probably heard some of these: “There ’s more to life than having fun all the
time,”“Can’t you think a little before you do something? ”or“Slow down
and enjoy life. ”In short, whether you are introverted or extraverted, someone
has probably asked you to become more of the other. Even the most extreme
extravert can sit still for a few minutes, and the most introverted person you
know occasionally cuts loose and has a good time with friends. But is it pos-
sible for an extravert to become permanently more introverted? Can you raise
your child to be less introverted or more extraverted?
The answer to these questions depends on what causes a person to be-
come an extravert or an introvert. Eysenck championed the role of geneticsin answering this question. As seen in the previous chapter, the exact natureof the physiological differences between extraverts and introverts is still being
investigated. Nonetheless, these inherited differences are said to remain fairly
constant throughout one ’s life and eventually develop into the adult behavior
styles of extraversion or introversion. Although little evidence for heritability
was available when Eysenck first introduced his theory of personality, today
an impressive body of work appears to support Eysenck on this point.
As described earlier, researchers often use the twin-study method to deter-
mine the role of genetics in the development of personality. Consequently,
much of the evidence for the heritability of extraversion –introversion comes
from research comparing correlations between pairs of MZ twins with correla-
tions between pairs of DZ twins. Studies using this procedure find consis-
tent evidence for a genetic component of extraversion –introversion (Baker &
Daniels, 1990; Eaves & Eysenck, 1975; Heath, Neale, Kessler, Eaves, &
Kendler, 1992; Neale, Rushton, & Fulker, 1986; Scarr, 1969). In fact, the
findings suggest such a strong genetic influence that some researchers are
convinced the heritability estimates for this personality variable are somehow
exaggerated (Plomin et al., 1990).
Nonetheless, two of these studies deserve special attention. One group of
researchers gave a version of the Eysenck Personality Inventory to 12,898adult twin pairs in Sweden (Floderus-Myrhed, Pedersen, & Rasmuson, 1980).
This number represents virtually all of the contactable twins born in Swedenbetween the years 1926 and 1958. Another team of researchers tested 7,144
adult twin pairs in Finland (Rose, Koskenvuo, Kaprio, Sarna, & Langinvainio,
1988). This is nearly every living twin in that country born before 1958.Several features of these samples make them particularly important. Not only
are the samples large, but they represent nearly every twin in these two coun-
tries. This means researchers don ’t have to worry about only a certain kind of
person volunteering to participate in the study.
When the within-pair correlations for DZ and MZ twins in these samples
were compared, considerable evidence fo r a genetic component for extraversion-
introversion was uncovered. As shown in Table 10.3, the MZ twins were more
like each other than were the DZ twins, which argues for a genetic influence.
Beyond this, the researchers in the Finnish study examined the amount of socialcontact between the members of the twin pairs as well as the amount of social
contact the twins generally engaged in. Although the researchers did find that
MZ twins were more likely to stay in communication with each other, thisExtraversion-Introversion 259