Suicide and suicidal behavior is highly familial, and appears to be familially transmitted independently from the familial transmission of psychiatric disorder per se (1). Adoption, twin, and family studies support the view that the etiology of the familial transmission of suicidal behavior is at least in part genetic, and may be mediated by the transmission of intermediate phenotypes, such as impulsive aggression. In addition, there may be environmental causes for familial transmission, including imitation, and the intergenerational transmission of family adversity. In this review, we cover the evidence supporting the familial transmission of suicidal behavior, possible genetic and environmental explanations for this phenomenom, describe putative intermediate phenotypes, and discuss the contributory roles of early child-rearing and concurrent familial environmental stressors to suicidal risk. A better understanding of the mechanisms for the familial transmission of suicidal behavior can help to shed light on etiology, identify individuals at high risk for the development of incident suicidal behavior, and frame targets for intervention and prevention.Adoption studies (see Table 1) Three adoption studies have been conducted, all using the same Danish adoption registry. Kety et al., in a study designed to examine the genetics of schizophrenia and mood disorders, found a non-significant trend towards higher concordance for suicide in biological, compared to adoptive relatives of adoptees who committed suicide (2). Subsequently, a second study compared the rates of suicide among the biological and adoptive relatives of adoptees who committed suicide vs. biological and adoptive relatives of a matched living adoptee control group in Denmark (3). This study found a six-fold higher rate of suicide in the biological relatives of the suicide vs. those of the control adoptees, and an absence of suicide among the adopted relatives of the suicide vs. control adoptees supporting a genetic rather than environmental etiology (see Table 1). The rate of suicide was higher in the biologic relatives of suicide adoptees regardless of whether the adoptees were psychiatric patients or not. However, it was not possible to determine if the genetic liability to suicide was attributable to the transmission of major psychiatric disorders or to a suicide diathesis per se.In a third adoption study using this registry, a comparison of biological and adoptive relatives of adult adoptees with mood disorder and matched unaffected adoptees were examined (4), revealed a 15-fold increase in suicide among the biological relatives of the mood-disordered adoptees vs. those of the unaffected adoptees (4). This finding supports the role of mood disorder in the genetics of suicide. However, the greatest increased risk for suicidal behavior was found in the relatives of those probands with "affect reaction," a diagnosis akin to Corresponding Author: Dr. David Brent, BrentDA@upmc.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript th...