44 CHAPTER 2
In the fi rst case, we keep the environment the same for the two trees. Un-
fortunately for them, however, it is not a very friendly environment: The soil is
bad, the trees are in the shade, and there isn’t much water. Both trees produce
small apples. In this case, the environment overshadows the genetic infl uence for
large apples.
In the second case, the trees are luckier. We keep the environment the same for
the two trees, but the soil is rich, the trees are in the sun, and they receive plenty of
water. What happens? The tree with genes for large apples produces larger apples
than the tree with genes for small apples.
In the third case, the tree with genes for large apples is planted in the impov-
erished environment, and the tree with genes for small apples is planted in the
favorable environment. Now, the tree that has genes for small apples might produce
bigger apples than the tree with genes for large apples because the environmental
conditions have favored the former and acted against the latter.
As this example makes clear, for trees and other organisms—including
people—the infl uence of genes must be described in relation to the environment
in which they function. In other words, genes and environment interact through
feedback loops—and in fact, that’s why the phenotype is described as the product
of genes in a specifi c environment. The same genes can have different effects in
different environments. A research fi nding of a certain degree of genetic infl uence
on a disorder in one environment does not necessarily have any relationship to
the degree of genetic infl uence on the disorder in other environments. For ex-
ample, the fact that genes can predispose an individual to alcoholism has differ-
ent effects in the alcohol-embracing culture of France and the alcohol-shunning
culture of Pakistan.Heritability
Behavioral genetics characterizes the relative infl uence of genetic factors in terms
of the heritability of a characteristic. Heritability is an estimate of how much of
the variation in that characteristic within a population (in a specifi c environment)
can be attributed to genetics. For example, the heritability of generalized anxiety
disorder (which is characterized by anxiety that is not associated with a particular
situation or object, as we will discuss in detail in Chapter 7) is about .32 in citizens
of Western countries (Hetteman, Neale, & Kendler, 2001). This means that about
a third of the variation in generalized anxiety disorder in this population is geneti-
cally determined. Note that this estimate of .32 does not mean that any particular
person’s likelihood of developing the disorder is determined 32% by genes and 68%
by environment.
How do researchers estimate the heritability of a given psychological disor-
der? There is no sure-fi re research method, because many variables can affect the
results. For example, if researchers fi nd a similar prevalence of a mental disorder
in children and their parents, can they assume genetic inheritance? Not neces-
sarily; they would have to rule out any effects of the environment that might be
operating. As an extreme example, suppose that for generations, the members of
a given family always prepare their meals in lead pots, and hence each generation
of children becomes mentally retarded because of lead poisoning. This mental
retardation is not a result of genetics. Furthermore, it is diffi cult to assess “the
environment” for a given person. The environment must be understood not in
objective terms, but rather in terms of how situations and events are perceived
and understood. For instance, for siblings in a given family, does having divorced
parents constitute the same environment? Not exactly: A child’s age at the time
of parents’ divorce can infl uence how the child experiences the divorce. A pre-
schooler might believe he or she somehow caused the divorce, whereas an older
child—who is more mature cognitively, emotionally, and socially—is less likely
to make that inference (Allison & Furstenberg, 1989; Hoffman, 1991). Research-
ers can entirely avoid such age effects between siblings by studying twins.David Young-Wolff/PhotoEdit
In Western countries, genes account for about .32
of the variation in generalized anxiety disorder;
in other words, in this context, this disorder is
about 32% heritable. This does not mean that the
likelihood that a given person living in a Western
country will develop generalized anxiety disorder
is determined 32% by the genes and 68% by the
environment; it means that about a third of the
variation across the whole population of that
environment (Western countries) is determined
by genetics.Heritability
An estimate of how much of the variation in a
characteristic within a population (in a specifi c
environment) can be attributed to genetics.