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Twin studies suggest a genetic component in diurnal types. In 1981, a questionnaire sent to the Older Finnish Twin Cohort yielded responses from 2836 adult monozygotic (MZ) and 5917 like-sexed dizygotic (DZ) twin pairs with four category self-report on diurnal type. We used structural equation modelling to estimate genetic and environmental components of variance in morningness and eveningness. The model fitting was best when the morningness and the eveningness were analysed together. The ADE-model (including additive genetic, dominant genetic and non-shared environmental effects) fitted best to the data. ADE-models for men and women separately did not differ in a statistically significant manner from the combined model, and similarly ADE-models for young and old age groups separately did not differ either. The estimate for overall genetic effect (broad sense heritability) was 49.7% (95% confidence interval 46.4-52.8), with the remainder accounted for by environmental factors not shared by siblings. The variance component estimates for the underlying liability to diurnal type were 11.7% (95% CI 0-23.7) for additive genetic factors, 38.0% (24.7-51.3) for genetic factors due to dominance. Genetic effects thus account for about one-half of the interindividual variability in diurnal type in adults.
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