OBJECTIVE: To disentangle the influences on health of selection processes related to genetic and rearing environmental factors from factors related to marriage benefits. We compared health status among same-sex male and female twin pairs who lived together during childhood and were discordant or concordant on adult marital status. METHODS: A cross-sectional survey of a random sample of middle-aged Danish twins was conducted in 1998 to 1999. This study included 1175 same-sex twin pairs (52.5% monozygotic (MZ) and 47.5% dizygotic (DZ)). Data were obtained on adult marital status and on height, body mass index (BMI), depression symptoms, self-rated health, cognitive function, physical activity, smoking, and alcohol intake. RESULTS: Among all 2350 individual twins, men who were divorced/widowed or never married had higher depression scores, lower cognitive test scores, lower physical activity scores, and were also less often moderate drinkers and nonsmokers compared with married men. Divorced/widowed women had higher depression scores and those divorced/widowed or never married were more often smokers than married women. Within twin pairs discordant on marital status, the divorced/widowed twin had higher average depression scores and was more likely to be a smoker. Never married twins had lower physical activity scores and never married male twins had higher BMI and higher depression scores than their married co-twin. CONCLUSION: This study suggests that the relationships of adult divorce with depression and smoking in Danish twins are due to the stressful effects of marital dissolution, but that marital differences in other health and behavioral outcomes are most consistent with selection effects related to genetic or rearing environmental factors.