B. Genetics in Modern Biological Theories
Efforts to find a genetic explanation for violence and aggression have been met with strong resistance, primarily because of painful memories of how research linking biology and crime were used in the past (eugenics). In 1992, a conference related to the Human Genome Project at the University of Maryland had its federal funding withdrawn for attempting to discuss any particular linkage between genes and violence (Murphy & Lappé, 1994). Objections by groups who believed that any such research would be used to oppress poor and minority populations overpowered the quest for knowledge.
Although genetic research began with Mendel’s laws of inheritance, our understanding of how genes influence our behaviors is still evolving. Discovery of the genetic code in the mid-1950s took us beyond recognizing that genes were involved in heredity to a greater understanding of the process through which hereditary traits are passed from one generation to another. Part of this discovery process was the clarification of the structure and function of chromosomes, which carry human genetic material.
Human cells normally have 22 pairs of chromosomes, plus a pair of chromosomes that determines sex, for a total of 46. Sex chromosomes are termed X and Y. Females carry a combination of XX, and males carry a combination of XY. During conception, the male’s sperm carries genetic material to the female’s egg. If the sperm that fertilizes a female egg is carrying a Y chromosome, the resulting embryo will develop into a male fetus (XY). If the sperm is carrying an X chromosome, the resulting embryo will develop into a female fetus (XX).
During this process, however, things can develop abnormally. For example, during the process, some men are left with an extra Y chromosome (XYY). Erroneously termed XYY syndrome, a “supermale” carrying this chromosomal pattern usually has a normal appearance and will probably never know that he carries an extra Y chromosome, unless he is genetically tested for some other reason. Given the Y chromosome’s association with the male sex and with increased production in testosterone, many claims have been made in the research literature that XYY males are more aggressive and more violent. This supposition has not been supported with scientifically valid research.
Scientific progress made inquiry into genetic correlates of behavior more precise and less speculative. Although scholars are reluctant to associate criminal behavior with any specific gene, researchers continue to investigate the inheritability of behavioral traits. Some of the most promising work involves the study of twins and adoptees.
2. Twin Studies
Since Galton’s work with twins, twin studies have become more sophisticated and have attempted to respond to methodological criticisms. Distinctions between fraternal (dizygotic [DZ]) and identical (monozygotic [MZ]) twins have contributed to the sophistication of this type of research. DZ twins develop from two eggs and share about half of their genetic material, whereas MZ twins develop from a single egg and share all of their genetic material.
Twin studies attempt to control for the impact of the social environment, hypothesizing that these environments are similar for twins. Twins generally are raised in the same social environment, so the impact of the social environment is considered to be equal and consistent (and thus controlled). Therefore, any greater similarity between identical twins than between fraternal twins would provide evidence for a genetic link.
One of the earlier and simpler twin studies was conducted in the 1920s by Johannes Lange (1929). He studied 30 pairs of twins who were of the same sex. Seventeen of these pairs were DZ twins, and 13 of these pairs were MZ twins. At least one of each twin pair was known to have committed a crime. However, Lange found that both twins in 10 of the 13 MZ twin pairs were known criminals, compared with both twins in only 2 of the 17 DZ pairs.
More sophisticated and extensive studies have followed. In 1974, Karl O. Christiansen evaluated the criminal behavior of 3,586 twin pairs born in Denmark between 1881 and 1910. He found that the chance of one twin engaging in criminal behavior when the other twin was criminal was 50% among the MZ twin pairs but only 20% among the DZ twin pairs. The correlation between the genetic closeness of the biological relationship and crime was especially true for serious violent crime and for more lengthy criminal careers.
These findings were supported by additional work on the self-reported delinquency of twins in the 1980s and 1990s by David C. Rowe and his colleagues. This research found that MZ twins were more likely than DZ twins to both be involved in delinquent activity. Moreover, MZ twins reported more delinquent peers than did DZ twins (Rowe, 1983). The work of Rowe and his colleagues supported a genetic component to delinquency but also provided evidence of a social component.
Although twin studies have provided some support for a genetic component to behavior, it is difficult to separate the influence of genetics from the influence of social factors. There also are theoretical problems with the assumption that twins raised in the same home are subject to the same treatment and the same social environment. Even scholars who study the link between criminal behavior and genetics are cautious with their conclusions, arguing that these types of studies reveal only that the similarities between twins have some impact on behavior. Whether these similarities are genetic, social, or some combination of the two is still open for debate. Studies of adopted individuals constitute one attempt to resolve this issue.
3. Adoption Studies
In adoption studies, the behavior of adoptees is compared with the outcomes of their adopted and biological parents. The aim is to separate out the impact of the environment from the influence of heredity. This research asks whether a child will exhibit traits of the adopted parents or of the biological parents.
Research indicates that an adoptee with a biological parent who is criminal is more likely to engage in property crime than other adoptees and that this effect is stronger for boys. The findings, from a study of 14,427 Danish children adopted between 1924 and 1947, provide evidence that there may be a genetic factor in the predisposition to antisocial behavior (Mednick, Gabrielli, & Hutchins, 1984). Studies in both Sweden and in the United States confirm these conclusions.
A meta-analysis of adoption studies, conducted by Walters and White (1989), reinforced the importance of adoption studies as the best way to determine the impact of both environment and genetics on criminal behavior but also emphasized the theoretical and methodological difficulties inherent to this approach. Knowing, for example, whether an adoptive parent has a criminal history provides no information on the social environment provided in the adoptive parent’s home. The definitions of crime and criminality also widely vary in these studies and can be challenged. For example, one study may consider as criminal behaviors perhaps best classified as antisocial (e.g., using bad language, adultery). Furthermore, these studies do not account for the quantity or quality of social interactions experienced within the various settings (adoptive vs. biological). Finally, the determination that someone is a criminal simply on the basis of a conviction or incarceration is problematic and does not consider undetected criminal behaviors.
According to researchers who worked on the Human Genome Project, however, twin and adoption studies are the best source for evaluating individual differences in human behavior. Recent studies have consistently demonstrated that genetic variation substantially contributes to behavioral variation across all types of behavior. Two primary conclusions are derived from these studies: (1) Nearly all of the most frequently studied behaviors, characteristics, and conditions (e.g., cognitive abilities, personality, aggressive behavior) are moderately to highly heritable, and (2) nonshared environments play a more important role than shared environments and tend to make people different from, instead of similar to, their relatives.
Most biological scholars now cautiously conclude that there may be a genetic predisposition toward criminal behavior but that the manifestation of these predispositions is dependent on social and environmental factors. However, belief (or not) in a genetic link to criminality does not preclude other potential biological explanations of crime.