than their biological parents, genetic and environmental influences are not
confounded. Think of a family in which parents raise one child they adoptedand one they gave birth to. Which child should have a personality similar to
the parents ’? If genes are playing a role, we would expect the biological off-
spring to be more like the parents because that child shares not only the envi-
ronment but also some genes with the parents. In fact, this is what researchers
find (Scarr, Webber, Weinberg, & Wittig, 1981). However, calculations
with the data from these studies indicate the genetic influence is less thanthat suggested by the twin-study data. In fact, data from adoption studies
suggest that the heritability of personality is about half what the twin-study
data suggest (Plomin et al., 1990; Plomin, Corley, Caspi, Fulker, & DeFries,1998).
But the adoption situation provides even more opportunities to test the
genetic-environmental influence question. What would you expect if youcompared the personalities of the adopted children with those of their biolog-
ical mothers? The children have shared no environment with the mothers but
are still linked by genes. When the personality scores of adopted children arecompared with those of their adoptive parents and their biological mothers,
the children look more like the biological mothers, whom they have never
known (Loehlin, Willerman, & Horn, 1982, 1987). Although the strength of
the relationship is also weaker than that suggested by the twin-study data, we
have evidence from yet another source that genetics plays at least some role in
the formation of adult personalities.
It is also possible to combine the twin-study and adoption situations.
Although rare, some researchers have taken advantage of situations in whichMZ twins are separated from their parents at birth and in addition are raised
in two different households. The twins in these pairs share genes but not en-
vironments. These twins are then compared with MZ twins raised in the
same household, who share both genes and environments. A summary of thefindings from studies using this method is shown in Table 10.2. As you can
see, the MZ twins tend to be quite similar to each other regardless of whether
they are raised with or separated from their twin brother or sister (Rowe, 1987).The obvious explanation for this similarity is that the twins ’genes shaped
their personalities in a similar manner regardless of the environments theygrew up in.
TABLE10.2
Correlations for Twins Raised Apart and Twins Raised Together
Identical Twins
Raised ApartIdentical Twins
Raised Together
Extraversion .61 .51
Neuroticism .53 .50Intelligence .72 .86
Source: From “Resolving the person-situation debate: Invitation to an interdisciplinary dialogue, ”by D. C. Rowe,
American Psychologist , 1987, 42, 218 –227. Reprinted by permission of the American Psychological Association.256 CHAPTER 10 The Biological Approach