Journal: British Journal of Psychiatry. 2006 Sep;189:247-53. Authors and Affiliation: Fowler TA, Rice F, Thapar A, Farmer A. Section of Child and Adolescent Psychiatry, Department of Psychological Medicine, Cardiff University, Heath Park, Cardiff, Wales CF14 4XN, UK. email@example.com . PMID: 16946360 [PubMed – in process] BACKGROUND: Medically unexplained disabling fatigue in young people is familial and frequently associated with depressed mood. AIMS: To examine the degree of sharing of genetic and environmental influences on the symptoms of depression and fatigue in this age group. METHOD: The parents of twins aged 8-17 years, derived from a population-based register, completed a questionnaire regarding lifetime-ever disabling fatigue in both twins. Twins aged 11 years or over completed the Mood and Feelings Questionnaire. The genetic and environmental influences on fatigue and the relationship with depression were examined using bivariate genetic analysis. RESULTS: Parent-rated data were obtained for 1468 twin pairs (65%) and self-rated data from 930 older twin pairs (58%). Bivariate analysis of fatigue and depression suggested that genetic and environmental influences on disabling fatigue were mainly specific to fatigue. CONCLUSIONS: Unexplained disabling fatigue in childhood is substantially familial and has mainly an independent aetiology from depression.