How does IQ affect onset of smoking and cessation of smoking--linking the Swedish 1969 conscription cohort to the Swedish survey of living conditions.

OBJECTIVE: To examine the association between intelligence quotient (IQ) measured at ages 18 to 20 and onset of smoking, and the association between IQ and smoking cessation.
METHODS: Data on IQ, smoking, mental health, and social background among 49,321 Swedish men born 1949 to 51, collected at conscription for military service in 1969, were used. The association between IQ and smoking cessation was investigated among those 694 members of the full cohort also interviewed in the Swedish Level of Living Conditions study 1981 to 2002.
RESULTS: Lower IQ measured at ages 18 to 20 was weakly associated with increased prevalence of smoking, independently of indicators of mental illness and social misbehavior measured in late adolescence. By contrast, smoking cessation later in life among those who smoked at ages 18 to 20 was not associated with IQ. Among smokers, lower IQ was significantly associated with a lower level of smoking after adjusting for other factors.
CONCLUSION: Low IQ was associated with an increased prevalence of smoking in adolescence. However, the main part of this association disappeared after adjustment for measures of mental health and social function in early life. IQ was not associated with likelihood of quitting smoking.