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Causes of schizophrenia
The Genetic
Explanation
Learning Objectives
By the end of this lesson you will:
Be able to outline how the genetic
approach explains schizophrenia
 Have many evaluative points at your
disposal for the exam
 Have revisited key aspects of the
research methods (AO3) content

Starter
In pairs, discuss how the biological
approach may explain schizophrenia
i.e. what causes schizophrenia?
Genetic explanations
 Biochemical explanations
 Brain structure
 Season of birth

Genetic explanations
Schizophrenia runs in families
 But we can’t assume it’s inherited; eg. could
be that families share same disadvantaged
background..? Environmental Factors!
 Research suggests genetic factors are
important:
 Family studies
 Twin studies
 Adoption studies

A graph to show the
genetic risk of developing
schizophrenia.
Source: Zimbardo et al
(1995)
These concordance rates suggest that
schizophrenia is caused by our genes.
Problems with family studies
Although Kendler et al (1985) found that 1st degree
relatives of those with schizophrenia are 18 times
more at risk than the general population, family
studies:
 Are often inconclusive as they’re conducted
retrospectively – compare a cross-section of
people already diagnosed
 Therefore can’t easily differentiate between
genetic and environmental influences
 But prospective (longitudinal studies) can be
used to provide more reliable data…

The Copenhagen High-Risk Study
(Kety et al. 1962)
Kety and collegues identified 207
offspring of mothers diagnosed with
schizophrenia (high risk) along with a
matched control of 104 children with
‘healthy’ mothers (low risk) in 1962
 Children aged between 10-18 years at
start of study
 Matched on age, gender, parental socioeconomic status and urban/rural
residence

The Copenhagen High-Risk
Study (Kety et al. 1962)
Follow-up studies conducted in 1974 and 1989
Results:
 Schizophrenia diagnosed in 16.2% of high risk
group compared to 1.9% in low risk group
 Schizotypal personality disorder diagnosed in
18.8% of high risk group vs 5% of low risk
group
(Schizotypal Personality = A disorder characterized by eccentric behaviour
and anomalies of thinking and affect which resemble those seen in
schizophrenia, thought no definite and characteristic schizophrenic
anomalies have occurred at any stage )
The Copenhagen High-Risk
Study (Kety et al. 1962)
Combining the figures for the 2
disorders = 35% in high-risk group
and 6.9% in low-risk group
 Similar findings found in the New
York High Risk Project = 25 yr
follow-up (Erlennmeyer-Kimling et al.
1997)

Twin Studies

Compare concordance rates for identical
(MZ) and fraternal (DZ) twins
 Both share the same environment but only
MZ twins have identical genetics
 Many studies conducted – all show much
higher concordance rate in MZ than DZ
twins
 To separate environment from genetic
influences, researchers have sought out
MZ twins reared apart where at least 1 has
been diagnosed with schizophrenia
Gottesman & Shields (1982)

Used the Maudsley twin register and found 12
MZ twins who had been raised apart where at
least one of the pair had schizophrenia
 In 58% of cases (7 out of the 12 MZ twin pairs
reared apart) it was found that the other twin
also developed schizophrenia
 AO2: so even when there is no shared
upbringing / environment, we still see high
concordance for MZ twins. Points to factors
related to NATURE not NURTURE.
AO2 of twin studies

Twins often reared in same environment so
cant separate genetic and environmental
factors
 The number of MZ twins reared apart are few
in number – therefore cannot easily generalise
on such a small sample
 Reason MZ twins reared apart may be due to
problems in the family which could have been
a starting point for psychosis
 Even twins reared apart shared the same
environment in the womb before birth =
environmental factors cant be discounted
Adoption Studies

More effective in separating effects of genetic
and environmental factors
 Look at adopted children who later develop
schizophrenia and compare to biological
parents
The Finnish Adoption Study (Tienari, 1969))
 Identified adopted offspring of biological
mothers with schizophrenia (112 cases)
 Matched control group (135 adopted offspring of
non-schizophrenic biological mothers)
The Finnish Adoption Study (Tienari, 1969)
Adoptees ranged from 5-7 yrs at the start
of the study – all had been separated from
mothers before 4
 Study checked children again in 1987
 Reported 7% of high risk group developed
schizophrenia compared to 1.5% of
controls

AO2 of Adoption studies
Data provided by prospective studies
suggest a strong genetic link for
schizophrenia
 Longitudinal studies are hampered as
diagnostic criteria for schizophrenia is
constantly being updated and changed

AO2 of genetic explanations

Twin, adoption and family studies
continues to provide reliable evidence that
the degree of risk increases with degree of
genetic relatedness
 No twin study has yet shown 100%
concordance in MZ twins
 Studies conducted so far don’t tell us which
genes might be important for the
transmission of schizophrenia.
= genetic risk
?
symptoms