All suicides were ascertained for a 100-year period (1880 to 1980) in a study of mental illness among the Old Order Amish. The majority (92%) of the 26 cases were diagnosed with a major affective disorder and were situated in multigenerational families with heavy loading for bipolar, unipolar, and other affective-spectrum illnesses. The suicides clustered in four primary pedigrees, and the role of inheritance was suggested by the way in which suicides followed the distribution of affective disorders in these kinship lines. We believe these extended pedigrees provide presumptive evidence of genetic factors in both suicides and affective disorders.
Egeland JA, Sussex JN. Suicide and Family Loading for Affective Disorders. JAMA. 1985;254(7):915-918. doi:10.1001/jama.1985.03360070053022