Dr. Mark Weiser and colleagues from Sheba Medical Center in Tel Hashomer found that young men who smoked a pack of cigarettes a day or more had IQ scores 7.5 points lower than non-smokers.
“Adolescents with poorer IQ scores might be targeted for programmes designed to prevent smoking,” they conclude in the journal Addiction.
While there is evidence for a link between smoking and lower IQ, many studies have relied on intelligence tests given in childhood, and have also included people with mental and behavioral problems, who are both more likely to smoke and more likely to have low IQs, Weiser and his team noted.
To better understand the smoking-IQ relationship, the researchers looked at 20,211 18-year-old men recruited into the Israeli military.
The group did not include anyone with major mental health problems, because these individuals are disqualified from military service.
According to the researchers, 28 per cent of the study participants smoked at least one cigarette a day, around 3 per cent said they were ex-smokers, and 68 per cent had never smoked.
The smokers had significantly lower intelligence test scores than non-smokers, and this remained true even after the researchers accounted for socioeconomic status measured by how many years of formal education a recruit’s father had completed.
The average IQ for non-smokers was about 101, while it was 94 for men who had started smoking before entering the military.
IQ steadily dropped as the number of cigarettes smoked increased, from 98 for people who smoked one to five cigarettes daily to 90 for those who smoked more than a pack a day.
IQ scores from 84 to 116 are considered to indicate average intelligence.
Recruits aren’t allowed to smoke while intelligence tests are administered, the researchers said, so it is possible that withdrawal symptoms might affect smokers’ scores.
To address this issue, they also looked at IQ scores for men who were non-smokers when they were 18 but started smoking during their military service.
These men also scored lower than never-smokers, 97 points, on average) “indicating that nicotine withdrawal was probably not the cause of the difference,” the researchers said.
The researchers also compared IQs for 70 pairs of brothers in the group in which one brother smoked and the other did not. Again, average IQs for the non-smoking sibling were higher than for the smokers.
The findings suggest that lower IQ individuals are more likely to choose to smoke, rather than that smoking makes people less intelligent, Weiser and his team conclude.