Psychological Medicine, 2000, 30, 1051–1061. Printed in the United Kingdom  2000 Cambridge University Press The genetic aetiology of somatic distress N.A. GILLESPIE,  G.ZHU, A. C.HEATH, I.B.HICKIE  N. G. MARTIN From Queensland Institute of Medical Research, University of Queensland, Joint Genetics Program, Brisbane, Queensland, School of Psychiatry, University of New South Wales, Academic Department of Psychiatry, St George Hospital and Community Health Service, Sydney, New South Wales, Australia ; and Department of Psychiatry, Washington University School of Medicine, St Louis, MO, USA ABSTRACT Background. Somatoform disorders such as neurasthenia and chronic fatigue syndrome are characterized by a combination of prolonged mental and physical fatigue. This study aimed to investigate the heritability of somatic distress and determine whether this dimension is aetiologically distinct from measures of depression and anxiety. Method. Measures of anxiety, depression, phobic anxiety, somatic distress and sleep difficulty were administered in a self-report questionnaire to a community-based sample of 3469 Australian twin individuals aged 18 to 28 years. Factor analysis using a Promax rotation, produced four factors : depression, phobic anxiety, somatic distress and sleep disturbance. Multivariate and univariate genetic analyses of the raw categorical data scores for depression, phobic anxiety and depression were then analysed in Mx1.47. Results. Univariate genetic analysis revealed that an additive genetic and non-shared environmental (AE) model best explained individual differences in depression and phobic anxiety scores, for male and female twins alike, but could not resolve whether additive genes or shared environment were responsible for significant familial aggregation in somatic distress. However, multivariate genetic analysis showed that an additive genetic and non-shared environment (AE) model best explained the covariation between the three factors. Furthermore, 33 % of the genetic variance in somatic distress was due to specific gene action unrelated to depression or phobic anxiety. In addition, 74 % of the individual environmental influence on somatic distress was also unrelated to depression or phobic anxiety. Conclusion. These results support previous findings that somatic symptoms are relatively aetiologically distinct both genetically and environmentally from symptoms of anxiety and depression. INTRODUCTION Somatoform disorders such as neurasthenia and chronic fatigue are characterized by a com- bination of prolonged fatigue, and disabling neuropsychological and neuromuscular symp- toms (Lloyd et al. 1990; Angst & Koch, 1991; Hickie et al. 1995). Differences between these syndromes are qualitative and reflect variations in duration criteria rather than symptom con- structs (Hickie et al. 1997). The ICD-10 (World Health Organization, 1992) classification system  Address for correspondence : Dr Nathan A. Gillespie, Epi- demiology Unit, Queensland Institute of Medical Research, Post Office, Royal Brisbane Hospital, Brisbane, QLD 4029, Australia. includes a formal diagnosis of neurasthenia based on mental and physical fatigue of at least 3 months duration. However, despite current international and epidemiological interest in this disorder, DSM-IV continues to include pro- longed fatigue syndromes within the ‘Un- differentiated Somatoform Disorders-300.81 ’ category (American Psychiatric Association, 1994) and largely discounts the notion of discrete syndromes like neurasthenia. Critics of the independence of somatic distress disorders base their objections on the co- morbidity of such syndromes with conventional measures of anxiety and depression (Wessely & Powell, 1989 ; Goldberg & Bridges, 1991). 1051