To estimate the contribution of genes and shared family environment to the liability to DSM-IV major depression and to examine the influence of certain proband characteristics on twin concordance.
We studied 177 probands with major depressive disorder ascertained via the Maudsley Hospital Twin Register (London, England) and their same-sex cotwins. Diagnostic assessments were carried out blind to zygosity and information on the other member of the twin pair. Probandwise concordances were used to compute correlations in liability, and model fitting was performed using maximum likelihood procedures.
The probandwise concordance was 46% in monozygotic (n=68) and 20% in dizygotic (n=109) twins, a statistically highly significant difference. There was no evidence of a sex difference in heritability or of shared environmental effects. Depending on the assumed population risks for DSM-IV, major depression estimates of heritability were between 48% and 75%. A duration of longest episode of less than 13 months, multiple episodes, and an endogenous rather than neurotic pattern of symptoms, as established by the International Classification of Diseases, Ninth Revision, in the proband were associated with a trend toward a higher monozygoticdizygotic concordance ratio. Using log-linear analysis, only the association between duration of episodes and monozygotic-dizygotic concordance ratio was significant.
Liability to DSM-IV major depression has a substantial heritable component, and there is no evidence of an effect of shared family environment. Some proband characteristics, especially shorter duration of episodes, may be associated with a larger degree of genetic determination.