"It is better to be hated for what you are then to be loved for what your not"
-Andre Gide

Twin Studies

In the late nineteenth century, criminologist Cesare Lombroso posed the theory that some people were evolutionary throwbacks who were biologically inclined towards criminal behavior and antisocial conduct. This theory, influenced by the theories of Darwin that were gaining popularity at the time, was based on the mistaken idea that "born criminals" have certain primitive features, such as long arms, a sloped forehead, and unusual faces, which indicated genetically determined antisocial tendencies. Although it was proven mistaken, Lombroso’s theory served to discredit many later theories and research on the heredity of crime (Berkowitz 387).

In the struggle to prove that there is some validity in genetic theories of aggression, one of the methods that researchers have used is twin studies. Monozygotic, or identical twins, because they are formed from one egg, have the same genetic make up. Dizygotic, or fraternal twins, are formed when two eggs fertilized at the same time, and contain genetic material no more similar than that of regular brothers and sisters (388). The degree of similarity in a pair of twins with respect to the presence or absence of a particular genetic pattern is called concordance. By studying the concordance rates of crime between identical twins and their fraternal counterparts, one may find how much significance genetic material has in leading humans towards criminal and aggressive behavior. One such study of these rates from the 1930’s found an average concordance rate of 75% for monozygotic and 24% for dizygotic twins. Later investigations with more precise methods of determining whether twins were mono- or dizygotic reported 48% concordance for mono- and 20% for dizygotic twins. Whatever the numbers, there is definitely some genetic influence on crime rates (388-9).

Meanwhile, oopponents to twin studies argue that identical twins have closer relationships than fraternal twins, so they influence each other to a greater extent and are more likely to take on each others behavioral styles and antisocial actions. This could create error in the studies. Also, because identical twins evoke similar reactions from other persons, they may grow up in a more psychologically similar environment than fraternal twins, causing a stronger environmental influence on their similarities (408).