So far, I have been outlining studies of the effect of adoption on intelligence and
academic achievement. The effect of adoption on criminality has also been studied
extensively. The largest sample consisted of nearly everyone adopted in Denmark between
1927 and 1947, over 14,000 adoptees. At birth, all the adoptees were taken from their
biological mothers. A little over a quarter were immediately placed in their adopting homes;
the rest were put in orphanages. Of the latter, a little over half were placed in their adopting
homes before they were one. The age at which they were adopted had no effect on their
subsequent criminality. I will quote from the most accessible summary, in Science12.
(Science is the journal of The American Association for the Advancement of Science):
If neither the biological nor the adoptive parents are convicted [of a crime],
13.5 percent of the sons are convicted. If the adoptive parents are convicted
and the biological parents are not, this figure rises to only 14.7 percent.
However, if the adoptive parents are not convicted and the biological parents
are, 20.0 percent of the sons are convicted.
So adoptees’ rate of criminality correlated 5.4 times more closely with the rate of
criminality of their biological parents, whom they never knew, than with the rate of
criminality of the parents who raised them. But the force of genetics on criminality is
even stronger than that because, “The mean number of convictions for the chronic[ally
criminal] adoptee increases as a function of biological parent recidivism.” This is
extremely important because, although most criminals commit only one crime, most
crimes are committed by chronic criminals. Only 4.1% of these adoptees were convicted
three or more times, but they committed 70% of the crimes.
