for excessive aggression
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).
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).