Smoking lowers IQ

Young people who smoke regularly are likely to have markedly lower intelligence levels than non-smokers.

Previous studies indicate that people with lower intelligence quotient (IQ) scores are more likely to become cigarette smokers. However, IQ scores of siblings at odds for smoking and of adolescents who began smoking between ages 18 - 21 years have not been studied systematically. To study this, researchers made 20,221 eighteen years old Israeli military recruits complete a smoking questionnaire. The cognitive functioning of the participants was assessed by the military using standardised tests equivalent to IQ tests.

Twenty-eight percent of the teenagers polled smoked one or more cigarettes a day, three percent admitted to having smoked in the past, while 68 percent of the young men had never smoked.

Those who smoked a pack or more of cigarettes a day averaged an IQ seven and a half points lower than those who did not smoke. An average smoker was found to have an IQ of 94, while non-smokers of the same age averaged 101. Those who smoked more than a pack a day had particularly low IQs of around 90. The average intelligence IQ score ranges from 84 to 116 points. Crucially, brothers scored differently depending on whether or not they smoked. Despite similar environmental conditions, non-smoking siblings achieved higher IQs than their smoking brothers.

It is unclear whether smoking causes IQ levels to drop or whether less intelligent people are simply more inclined to smoke.  Still, the study shows a relationship between low IQ and smoking.