke pot. Pot makes you dumb.

Separated at birth: Experiments on identical children are highly controversial. © CNN

Which statement is right? Both? Neither? The argument among experts remains as fierce as ever. Despite extensive research, the truth seems to be tantalisingly still just beyond our reach.

Are we born as we are? Or do we become who we are? The question is as old as civilisation.

The photograph at the top of this story is only famous because of that question. It is from a documentary film Three Identical Strangers released last year. It tells the story of Edward, David and Robert, identical US triplets born to a teenage single mother and separated at birth in 1961.

Psychologists wanted to know the answer to the question of nature versus nurture. In a secretive experiment, they decided to send the triplets to different families for adoption and see how they turned out.

Unwittingly, they created a tragedy. (Look into their eyes.) When the brothers discovered each other at 19, they were thrilled. But, in a few years, joy turned to misery. Edward committed suicide. David and Robert are estranged.

And the psychologists decided they could not conclude whether nature or nurture was dominant.

Today, a comparable riddle surrounds research into the effects of marijuana. There have been calls for similar experiments on identical siblings. What would happen if one was a pot smoker and the other wasn’t?

Scientists have linked teenage marijuana use with a host of undesirable outcomes: difficulty in paying attention, weaker memory, and lower verbal ability and intelligence.

But is the drug itself to blame?

Two long-term studies of twins suggest that other factors are at fault, at least as far as vocabulary skills are concerned.

In one study, children who went on to become marijuana users were not as bright to begin with as their peers who avoided it.

And in both studies, drug-using teens fared no worse on IQ tests than their non-using twins in the same household, suggesting that some other factor was to blame, the authors wrote.

Something in the family environment perhaps? Or simply the fact that kids who gravitate toward pot use may be less motivated to try hard in school?

Results of the two studies, described in a paper in Proceedings of the National Academy of Sciences, surely, will not settle the debate.

One of the studies started with 2,277 Minnesota youths aged 11 to 12, 93% of whom were white. The other tracked 789 adolescents from the Los Angeles area, aged 9 to 10, three-quarters of whom were racial minorities or of mixed ethnicity.

By the time they reached their late teens, 36% of the Minnesotans said they used marijuana. A subset of them was included in the study because they had shown disruptive behaviour and “academic disengagement”.

In the California group, 60% of the teens reported they had used the drug, but they were 20 when surveyed — about two years older than their Minnesota counterparts.

Dumb people smoke pot. Pot makes you dumb. Which statement is right? Both? Neither?

Causal effects

Mostly, that dumb people smoke pot, says the writer Alex Tabarrok. Twin studies do not show that cannabis reduces cognition (as above). Specifically, people with poor executive function when young are far more likely to become heavy cannabis users.

Not good enough, says Madeline Meier, an Arizona State assistant Professor of Psychology. She led a previous study in which participants were followed up to age 38, in which marijuana use did appear to lead to cognitive decline. “The problem is that for some teens,” she says, “low-level cannabis use leads onward to long-term dependence on cannabis when they become adults.”

You Decide

  1. It is wrong to do experiments on people when they are children?
  2. If A causes B, and B causes C, then does A cause C?


  1. What characteristics were you born with? Make a list of five things that you think were hard-wired in your nature.
  2. Think about something you are good at and that you like doing. Are you good at it because you like it. Or do you like it because you are good at it?Write your answer on one side of paper.

Some People Say...

“Does drinking lots of alcohol make you dumb or do dumb people drink lots of alcohol?”

Anonymous comment on an internet forum

What do you think?

Q & A

What do we know?
An important study of 2,235 British teenagers, between ages 8 and 16, was published in 2016. After adjusting for a range of 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.”
What do we not know?
Whether smoking cannabis can make the mundane more tolerable. The theory goes that people who would otherwise make more of themselves because they’re bored or stuck in a rut find pot makes their present situation more “acceptable”. This is conjecture.

Word Watch

Nature versus nurture
The extent to which particular aspects of behaviour are a product of either inherited/genetic or acquired/learned influences.
One of two or more individuals having one or both parents in common.
IQ tests
An intelligence quotient (IQ) is a total score derived from several standardised tests designed to assess human intelligence.
Proceedings of the National Academy of Sciences
A peer-reviewed multidisciplinary scientific journal. It is the official journal of the National Academy of Sciences, published since 1915.


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