Genetic and non-genetic subtypes of major depressive disorder

We aimed to examine the hypotheses that major depression is aetiologically heterogeneous consisting of a mixture of 'genetic' and 'non-genetic' forms or, alternatively, a mixture of one form that is 'pure' depression and another that has a familial relationship with alcohol dependency or other disorders.

One hundred and eleven twin pairs (44 monozygotic, 67 dizygotic) where the proband had received treatment for DSM-IV major depression were ascertained via a hospital register. Family history information on parents and siblings was obtained from the proband, co-twin or both. Diagnoses on parents and siblings were made blind to twin zygosity or concordance and compared in the relatives of concordant versus discordant twins.

The lifetime prevalence and age-corrected risk of depression were no different in the relatives of concordant and discordant twin pairs. There was a marginally significant increase in the rate of alcohol abuse or dependence among the relatives of concordant twins but no difference between concordant and discordant pairs in respect of other axis I diagnosis among family members.

The results argue against genetic heterogeneity and suggest that major depression cannot usefully be divided into genetic and non-genetic forms or into 'pure' depression and depression associated with other disorders such as alcohol dependency.