Data from more than 20,000 18-year-old Israeli men has revealed that IQ scores are lower in male adolescents who smoke compared to non-smokers, and in twin brothers who smoke compared to their non-smoking brothers. The average IQ for a non-smoker was about 101, while the smokers' average was about 94, with those who smoked more than a pack a day being lower still, at about 90. 28% of the sample smoked one or more cigarettes a day, 3% identified as ex-smokers, and 68% said they never smoked.
(2010). Cognitive test scores in male adolescent cigarette smokers compared to non-smokers: a population-based study.
Addiction. 105(2), 358 - 363.