450 Part V Biological/Evolutionary Theories
way of increasing their dopamine release and making up for deficient levels of
dopamine.
In the mid-1990s, researchers presented the first genetic evidence to support
this theory. The gene DRD4 is involved in dopamine production in the limbic
system, and the longer the gene sequence, the less efficient dopamine production is.
In other words, long versions of the DRD4 gene are associated with less efficient
dopamine production. If the theory is correct, people who seek out thrills and novel
experiences should have the longer form of this gene, and that is exactly what the
research has shown (Ebstein et al., 1996; Hamer & Copeland, 1998). In fact, con-
sistent with human research, DRD4 differences also affect exploratory behavior
like curiosity and seeking novel experiences in nonhuman animals such as birds
(Verhulst et al., 2016), apes (Shimada et al., 2004), and dogs (Ito et al., 2004). An
exciting aspect of this finding is that it was the first to demonstrate a specific
genetic influence on a normal (nonpathological) personality trait.
The second method used by behavioral geneticists for untangling the effects of
genetics and environment on personality was first discussed in Chapter 14 (Eysenck),
namely twin-adoption studies. Recall that the conclusion from this research is that
between 40–60% of our personality differences stem from genetic influence
(Bouchard & Loehlin, 2001; Caspi, Roberts, & Shiner, 2003; Kruger & Johnson, 2008;
Loehlin et al., 1998; Plomin & Caspi, 1999; Tellegen et al., 1988; Vukasović & Bratko,
2015). This line of research therefore suggests that about half of the differences that
exist between people in their personalities is attributable to genetics, leaving the other
half influenced by environment or other unknown factors. Interestingly, personality
disorders such as avoidant personality and obsessive-compulsive personality also
appear to be primarily under genetic influence. Gjerde and colleagues (2015), for
example, reported 67% of differences in avoidant personality and 53% of differences
in obsessive-compulsive personality can be explained by genetic factors. These results
are consistent with the view that personality, intelligence, motivation, and other psy-
chological qualities are products not only of biological or environmental forces alone,
but also of the interaction between the two. In short, personality differences are created
by both nature and nurture.Animal Personality
Most of you who have ever owned pet cats or dogs would readily agree that your
pets have their own unique personalities. One of your authors (GJF), for example,
currently has two sibling cats, a male (Scooter) and a female (Belle). These two
cats could hardly be more different in terms of their behavior and personality.
Scooter is absolutely curious and sociable. He explored every inch of his new home
on the first day and forces himself into every one of our activities—eating, watch-
ing TV, working at the computer, and sleeping. Not once has he been afraid of
any situation. He approaches everything with glee and wonder. Belle, however,
has been anxious and shy from the beginning. It took her about 3 days to come
out of hiding and become comfortable with her new home. She plays, but not so
much with strangers. She loves playing with her brother, however, and they regu-
larly tease and chase each other. Now she hangs out with the family and enjoys a
good massage, but she is still skittish to sudden approaches.