 |
 |
 |
 |
|
 |
 |
172
7
The most compelling evidence of biological factors for criminality comes from two studies — one of
100 twins, the other of adopted boys. Since the 1920's it has been understood that twins may develop from a
single fertilized egg, resulting in identical genetic endowments
—
identical twins
—
or from a pair of
separately fertilized eggs that have about half their genes in common — fraternal twins. A standard
105 procedure for estimating how important genes are to a trait is to compare the similarity between identical
twins with that between fraternal twins. When identical twins are clearly more similar in a trait than
fraternal twins, the trait probably has high heritability.
8
There have been about a dozen studies of criminality using twins. More than 1,500 pairs of twins
110 have been studied in the United States, the Scandinavian countries, Japan, West Germany, Britain and
elsewhere, and the result is qualitatively the same everywhere. Identical twins are more likely to have
similar criminal records than fraternal twins. For example, the late Karl 0. Christiansen, a Danish
115 criminologist, using the Danish Twin Register, searched police, court and prison records for entries
regarding twins bom in a certain region of Denmark between 1881 and 1910. When an identical twin had a
criminal record, Christiansen found, his or her co-twin was more than twice as likely to have one also than
120 when a fraternal twin had a criminal record.
9
In the United States, a similar result has recently been reported by David Rowe, a psychologist at the
University of Oklahoma, using questionnaires instead of official records to measure criminality. Twins in
125 high school in almost all the school districts of Ohio received questionnaires by mail, with a promise of
confidentiality as well as a small payment if the questionnaires were filled out and returned. The twins
were asked about their activities, including their delinquent behavior, about their friends and about their co-
130 twins.The identical twins were more similar in delinquency than the fraternal twins. In addition, the twins
who shared more activities with each other were no more likely to be similar in delinquency than those
who shared fewer activities.
135 11 No single method of inquiry should be regarded as conelusive. But essentially the same results are
found in studies of adopted children. The idea behind such studies is to find a sample of children adopted
early in life, cases in which the criminal histories of both adopting and biological parents are known. Then,
140 as the children grow up, researchers can discover how predictive of their criminality are the family histo-
ries of their adopting and biological parents. Recent studies show that the biological family history
contributes substantially to the adoptees' likelihood of breaking the law.
145 12 For example, Samoff Mednick, a psychologist at the University of Southern California, and his
associates in the United States and Denmark have followed a sample of several thousand boys adopted in
Denmark between 1927 and 1947. Boys with criminal biological parents and noncriminal adopting parents
150 were more likely to have criminal records than those with noncriminal biological parents and criminal
adopting parents. The more criminal convictions a boy's natural parents had, the greater the risk of
criminality for boys being raised by adopting parents who had no records. The risk was unrelated to
155 whether the boy or his adopting parents knew about the natural parents' criminal records, whether the natu-
ral parents committed their crimes before or after the boy was given up for adoption, or whether the boy
was adopted immediately after birth or a year or two later. The results of this study have been confirmed in
160 Swedish and American samples 160 of adopted children.
13 Because of studies like these, many sociologists and crimi-nologists now accept the existence of
genetic factors contributing to criminality. When there is disagreement, it is about how large the genetic
165 contribution to crime is and about how the criminality of biological parents is transmitted to their children.
14 Both the twin and adoption studies show that genetic contributions are not alone responsible for
170 crime
—
there is, for example, some increase in criminality among boys if their adopted fathers are
criminal even when their biological parents are not, and not every co-twin of a criminal identical twin
becomes criminal himself. Although it appears, on average, to be substantial, the precise size of the genetic
175 contribution to crime is probably unknowable, particularly since the measures of criminality itself are now
so crude.
15 We have a bit more to go on with respect to the link that transmits a predisposition toward crime
from parents to children. No one believes there are «crime genes», but there are two major attributes that
180 have, to some degree, a heritable base and that appear to influence criminal behavior. These are in-
telligence and temperament. Hundreds of studies have found that the more genes people share, the more
likely they are to resemble each other intellectually and temperamentally.
185 16 Starting with studies in the 1930s, the average offender in broad samples has consistently scored 91 to
93 on 195 I.Q. tests for which the general population's average is 100. The typical offender does worse on