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Researchers: Smokers are Stupid (9/15/06)
It's been known for some time that smokers tend to be less educated and make less money than non-smokers. Now, Canadian scientists report that not only is lower IQ a predictor of smoking, but smoking itself may reduce intelligence.
To gather their data, the researchers followed 112 children from infancy to early adulthood, testing their IQ and mental performance at 9-12 years old and again at 17-21 years old. They found that on average, the people who had not become smokers had IQs 10% higher than their smoking (9 cigarettes or more a day) peers. At first glance, this could simply be the result of smarter people being less likely to take up a dangerous and expensive addiction. To try to answer the question of cause and effect, the researchers examined their early test results, taken when the people being studied were likely too young to have started smoking yet.
Perhaps not surprisingly, the children that grew up to be smokers really were a little less intelligent (on average) even before their first cigarette. But, pre-existing differences didn't explain all of the gap in adult IQ. Instead, they found that the adult smoker's IQs had declined since childhood (by about 4%), even after adjusting for other known confounds such a marijuana use. (The non-smokers IQs had not gone down over time.)
Whether this decline in IQ was due just to smoking cannot be definitively determined from this research; it's possible that there is some other factor that both increases the risk of smoking and reduced intellectual performance. There is some supporting evidence that's also good news for smokers, though; those that had been smokers but quit returned to their pre-use IQs.
That smoking could alter cognitive performance isn't really a surprising idea: Most (perhaps all) psychoactive drugs, if used chronically, subtly alter the brain. IQ (and even brain density) goes down in heavy long-term drinkers. Heavy MDMA ('ecstasy') use is associated with increased anxiety and depression, etc. When you keep kicking your brain with a drug, your brain will eventually kick you back.