factor alone was not sufficient to explain the differences in MZ and DZ corre-
lations on the extraversion-introversion measure.
Another study takes the twin-study method one step further (Pedersen,
Plomin, McClearn, & Friberg, 1988). As in the earlier studies, the investigators
compared MZ and DZ twins who grew up together. However, these research-
ers also located 95 pairs of MZ twins and 220 pairs of DZ twins reared apart.
Again, a positive correlation between the scores of identical twins separated atbirth and reared in different environments would provide strong evidence for a
genetic component. And indeed, as shown in Table 10.4, there was a relatively
strong correlation between the scores of MZ twins reared in separate environ-ments, albeit not as strong as that for MZ twins reared together.
In short, extraversion appears to have one of the strongest genetic
components of any personality variable studied. Moving forward, recent scien-tific and technological advances have provided researchers with new ways to
examine the question of heritability and extraversion (Canli, 2006). One team
of researchers conducted genome-wide scans on adolescents to see which chro-
mosomes were related to various personality measures (Gillespie et al., 2008).
They found links between extraversion and chromosomes 2, 3, 8 and 12.
We can conclude from all this research that how introverted or extraverted
you are probably is strongly influenced by the set of genes you inherited. Thisis not to say that you can ’t be more outgoing at times if you are highly intro-
verted or learn to stop and introspect for a few minutes if you ’re an extravert.TABLE10.3
Within-Pair Extraversion Correlations for MZ and DZ Twins
Males Females
MZ Twins DZ Twins MZ Twins DZ Twins
Swedish sample .47 .20 .54 .21
Finnish sample .46 .35 .48 .14
Source: From Floderus-Myrhed et al. (1980) and Rose et al. (1988).
TABLE10.4
Within-Pair Correlations of Extraversion Scores for Twins
Reared Apart and Together
Twins Reared Apart Twins Reared Together
MZ Twins DZ Twins MZ Twins DZ Twins
.30 .04 .54 .06
Source: From “Neuroticism, extraversion, and related traits in adult twins reared apart and reared together, ”
by N. L. Pedersen, R. Plomin, G. E. McClearn, and L. Friberg, Journal of Personality and Social
Psychology , 1988, 55, 950 –957. Reprinted by permission of the American Psychological Association.260 CHAPTER 10 The Biological Approach