behavior appears to be on the order of 30–50 percent, family-environmental causes for suicidal behavior, such as abuse, must also be considered, as both independent factors and those that may interact with genetic vulnerability. Environmental influences are evidenced by the large shifts in rates of youth suicide in the United States and other Westernized countries over the last 20 years although the genetic makeup of the population has not changed appreciably over this short period of time.

The majority of those who complete and attempt suicide have evidence of at least one, and often more, major mental illness (see Chapter 3). The most common disorders associated with suicide and suicidal behavior are mood disorders, alcohol and substance abuse, and schizophrenia, all of which are familial disorders, which, on the basis of adoption and twin studies, have a strong genetic component (Cooper, 2001; McGuffin and Katz, 1989; McGuffin et al., 1991). Therefore, one set of genetic factors influencing suicide comprises those that predispose to the mental disorders that are associated with suicide (e.g., McGuffin and Katz, 1989).

However, the liability to mental disorder is not synonymous with the liability to suicide. For, while the majority of those who attempt and complete suicide have at least one mental illness, the converse is far from true—a very low proportion of mentally ill persons eventually kill themselves, and the majority never make as much as one suicide attempt (e.g., Bostwick and Pankratz, 2000; Murphy and Wetzel, 1990; Pokorny, 1983). One hypothesis, first advanced several decades ago, is that there are additional genetic factors relevant to suicide and suicidal behavior, perhaps related to a liability to impulsive behavior and aggression (Kety, 1986). The convergence of a mental disorder and aggression is associated with the greatest risk for suicidal behavior. Several studies have shown that mood disordered individuals with impulsive aggression are at much greater risk for suicidal behavior than are those without this trait (Mann et al., 1999). Furthermore, impulsive aggression contributes more to suicide and suicidal behavior in younger individuals (Conwell and Brent, 1995; Rich et al., 1986) than more mature adults.

Familial Aggregation of Suicidal Behavior

Even when suicidal behavior is familial, non genetic explanations must be considered. First, suicidal behavior in a relative can serve as a behavioral model for a family member, making imitation more likely to occur in subsequent generations. Second, there may be other familial factors that increase the liability to suicide such as parental psychopathology, lack of support, discord, and even frank abuse.

As discussed in Chapter 8, media presentation of suicide and suicidal behavior can result in imitation and contagion. This effect is most promi-

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