In short, some of the discrepancies between the results of twin studies and
studies using other methods might be attributed to methodological issues.
However, twin studies might produce higher estimates of heritability for an-
other reason. Research suggests that personality traits aren ’t passed down
from parents to child in a simple, direct manner. Rather, the inherited part
of personality is often the result of a complex combination of more than one
gene (Finkel & McGue, 1997; Plomin et al., 1998). That is, the genetic influ-
ence of some personality traits may not be seen unless a unique combinationof more than one gene is inherited. Researchers refer to these complex influ-
ences as nonadditive effects . DZ twins share many genes, but they may not
share the exact combination of genes that make up a specific personality trait.
However, because MZ twins have identical genes, they also share any unique
combinations of genes that come together to influence personality. Thus, non-
additive effects would show up in identical twins but not in fraternal twins. Ifthis is the case, it could explain why twin studies find evidence for a larger
genetic influence on personality than studies using other methods.
So where does this leave us? Exactly how or how much genes determine
our adult personalities remains an open question. Some of the answers to thisquestion may come from new methodological and technological developments
(Krueger, South, Johnson, & Iacono, 2008). For example, researchers are be-
ginning to identify connections between personality traits and DNA markers
for specific genes (Gillespie et al., 2008; Plomin & Crabbe, 2000). But regard-
less of what future discoveries tell us, at this point it seems foolish to ignore the
relatively strong case that genetics has an influence on personality.
EXTRAVERSION-INTROVERSION
Few personality variables have received as much attention from researchers
and theorists as extraversion and introversion. Clearly, this aspect of Hans
Eysenck ’s personality theory has drawn more attention than any other. As de-
scribed in Chapter 9, extraverts are less sensitive to stimulation than introverts.
This is why extraverts can drink more coffee than introverts without being
overtaken by the effects of caffeine. It also explains why it is not uncommon
to find extraverts at loud social gatherings or in the middle of a crowd, whereasintroverts seek out solitary activities and gravitate to a quiet corner at a party.
Space allows us to examine only three of the many topics investigated by
researchers in this area. First, we ’ll tie individual differences in extraversion-
introversion to the research covered in the previous section by looking at the
evidence for the heritability of this personality variable. Second, we ’ll look at
research examining one of the basic differences between introverts and extra-
verts postulated by Eysenck: preference for arousal. Third, we’ ll address the
question, Who is happier, introverts or extraverts?
The Heritability of Extraversion
If you are an introvert, it ’s likely you’ ve been given some of the following
pieces of advice: “You need to get out more often, ”“Why can ’t you be more
sociable? ”or“Loosen up and enjoy yourself a little. ”Extraverts have258 CHAPTER 10 The Biological Approach