Background Although previous studies indicate that people with lower intelligence quotient (IQ) scores are more likely to become cigarette smokers, IQ scores of siblings discordant for smoking and of adolescents who began smoking between ages 18–21 years have not been studied systematically.
Methods Each year a random sample of Israeli military recruits complete a smoking questionnaire. Cognitive functioning is assessed by the military using standardized tests equivalent to IQ.
Results Of 20 221 18-year-old males, 28.5% reported smoking at least one cigarette a day (smokers). An unadjusted comparison found that smokers scored 0.41 effect sizes (ES, P < 0.001) lower than non-smokers; adjusted analyses remained significant (adjusted ES = 0.27, P < 0.001). Adolescents smoking one to five, six to 10, 11–20 and 21+ cigarettes/day had cognitive test scores 0.14, 0.22, 0.33 and 0.5 adjusted ES poorer than those of non-smokers (P < 0.001). Adolescents who did not smoke by age 18, and then began to smoke between ages 18–21 had lower cognitive test scores compared to never-smokers (adjusted ES = 0.14, P < 0.001). An analysis of brothers discordant for smoking found that smoking brothers had lower cognitive scores than non-smoking brothers (adjusted ES = 0.27; P = 0.014).
Conclusion Controlled analyses from this large population-based cohort of male adolescents indicate that IQ scores are lower in male adolescents who smoke compared to non-smokers and in brothers who smoke compared to their non-smoking brothers. The IQs of adolescents who began smoking between ages 18–21 are lower than those of non-smokers. Adolescents with poorer IQ scores might be targeted for programmes designed to prevent smoking.