Are non-smokers smarter than smokers?

Cigarette smokers have lower IQs than non-smokers, and the more a person smokes, the lower their IQ, according to a study of over 20,000 Israeli military recruits.

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 programs designed to prevent smoking," conclude Weiser and colleagues 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.

The studies have also included people with mental and behavioural problems, who are both more likely to smoke and more likely to have low IQs, Weiser and his team say.
Military recruits

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% of the study participants smoked at least one cigarette a day, around 3% said they were ex-smokers, and 68% 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.