By comparing the similarity of monozygotic (identical) twins, who share all of their genes, to dizygotic (fraternal) twins, who share half of their segregating genes on average, the twin-study methodology facilitates the estimation of the proportion of the total variance in a trait that is attributable to the influence of genes, shared environment, and nonshared environment. Results of large population-based twin studies have shown that genetic influences are significant for virtually all psychological traits and disorders. Similarly, environmental factors also play an important role in the etiology of nearly every trait or disorder that has been studied, although nonshared environmental influences are often slightly stronger. Therefore, the question is no longer whether a trait is due to nature or nurture, but instead the extent to which each of these factors and the interactions between them influence the development of the trait. In the next section we turn to methods that can be used to identify the specific genes that influence these behaviors.
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