After examining the evidence from all of these sources, and no doubt
adding a bit of his own intuition, Eysenck (1982) asserted that about two-
thirds of the variance in personality development can be traced to biologicalfactors. Although the exact figure may not be this high, data from a con-
tinuing stream of studies suggests that extraverts and introverts differ on a
number of biological measures (Cox-Fuenzalida, Gilliland, & Swickert,2001; Doucet & Stelmack, 2000; Stelmack & Pivik, 1996). This is not to
say that environmental factors do not play a role. But, as the evidence re-
viewed in Chapter 10 makes clear, bi ology probably sets limits on how
much we can change an introverted friend into a highly sociable individual
or the likelihood of shaping an impulsive, outgoing child into a calm, easy-
going adult.
How do you spend your spare time? If you ’re an extravert, it probably never occurs to
you to take a long walk by yourself. If you ’re an introvert, you may rely on a long walk
to reduce your arousal level after an intense and active day.
Photo by Marlene SomsakHans Eysenck ’s Theory of Personality 229