Cigarette smokers have lower IQs than non-smokers and the more a person smokes, the lower his IQ, according to a new international study.
The study of more than 20,000 Israeli military recruits found young men who smoked a pack of cigarettes a day or more had IQ scores that were 7.5 points lower than non-smokers.
The research has been backed by Auckland University expert Dr Marewa Glover, who said it was proof of a successful campaign by the tobacco industry to target those with lower IQs by using devices such as cartoons and free samples.
"They already know that people with poorer cognitive functioning, and populations where that is concentrated, are going to be more vulnerable to marketing tactics that are not dependent upon literacy skills," said Dr Glover, the director of the Centre of Tobacco Control Research at the university.
The study, by Dr Mark Weiser and colleagues from Sheba Medical Center in Tel Hashomer, found 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.
The study was based on tests to get into the Israeli military.
"Adolescents with poorer scores on cognitive tests might be targeted for programmes designed to prevent smoking," the researchers concluded in the journal Addiction.
While there was evidence for a link between smoking and lower IQ, many studies had relied on intelligence tests given in childhood, and had also included people with mental and behavioural problems, who were both more likely to smoke and more likely to have low IQs, the Israeli team said.
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, about 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 socio-economic status measured by how many years of formal education a recruit's father had completed.
The findings suggested lower-IQ individuals were more likely to choose to smoke, rather than that smoking made people less intelligent, Weiser and his team concluded.
Dr Glover said known risk factors for smoking in New Zealand included whether a person's parents smoked, their socio-economic status and whether they were Maori or Pacific Islanders.
Anti-smoking group ASH said a survey of young people in 2007 found 57 per cent had never smoked, a lower proportion than in the Israeli study.
Maori women were twice as likely to smoke than women in the general population.