Crime Causation: Biological Theories

Twin and adoption studies have been employed to address this question, yielding mixed results. Relying on criminal arrest data, Cloninger and Gottesman reanalyzed the twin data collected by Christiansen and grouped subjects as either violent offenders or property offenders. Heritability for property offenses was found to be .78 while heritability for violent offenses was .50. Although the genetic effect for property offenses was greater than for violent offenses, the data suggest that violent offenses, as assessed by official crime statistics, may also have a heritable underlying component.

Two independent adoption studies, however, have failed to provide support for the hypothesis that violence is a heritable trait (Bohman et al.; Mednick et al.). The largest adoption study to date was carried out in Denmark by the present authors' research group (n = 14,427). Mednick, Gabrielli, and Hutchins had previously reported a significant relationship between the number of criminal convictions in the biological parent and the number of convictions in the adoptees. Subsequent statistical analyses revealed that this relationship held significantly for property offenses, but not significantly for violent offenses.

Perhaps a genetic predisposition toward violence may exist in the presence of some other unidentified mediator. A study in Oregon provided an important clue in that mental illness, particularly severe mental illness, may be genetically related to violence. In a classic study, Heston followed up a sample of forty-seven offspring born to schizophrenic mothers and compared them to a group of matched controls from the same orphanage. These offspring were separated from their mothers shortly after birth and placed in foster care or orphanages. Heston was primarily interested in determining if adopted-away offspring were at increased risk of becoming schizophrenic themselves. The findings supported the original hypothesis, as five of the forty-seven offspring became schizophrenic. An interesting finding is that an even greater number of the adopted-away offspring of schizophrenic biological mothers actually had been incarcerated for violent offenses. Eleven (23.4 percent) of the adoptees had been incarcerated for violent offenses. Since these offspring were not raised by their schizophrenic mothers, this suggested the possibility that mental illness and criminal violence may share a common genetic basis.

With the Heston study in mind, Moffit investigated the role of parental mental illness in the emergence of violent offending among the Danish adopted-away sons. When only the criminal behavior of the biological parents is considered, she found no increase in violent offending in the adoptees. A significant increase in the rate of violent offending is noted only among offspring whose biological parents were severely criminal (typically the biological father) and had been hospitalized one or more times for a psychiatric condition (typically the biological mother).

These findings suggest that a biological background positive for mental disorders appears to be associated with an increased risk of violent offending in the children. Other disorders in the biological parents may also increase the risk of violent offending in the adopted-away offspring. One such disorder that may elevate the risk of violent offending in children is the presence of alcoholism in the biological parents.