Major Movements in Psychology

Sociobiology and Evolutionary Psychology

How do twin studies help illuminate the role of genetics?

Twin studies have largely focused on IQ tests, comparing monozygotic (identical) and dizygotic (fraternal) twins. Monozygotic twins grew from the same fertilized egg and share 100 percent of their genes. Dizygotic twins grew from two separate eggs and therefore share only 50 percent of their genes. Twins reared together and twins reared apart have also been compared. These studies show that intelligence does have a significant genetic component, as monozygotic twins score much more similarly on IQ tests than do other types of siblings. However, these studies have been criticized because while the percentage of genes that differ from one subject to another has been carefully measured, the degree that environments differ between subjects is not clear at all. Moreover, there is considerable evidence showing that many environmental factors, such as socio-economic status, years of education, and mother’s level of education also have very strong influences on IQ.


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