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Behavior Genetics

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Research in behavior genetics has shown that almost all personality traits have both biological and environmental bases. One such trait is
Intelligence. An alternative approach on which biology leaves its signature is Evolutionary Psychology.

Genes Versus Environment

Behavior genetics is a field in which variation among individuals is separated into genetic versus environmental components. The most common research methodologies are family studies, twin studies, and adoption studies.

Environmental influences can be divided into two classes, shared and nonshared (or unique) environment. Shared environment is the environment shared by siblings reared in the same family. This includes such variables as socioeconomic status and parent education. Nonshared environment is the environment unique to the individual. This includes such variables as peer group.

It is important to note that there is no single gene for intelligence, personality traits, behavior, or even height. Rather, such complex characteristics are polygenic, i.e., they are influenced by multiple genes. The research methodologies mentioned do not tell us which genes are involved, only the relative influence of all genes as opposed to environment. Also, heritability (genetic influence) is a population value; knowing that height, for example, is 90 percent heritable does not tell us that 90 percent of any one person's height is due to genetic influences. Three models of genetic transmission from parents to children exist, one passive model and two evocative models (Reiss, 1997).


Family Studies

Children share 50 percent of their genes with each parent. Therefore, for genes to be influential whatsoever, the trait in question must run in families. Obviously, a trait could be environmentally transmitted rather than inherited. For example, last names tend to run in families, but no one would claim that last names are genetic. Thus, running in families is a necessary, though not sufficient, condition for a trait to be genetic.

Twin Studies

Monozygotic (MZ, identical) twins share 100 percent of their genes, while dizogotic (DZ, fraternal) twins share only 50 percent of their genes (the same percentage as non-twin siblings). Therefore, to the extent that genes are influential, identical twins should be more alike than fraternal twins.

Adoption Studies

If shared environment is influential, then sibling reared in the same family should be more similar than adopted away siblings (siblings reared apart).


Behavior genetics studies of adult personality make one thing abundantly clear: genes are important, and unique environment is important, but shared environment is not important at all (Eysenck, 1990). This conclusion could be devastating to theories such as psychoanalysis that place a premium on how parents treat their children (assuming that parents tend to treat all their children alike).

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Eysenck, H. J. (1990). Genetic and environmental contributions to individual differences: The three major dimensions of personality. Journal of Personality, 58, 245-261.

Reiss, D. (1997). Mechanisms linking genetic and social influences in adolescent development: Beginning a collaborative search. Current Directions in Psychological Science, 6, 100-105.

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Last modified May 1998

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