Background. Fatigue is a complex symptom associated with many physiological, psychological and pathological processes. Its correlates and typology remain inadequately understood.

Method. These data were from two large, longitudinal twin studies. Trained interviewers enquired as to the presence of a [ges ]5 day period in the previous year of fatigue or tiredness that interfered with daily activities. A range of potential correlates was assessed in a structured interview: demography; health beliefs; the presence of nine physical disorders; mood, anxiety and addictive disorders; neuroticism and extraversion; recollections of parental rearing; and nine stressful life events. Statistical analyses included logistic regression, CART, MARS, latent class analysis and univariate twin modelling.

Results. Data were available for interfering fatigue (IF) on 7740 individual twins (prevalence 9·9% in the previous year). IF was significantly associated with 42 of 52 correlates (most strongly with major depression, generalized anxiety disorder, reported major health problems and neuroticism). Multivariate analyses demonstrated that IF is a highly complex construct with different sets of correlates in its subtypes. There were two broad clusters of correlates of IF: (a) major depression, generalized anxiety disorder and neuroticism; and (b) beliefs of ill health coexisting with alcoholism and stressful life events. Twin analyses were consistent with aetiological heterogeneity – genetic effects may be particularly important in women and shared environmental effects in men.

Conclusions. IF is a complex and common human symptom that is highly heterogeneous. More precise understanding of the determinants of IF may lead to a fuller understanding of more extreme conditions like chronic fatigue syndrome.