academic achievement, personality and temperament (including such traits as aggression and hostility), psychopathology, and even vocational interests and social attitudes (Plomin et al., 1989). Hence, a research finding that criminal or violent behavior had some heritable component would come as no surprise—especially since violent and criminal behaviors are themselves correlated with some of the other behaviors for which genetic relationships have been established. Beyond confirming the existence of heritability in violent behavior, the more interesting intellectual challenges are

1. isolating the precise nature of the mechanisms through which an individual's propensity to engage in or refrain from violent behavior may be inherited;

2. using quantitative methodology to control for heritable influences so that conclusions about environmental influences on violent behavior can be clarified; and

3. quantifying the genetic effect in terms of its importance or triviality in explaining human behavior and the magnitude of its correlation with risk factors for violence.

On the first challenge, quantitative genetic studies have not isolated any simple genetic syndrome, either Mendelian or chromosomal, that is invariably associated with violence or, more broadly, with antisocial behavior. Like inherited propensities for other behaviors, a genetic liability toward violence is likely to involve many genes and substantial environmental variation. The existence of such mechanisms may well be confirmed by future quantitative genetic research, but knowledge of their precise nature must await progress in detecting genes—and markers linked to them—that account for small variations in behavior, a problem in molecular biology that lies beyond the scope of this paper.

The second challenge suggests a more promising line of research than the reiteration of long-standing, sterile ''nature versus nurture" debates—that genetic research designs may clarify environmental effects. This can best be illustrated by a hypothetical example. Suppose that a propensity toward violent behavior is transmitted from parent to offspring by two mechanisms: one operating through the genes and the other through social learning. How can these two mechanisms be detected and quantified in a study of intact nuclear families? If the parent-offspring correlation is interpreted solely in terms of social learning, then the environmental transmission will be overestimated. On the other hand, if the correlation is interpreted solely in terms of genetic transmission, then the social learning of aggression will be over-looked.