Studies of patterns of major depression in families suggest that 50 to 70% of the predisposition to major depression is caused by genes. People with recurrent depression and earlier ages of onset have more relatives with depression. There are probably at least several interacting genes, rather than a single gene as in some disorders. The identification of predisposing genes is likely to lead to greater understanding of the brain mechanisms involved in severe depression. This could lead to the discovery of new treatments.