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Aim: A first aim of this thesis was to examine the relationship between intelligence in early adulthood and health outcomes, mainly cardiovascular morbidity and mortality, CVD, later in life using register-based data on Swedish men. A second aim was to increase understanding of the pathways between intelligence and CVD morbidity/mortality by studying the association between intelligence and smoking habits and nicotine dependence, who might act as mediators of the association between intelligence and mortality. Material and methods: The study populations in this thesis were created through record linkage of several national registers where IQ was collected from the Military Conscription Register. The analyses were based on all Swedish men born 1951 to 1984, depending on the paper, that went through conscription examinations. Also, for paper II parents of the men were analysed. Paper IV and V were based on Swedish male twins. The association between IQ and mortality was analyzed with Cox proportional hazards regression and conditional logistic regression models. The association between IQ and smoking status was analysed with linear, logistic and polytomous regression models and the association between IQ and nicotine dependence with quantitative genetic analyses. Results: In general, paper I-III support previous research about inverse associations between IQ and CVD morbidity/mortality. In addition to previous research our results revealed that IQ was associated with coronary heart disease, CHD, independently of socioeconomic position, SEP, (paper I) and that IQ was inversely and significantly associated with major subtypes of stroke (paper III). The strongest association found for hemorrhagic stroke. Further, the effect of IQ on mortality was found to be present also when offspring IQ was used as a proxy for parental IQ (paper II). In paper IV and V when the association of IQ and smoking and nicotine dependence was studied no evidence of a causal association was foun

It has been suggested that certain health behaviours, such as smoking, may operate as mediators of the well-established inverse association between IQ and mortality risk. Previous research may be afflicted by unadjusted confounding by socioeconomic or psychosocial factors. Twin designs offer a unique possibility to take genetic and shared environmental factors into account. The aim of the present national twin Study was to determine the interrelations between IQ at age 18, childhood and attained social factors and smoking status in young adulthood and mid-life. We studied the association between IQ at age 18 and smoking in later life in a population of 11 589 male Swedish twins. IQ was measured at military conscription, and data on smoking and zygosity was obtained from the Swedish Twin Register. Information on social factors was extracted from censuses. Data on smoking was self-reported by the twins at the age of 22-47 years. Logistic regression models estimated with generalised estimating equations were used to explore possible associations between IQ and smoking among the twins as individuals as well as between-and within twin-pairs. A strong inverse association between IQ and smoking status emerged in unmatched analyses over the entire range of IQ distribution. In within-pair and between-pair analyses it transpired that shared environmental factors explained most of the inverse IQ-smoking relationship. In addition, these analyses indicated that non-shared and genetic factors contributed only slightly (and non-significantly) to the IQ-smoking association. Analysis of twin pairs discordant for IQ and smoking status displayed no evidence that non-shared factors contribute substantially to the association. The question of which shared environmental factors might explain the IQ-smoking association is an intriguing one for future research., authorCount :6