BOX7-2 Behavioral GeneticsBehavioral genetics is a research specialty in which both
genetic and environmental influences on the develop-
ment of behavior are evaluated. Proteins are produced
and regulated by genetic codes, and proteins interact
with physiological intermediaries (hormones, neuro-
transmitters, structural properties of the nervous system)
to produce behavior (Plomin, DeFries, & McClearn, 2008).
The genetic makeup of an individual, referred to as the
genotype, is fixed at birth. A person’sgenotypeispassed
down from the biological parents. Thephenotyperefers
to observable characteristics of an individual, and a per-
son’s phenotype can change. Intelligence and even men-
tal disorders are phenotypic characteristics that may
change over time. The phenotype is a product of
the genotypeandthe environment.
One of the most powerful (in the explanatory
sense) research designs used in behavioral genetics is
the twin method. This method involves comparing
monozygotic (MZ) twins, who are genetically identical,
withdizygotic (DZ) twins, who share only about 50%
of their genetic material, on the behavior or charac-
teristic of interest. The similarity among twin pairs is
typically presented in the form of aconcordance rate
orsimilarity index. In its simplest form, a concordance
rate is the percentage of instances across all twin pairs
in which both twins exhibit similar behaviors or char-
acteristics. A concordance rate or similarity index for
the MZ twin sample that is significantly greater than
that for the DZ twin sample suggests that genetic
influences play an important role in the development
of that set of behaviors or features.
Because MZ twins are identical and, if reared
together, may be treated more similarly than DZ twins,
one could argue that the higher concordance rate for MZ
twinsthanDZtwinsmayhaveasmuchtodowithenvi-
ronmental influences as genetic influences. An even more
informative method used in behavioral genetic studies
involves sampling MZtwins reared together(MZT), MZ
twins reared apart(MZA), DZ twins reared together (DZT),
and DZ twins reared apart (DZA). In this way, it is easier to
separate genetic and environmental influences. For
example, the following findings would suggest genetic
influences in the manifestation of the behaviors or fea-
tures under study: (a) the concordance rates for MZT and
MZA twins are significantly greater than those for DZT
and DZA twins, respectively; (b) the concordance rate for
MZA twins approaches that of MZT twins; and (c) the
concordance rate of DZA twins approaches that of DZT
twins. These findings wouldsuggest that genetics plays
an important role because similarity/concordance is afunction of the amount of genetic material shared, and
being raised in different environments does not have an
appreciable effect on similarity.
We have presented only a brief and rather sim-
plistic overview of behavioral genetics. Interest in this
field has waxed and waned over the years, and at
times the field has been the target of attacks from a
variety of groups (Plomin et al., 2008). However, it is
important to keep several points in mind. First,
concordance rates or similarity indices less than 100%
(or 1.00)necessarilyimplicate environmental influ-
ences. Therefore, behavioral genetics methods are
tools for identifying and quantifying environmental
as well as genetic factors in behavior. Second, finding
that a behavior or characteristic is genetically
influenced does not mean that it is immutable or
unchangeable. Geneticsandthe environment interact
in complex ways to produce behavior.The twin method compares similarity of characteristics
or features among monozygotic twins to that of dizy-
gotic twins.Radius Images/Jupiter Images202 CHAPTER 7