Psychobiology of Crime

Genetics

The bad seed idea of criminality assumed a solely genetic basis for crime, ignoring the influence of socioeconomic class, peer influences, and disorganized and [Page 1225]abuse family influences in criminal behavior. Conversely, strictly environmental explanations ignored the appearance of criminal behavior in some individuals growing up in economically comfortable and stable family environments. A more apt analogy for the origins of crime may be bad seed in bad soil with inadequate rain and sunshine. Such conditions may be optimal for weeds.

Heritability is the proportion of variance in a trait that is due to genetic rather than environmental factors. One way of assessing it is to compare concordance of the trait in identical compared to fraternal twins. Identical twins share all their genes whereas fraternal twins share only half of their genes (that is, those genes that actually have more than one form and therefore contribute to individual variations). A recent review of twin studies of adult criminality between 1929 and 1993 found an overall concordance for crime of 44 percent in identical compared to 22 percent in fraternal twins. This pattern shows a moderate heritability of 46 percent with no influence of shared family environment, but an equal influence (54 percent less some percent due to error of measurement) of nonshared environment. Nonshared environment is that outside the home that may differentially affect the twins such as different friends and other associations. Sharon Ishikawa and Adrain Raine compared the results excluding studies conducted during the 1930s in Germany, or studies with small sample sizes, or studies using physical resemblance rather than blood typing to determine zygosity, and the results were the same for the remaining twins.

The results for twin studies of juvenile crime are in marked contrast to those for adult crime. In most studies of juveniles, the concordances are high and nearly the same for identical and fraternal twins. This pattern would indicate a strong effect for shared environment and a weak or absent one for heredity. The explanation may be that the larger numbers of juveniles engaging in delinquency because of neighborhood, family, and peer influences obscures the smaller number who go on to become adult criminals. Most preadult delinquency is often of a transient nature and disappears with the assumption of adult responsibilities. Those with persistent criminality are the ones influenced relatively more by genetic predisposition. In order to qualify for the adult diagnosis of antisocial personality disorder (APD) one must have shown conduct disorder before the age of 15 and persisting antisocial behavior through adolescence and into adulthood at age 18. Those with APD may have more of the genetic component of criminality than other types of criminals. Their criminality is more impulsive, persistent, and unaffected by punishment and incarceration. They do not learn from experience.

Some have criticized twin studies because the tendency of people to treat identical twins more similarly than fraternal twins could enhance the genetic part of the equation. Adoption studies control this factor because the biological connection with the birth parents is free of a social influence, assuming parental separation at or near birth, whereas a genetic influence does not confound the social influence of the adopting parents. Studies of total populations of adoptees conducted in Denmark and Sweden and studies of adoptees in America have revealed significant relationships between criminality in adopted children and their biological parents even when there was no criminality in their adopting families. In some of these studies, particularly the one in Sweden, there was an interaction between the genetic influences of the biological parents and the social influence of criminality in the adopting parents, with the rate of criminality in the adopted children much higher when both factors were operative.

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