Genetic and non-genetic subtypes of major depressive disorder

Background We aimed to examine the hypotheses that major depression is aeti ologically heterogeneous consisting of a mixture of 'genetic' and 'non-gene tic forms or, alternatively, a mixture of one form that is 'pure' depressio n and another that has a familial relationship with alcohol dependency or o ther disorders. Method One hundred and eleven twin pairs (44 monozygotic, 67 dizygotic) whe re the proband had received treatment for DSM - IV major depression were as certained via a hospital register. Family history information on parents an d siblings was obtained from the proband, co-twin or both. Diagnoses on par ents and siblings were made blind to twin zygosity or concordance and compa red in the relatives of concordant Versus discordant twins. Results The lifetime prevalence and age-corrected risk of depression were n o different in the relatives of concordant and discordant twin pairs. There was a marginally significant increase in the rate of alcohol abuse or depe ndence among the relatives of concordant twins but no difference between co ncordant and discordant pairs in respect of other axis I diagnosis among fa mily members. Conclusions The results argue against genetic heterogeneity and suggest tha t major depression cannot usefully be divided into genetic and non-genetic forms or into 'pure' depression and depression associated with other disord ers such as alcohol dependency.