Abnormal Psychology

Anxiety Disorders 293

In addition, lower levels of the neurotransmitter acetylcholine are associated

with higher levels of anxiety (Degroot & Treit, 2002). Moreover, the systems that

produce acetylcholine and serotonin appear to be linked, so that increases in the

output of one are accompanied by decreases in the output of the other (File, Kenny, &

Cheeta, 2000); thus, reducing the serotonin level raises the level of acetylcholine,

which in turn decreases anxiety. This may be one reason why SSRIs, which lower

the levels of serotonin, effectively treat anxiety disorders. Moreover, to make the sit-

uation even more complicated, norepinephrine is also implicated in anxiety. Clearly,

anxiety does not result from a simple biochemical imbalance, and effective medica-

tions will need to target several different neurotransmitter substances.

Genetics

Researchers have discovered that some genes predispose individuals to develop a spe-

cifi c phobia (but not a particular one), whereas other genes underlie each specifi c type

of phobia (Kendler et al., 2001; Lichtenstein & Annas, 2000). According to one the-

ory, genetic differences may cause parts of the brain related to fear (in particular, the

amygdala) to be too reactive to specifi c stimuli; that is, the amygdala is “prepared”

to overreact to a specifi c stimulus, which leads to a specifi c phobia of that stimulus.

Some people’s brains may be more prepared than others’, and so they are more likely

to develop a specifi c phobia (LeDoux, 1996).

However, such a neural predisposition cannot be all there is to it: If it were, then

when one identical twin has a specifi c phobia, so would the other twin, which is not al-

ways the case. As we’ve noted before, the genes predispose, but rarely determine. Rather,

certain environmental events are necessary to trigger the disorder. Some theorists have

hypothesized that brain damage, such as can occur in an auto accident if someone isn’t

wearing a seatbelt and suffers a head injury, may be one such triggering event. And, in

fact, both specifi c phobia and panic disorder are more likely to develop if a person has

had traumatic brain injury. In one study, researchers found that 30 years after a brain

injury, over 8% of the people injured had a specifi c phobia (Koponen et al., 2002).

However, genes do not have equal effects for all specifi c phobias: The different

specifi c phobias appear to be infl uenced to different degrees by genetics and the en-

vironment (see Figure 7.11 for the heritabilities of types of specifi c phobias). Family

environment also has proven to be an important risk factor for specifi c phobias

(Kendler et al., 2001).

Can genes shed light on the sex differences for specifi c phobias? Perhaps to

a limited degree. Men and women have comparable genetic risk for animal pho-

bia but may have different risks for agoraphobia, situational phobias, and blood-

injection-injury phobia because different genes may underlie these disorders in

the two sexes (Kendler et al., 2002). However, over the long run, genetics may

only moderately infl uence whether a person will develop a specifi c phobia. Unique

experiences (such as falling out of a tree and then becoming afraid of heights) ap-

pear to be very important (Kendler, Karkowski, & Prescott, 1999). In fact, animal

and situational phobias are most likely to arise as a consequence of specifi c unique

experiences with that particular animal or situation.

The sum of the research fi ndings about neurological factors suggests that par-

ticular life experiences can lead to a particular specifi c phobia for people who—

through genes or other life experiences—are neurologically vulnerable (Antony &

Barlow, 2002; see Feedback Loops in Action section, below).

Psychological Factors

Life experiences always have their impact via how a person perceives and interprets

them. Thus, psychological factors play a key role in whether a person will develop a

specifi c phobia. Three primary psychological factors contribute to a specifi c phobia:

a tendency to overestimate the probability of a negative event’s occurring based on

contact with the feared stimuli, classical conditioning, and operant conditioning.

Faulty Estimations

As we saw with social phobia, people who have a specifi c phobia have a particular

cognitive bias—they believe strongly that something bad will happen when they

7.11 • Heritabilities of

Understanding Specifi c Phobias

Source: American Psychiatric

Association, 2000.

Figure 7.11g7

100

90

80

70

60

50

40

Heritability (percentage)^30

20

Blood-

injection-

injury

phobia

Animal

phobia

Situational

phobia

10

0

P S

N