Socioeconomic position and twins' health: A life-course analysis of 1266 pairs of middle-aged Danish twins

Background: The association between socioeconomic circumstances and health in adulthood could come about through processes that may be divided into factors experienced early in life and those experienced in later adulthood. In order to disentangle the influences on health of the early genetic, prenatal and rearing environmental factors from environmental factor later in life, we compared the health status among male and female twin pairs who lived together during childhood and were discordant or concordant on adult socioeconomic position. Methods: A cross-sectional survey among a random sample of middle-aged Danish twins was conducted in 1998-99. The study population included 1266 like-sex twin pairs [52.5% monozygotic (MZ) and 47.6% dizygotic (DZ)]. Data were obtained on childhood and adult social class and on height, BMI, grip strength, depression symptoms, self-rated health, cognitive function, physical activity, smoking, alcohol and food intake. Results: The expected associations between the individual twins' adult social class and health measures were observed. Among DZ male twins discordant on adult social class, the higher social class twin was on average significantly taller and had higher cognitive test scores. Among DZ female twins discordant on adult social class, the higher social class female twin was more physically active and had a higher cognitive test score. There were no significant health disparities or behavioural differences between the members of MZ twin pairs discordant on adult social class. For most health outcomes, the variability within twin pairs was related to zygosity (higher for DZ than for MZ) but not to social class. Conclusion: This study suggests that the relationship between adult social class and health outcomes in Denmark is due mainly to selection effects rather than a causal effect of social class exposures on health and behaviour.