Anxiety Disorders 319
to the traumatic event that precipitated their disorder. Moreover, this drug resulted in
more extreme biochemical and cardiovascular effects for the PTSD patients than for
the controls. However, neuroimaging studies have not documented abnormal activity in
the locus coeruleus when these patients are confronted with the relevant stimulus cues.
One possibility is that the locus coeruleus only creates abnormal levels of norepineph-
rine when the person is under larger amounts of stress than are induced by the usual
stimuli. Another possibility is that stress itself is another sort of trigger for panic attacks
in people with PTSD, operating along with images of the relevant stimuli.
Various types of evidence indicate that serotonin also plays a role in PTSD. For one,SSRIs can help treat the disorder, and apparently do so in part by allowing serotonin
to moderate the effects of stress (Corchs, Nutt, Hodd, & Bernik, 2009). In addition,
people who have certain alleles of genes that produce serotonin are susceptible to devel-
oping the disorder following trauma (Adamec, Holmes, & Blundell, 2008; Grabe et al.,
in press). However, research has shown that the effects of such genes may depend on a
combination of factors, such as stressful environmental events in combination with low
social support—and the same factors that affect whether people develop PTSD also af-
fect whether they develop major depressive disorder (Kilpatrick et al., 2007). Thus, the
effects of serotonin on PTSD are likely to be complex, and perhaps arise because of how
it affects more general aspects of brain function.
Genetics
Some people may have a genetic predisposition to develop PTSD (Shalev et al.,
1998; True et al., 1993). However, genes appear either to play a smaller role than
the environment in predicting PTSD (McLeod et al., 2001) or to be relevant only
in the context of complex interactions between genes and environment (Broekman,
Olff, & Boer, 2007). Consider a study of twin pairs where all twins served in the
military during the Vietnam War. Results of the study found that a twin was nine
times more likely to develop PTSD if he had been in heavy combat in Vietnam
than if he had not been in combat. Controlling for the amount of time in combat,
researchers found that genes account for about one third of the variance in PTSD
symptoms (True et al., 1993). Unique environmental experiences (such as being in a
fi erce battle) could account for the remaining variance.
Psychological Factors: History of Trauma,
Comorbidity, and Conditioning
Psychological factors that exist before the traumatic event occurs affect whether
a person will develop PTSD. Such factors include a history of depression or
other psychological disorders (Brewin, Andrews, & Valentine, 2000), and the
beliefs the person has about himself or herself and the world. Two specific
beliefs that can create a vulnerability for PTSD are considering yourself unable
to control stressors (Heinrichs et al., 2005; Joseph, Williams, & Yule, 1995) and
the conviction that the world is a dangerous place (Keane et al., 1985; Kushner
et al., 1992).
In addition, PTSD patients who have lower IQs tend to have more severe symp-toms (Kremen et al., 2007; Macklin et al., 1998; McNally & Shin, 1995). However,
this fi nding does not imply a causal relation in either direction. IQ might lead to
some other factor, which in turn is responsible for the severity of symptoms. For
example, one study found that children with IQs over 115 (more than 1 standard
deviation above average; see Chapter 3) were less likely to experience a traumatic
event by age 17 (Breslau, Lucia, & Alvarado, 2006). It may be that people with
lower IQs are less adept at avoiding traumatic events. Alternatively, perhaps they
believe that they cannot control traumatic events, and that’s why there is a correla-
tion between IQ and the severity of PTSD symptoms. In fact, perceiving that you
have no control over the traumatic event as it is taking place or that your life is at
risk during the event (whether or not the threat is that serious) can promote PTSD
(Foa, Steketee, & Rothbaum, 1989).
P SN