Genetic and environmental influences on individual differences in cortisol level and circadian rhythm in middle childhood

► We examine genetic and environment contributions to individual variation in diurnal cortisol. ► Cortisol level ascertained in the morning showed the highest additive genetic variance ► The diurnal rhythm had an additive genetic component, particularly across the morning hours. ► Shared environment factors account for the majority of variation in cortisol during all parts of the day.