Psychological Disorders 575If schizophrenia were entirely controlled by genes, identical twins would indeed
both have the disorder at a risk of 100 percent, not merely 50 percent. Obviously,
there is some influence of environment on the development of schizophrenia.
One model that has been proposed is the stress-vulnerability model, which
assumes that persons with the genetic “markers” for schizophrenia have a phys-
ical vulnerability to the disorder but will not develop schizophrenia unless they
are exposed to environmental or emotional stress at critical times in develop-
ment, such as puberty (Harrison, 1999; Weinberger, 1987). That would explain
why only one twin out of a pair might develop the disorder when both carry
the genetic markers for schizophrenia—the life stresses for the affected twin
were different from those of the one who remained healthy. The immune sys-
tem is activated during stress, and one recent study has found that in recent-on-
set schizophrenia (the early stages of the disorder), the brain’s immune system
secretes high levels of an inflammation-fighting substance, indicating a possible
infection (Söderlund et al., 2009). This leads to the possibility that schizophrenia
might one day be treatable with anti- inflammatory medications.
Both structural and functional neuroimaging have provided information
about how schizophrenia affects the brain, or how the brain operates in an indi-
vidual with schizophrenia. In one study, researchers using diffusion tensor imag-
ing (DTI), to Learning Objective 2.9, in addition to other neurological
testing, found that, when compared to healthy control participants, participants
with schizophrenia showed structural differences in two particular areas of the
brain (Nestor et al., 2008). Specifically, a white matter tract called the cingulum
bundle (CB) that lies under the cingulate gyrus and links part of the limbic sys-
tem, and another that links the frontal lobe to the temporal lobe, were found
to have significantly less myelin coating on the axons of the neurons within
the bundle. This makes these areas of the brain less efficient in sending neural
stress-vulnerability model
explanation of disorder that assumes a
biological sensitivity, or vulnerability,
to a certain disorder will result in the
development of that disorder under
the right conditions of environmental
or emotional stress.Nestor et al. (2008) used diffusion tensor imaging to
investigate schizophrenia. Two of the brain areas examined
were the cingulum bundle (CB, consisting of fibers
underlying the cingulate gyrus linking parts of the limbic
system) and the uncinate fasciculus (UF, neural fibers
linking the frontal lobe to the temporal lobe). The cingulum
bundle is depicted in the image above. For individuals with
schizophrenia, both the CB and UF fiber pathways were
found to have neurons with significantly less myelin, making
them less efficient in information transfer and resulting in
decreased memory and decision-making ability. Image
courtesy of Dr. Paul Nestor.Figure 14.3 Genetics and Schizophrenia
This chart shows a definite pattern: The greater the degree of genetic relatedness, the higher the risk of
schizophrenia in individuals related to each other. The only individual to carry a risk even close to that of
identical twins (who share 100 percent of their genes) is a person who is the child of two parents with
schizophrenia.
Based on Gottesman (1991).
General population (1%)
Spouses of patients (2%)
First cousins (2%)
Uncles/aunts (2%)
Nephews/nieces (4%)
Grandchildren (5%)
Half-siblings (6%)
Parents (6%)
Siblings (9%)
Children (12%)
Siblings with
schizophrenic parent (17%)
Dizygotic twins (17%)
Offspring of two
schizophrenic parents (46%)
Monozygotic twins (48%)Interactive