Context: Twin studies using selected samples have shown high heritability for Alzheimer disease (AD).
Objective: To evaluate genetic and environmental influences on AD in a fully ascertained population of older twins, including like- and unlike-sex pairs.
Design: Five-group quantitative genetic model: male monozygotic twins, female monozygotic twins, male dizygotic twins, female dizygotic twins, and unlike-sex twins.
Setting and participants: All twins in the Swedish Twin Registry aged 65 years and older. The study included 11,884 twin pairs, among whom were 392 pairs in which 1 or both members had AD.
Main outcome measures: All individuals were screened for cognitive dysfunction. Suspected cases of dementia and their co-twins received complete clinical diagnostic evaluations for AD. Estimates of heritability, shared environmental influences, and nonshared environmental influences, adjusting for age, were derived from the twin data.
Results: Heritability for AD was estimated to be 58% in the full model and 79% in the best-fitting model, with the balance of variation explained by nonshared environmental influences. There were no significant differences between men and women in prevalence or heritability after controlling for age. Within pairs concordant for AD, intrapair difference in age at onset was significantly greater in dizygotic than in monozygotic pairs, suggesting genetic influences on timing of the disease.
Conclusions: In the largest twin study to date, we confirmed that heritability for AD is high and that the same genetic factors are influential for both men and women. However, nongenetic risk factors also play an important role and might be the focus for interventions to reduce disease risk or delay disease onset.