Prolonged fatigue of ≥1 month duration and chronic fatigue of ≥6 months duration are both commonly seen in clinical practice, yet little is known about the etiology or epidemiology of either symptom. Chronic fatigue syndrome (CFS), while rarer, presents similar challenges in determining cause and epidemiology. Twin studies can be useful in elucidating genetic and environmental influences on fatigue and CFS. The goal of this paper was to use biometrical structural equation twin modeling to examine genetic and environmental influences on fatigue and to investigate whether these influences varied by gender. A total of 1,042 monozygotic twin pairs and 828 dizygotic twin pairs who had completed the University of Washington Twin Registry survey were assessed for three fatigue-related variables: prolonged fatigue, chronic fatigue, and CFS. Structural equation twin modeling was used to determine the relative contributions of additive genetic effects, shared environmental effects, and individual-specific environmental effects to the three fatigue conditions. In women, tetrachoric correlations were similar for monozygotic and dizygotic pairs for prolonged and chronic fatigue, but not for CFS. In men, however, the correlations for prolonged and chronic fatigue were higher in monozygotic pairs than in dizygotic pairs. About half the variance for both prolonged and chronic fatigue in males was due to genetic effects and half due to individual-specific environmental effects. For females, most variance was due to individual environmental effects.
Twin analyses of fatigue. Twin Res Hum Genet. 2007; 10(5):729-733.