Twin studies have shown that P has moderate heritability
Twin studies have shown that P has moderate heritability. Zuckerman (1989) reviewed the earlier studies of P or related traits and found a median heritability of 0.51. However, twin studies using an Eysenckian P scale have tended to estimate somewhat lower heritabilities than this, in the range 0.36–0.50 (Heath & Martin, 1990). Heath and Martin (1990) have also reported evidence that P items are aetiologically heterogeneous, although this study used an earlier version of the P scale which has subsequently been improved (Eysenck, Eysenck, & Barrett, 1985). The next step in genetic research on P and other personality traits is to move beyond heritability and find specific polymorphisms associated with these traits (Plomin & Colledge, 2001). A powerful approach is to examine associations between the trait in question and candidate genetic polymorphisms which are predicted to determine variation in the trait. Theories of the biological influences on P can suggest suitable candidate AZD0156 formula for this purpose. Zuckerman (1994) has presented a psychopharmacological model for P and N. This model is complex and involves many interacting actions. One of these hypothesized actions is that a low level of the neurotransmitter norepinephrine is associated with P. Norepinephrine is produced from dopamine by the enzyme dopamine-β-hydroxylase (DβH). Consequently, a low level of DβH is also hypothesized to be associated with P. In support of this hypothesis, Zuckerman (1994) cited studies showing a correlation of plasma DβH with sensation seeking or with clinical states which involve disinhibited sensation seeking. He noted that there were inconsistencies in the results of these studies, but attributed these to the influence of current emotional states on DβH. Variation in plasma DβH activity is highly heritable and in European Americans around 44% of the variation is accounted for by a 51021C->T polymorphism in the 5′ flanking region of the DBH gene (Zabetian et al., 2001). Zuckerman’s hypothesis suggests that this polymorphism is a candidate for associations with P. No studies to date have examined associations of DBH gene polymorphisms with personality traits. However, several studies have examined associations with psychiatric disorders. Wood, Joyce, Miller, Mulder, and Kennedy (2002) examined a 444G>A polymorphism in patients with major depression and found the GG genotype to be associated with lower scores for interpersonal sensitivity and paranoid ideation. Cubells et al. (2002) looked at several polymorphisms in relation to depression with psychotic features compared to depression without psychotic features. They focused particularly on the C-1021T polymorphism. This polymorphism was found to be associated with plasma DβH activity and plasma DβH differentiated the two groups, but there was no association of the polymorphism with depression subtype. There have also been several studies finding associations of a DBH Taq I polymorphism with attention-deficit/hyperactivity disorder (Daly, Hawi, Fitzgerald, & Gill, 1999; Roman et al., 2002) and with symptoms of this disorder in patients with Tourette’s syndrome (Comings et al., 1999; Comings et al., 1996). No studies to date have examined associations with personality traits. Another possible biological determinant of P is gonadal hormones. On the basis that males show higher mean P scores than females, Eysenck has proposed that testosterone plays a role. Zuckerman (1989) has also hypothesized that gonadal hormones are associated with sensation seeking. His summary of the literature is that “testosterone in normal males is primarily related to sociability, sensation seeking, heterosexual experience, provoked aggressiveness and lack of frustration tolerance. In populations of delinquents or adult criminals testosterone is related to social dominance and sensation seeking, but is also related to extreme levels of unprovoked physical aggression” (p. 409). These hypotheses would also suggest that genes that affect the production or action of testosterone, such as the androgen receptor (AR) are potential candidates for an association with P.