The risk of the disease the 17 incidence minus the 1

the risk of the disease (the 17% incidence minus the 1% baseline incidence) is greater than the risks of the
disease in the other two categories added together ([3% - 1%] + [9% - 1%]).
This was a landmark finding (that earned Kety a Noble-prize nomination) and was widely interpreted as
presenting the first evidence for a genetic component to a psychiatric disorder .
So what are the problems with this approach:
First, there’s the assumption that the adoptee shares ONLY genes with the biological parents. And
even if the infant was removed from the biological parents within milliseconds of birth, there’s still all that fetal
environment shared with the mother (stay tuned). Is there any way to correct for that? Yes – in the early
adoption paradigm, the adoptee shares only genes with the father, while sharing genes and prenatal
environment with the mother. So comparing adoptees with their biological fathers and their adoptive fathers
bypasses that issue of prenatal environment.
The second problem concerns the other assumption, namely that adoptees ONLY share environment
with adoptive parents. And this is clearly wrong. As shown in many studies, children are not placed with
adoptive parents randomly. Instead, attempts are made to match the two in terms of race, ethnicity, likely
appearance (in fact, placement of children with adoptive parents of very different races, ethnicities, etc., has
often been criticized as culturally inappropriate). In other words, adoptees and adoptive parents typically
share genes at a higher rate than chance.
So the adoption approach has problems; however, it is still one of the backbones of the field, and can
uncover instances of genetic influences, although it probably overestimates their importance. Since Kety, the
approach has been used to infer genetic influences on the incidence of other psychiatric disorders (most
notably, major depression), personality profiles, and vulnerability to substance abuse.
Identical genes/different environment
Thus, two mainstays of behavior genetics are studies of twins, and studies of adoptees. The dream
scenario for any behavior geneticist is to combine the two approaches – to study identical twins separated at
birth, and adopted into different households. Entire careers are built around studying the relatively small
number of people in this category, an enormously influential approach. This was pioneered by Thomas
Bouchard of the University of Minnesota.
This approach has often produced some very good data, and is basically state-of-the-art (this probably
reflects the fact that there are very few identical twins separated at birth; thus, it is not possible for all sorts of
marginal scientists to get a hold of some of these folks and study them [while in contrast, rats, and thus rat
studies are a dime a dozen]). These studies have suggested that there are large heritable components
(typically > 50% heritability) to aggressive behavior, IQ (which, to hammer in a truism, indicates how well
someone does on an IQ test, without necessarily telling much about intelligence) and aspects of personality