Although heredity may predispose an individual to violence, other environmental factors such as education and family background influence whether a predisposition to violence will translate into violent acts. Researchers of the link between biology and crime find themselves caught in a bitter controversy, particularly since violence has become a significant public health threat and the United States is the most violent country in the industrialized world. Factors in society, such as gun availability, economic inequity, and a violence-saturated culture, are not rooted in biology. Nonetheless, susceptibility to violence may be partly genetic. Some studies of identical twins who have been reared apart suggest that when one twin has a criminal conviction, the other twin is more likely to have committed a crime than is the case with fraternal twins. Other research with adopted children indicates that those whose biological parents break the law are more likely to become criminals than are adoptees whose natural parents are law- abiding. While no one believes there is a single "criminal gene" that programs people to commit violent acts, an individual's genetic makeup may give a subtle nudge toward violence. Researchers are also trying to find hereditary personality traits that may make people become more physically aggressive. Heredity and environmental factors must both be considered in explaining violence. 1 figure