types of human twins. Some twins are monozygotic (MZ); that is, the two ba-
bies come from the same fertilized egg. These are the twins that look alike
physically, the ones we commonly call identical twins. The important point for
researchers is that MZ twins have identical genes. The other type, dizygotic
(DZ) twins, come from different eggs. These two babies, commonly called fra-
ternal twins, are no more alike genetically than any two siblings.
The logic behind the twin-study method is illustrated in Figure 10.1. We
assume that two same-sex DZ twins and two MZ twins (who are always thesame sex) share very similar environments. That is, in studies using this method,
twin pairs, regardless of type, are the same age and the same sex and live in thesame house under the same rules. Therefore, the extent to which the environ-
ment is responsible for their personalities is going to be about the same for both
types of twin pairs. However, if there is also a genetic influence on personality,we would expect the MZ twins to be more like each other than are the DZ
twins because the MZ twins also have identical genes, but the DZ twins do not.
Researchers using the twin-study method give personality trait measures
to both members of both kinds of twins. They then look at how similar the
twin brothers and sisters are on the traits. If trait scores for the MZ twin
pairs are more highly correlated than the scores for the DZ twin pairs, we
have evidence for genetic influence on personality. Because the environmental
influence is roughly the same for both kinds of twins, it is assumed that the
MZ twins are more alike because they also have identical genes.
Identical twins not only share physical features but also have similar personalities.
Researchers attribute this similarity in part to genetic influences although the extent ofgenetic influence on personality continues to be debated.254 CHAPTER 10 The Biological Approach