What have twin studies revealed about the causes of schizophrenia?
Introduction
Schizophrenia has been diagnosed in ‘around 1% of the population’ (Workman, 2012), which
means that it affects a great number of people globally. Therefore, it is important to find more out
about the causes of Schizophrenia to aid developments in areas of diagnosis and in treatment.
One of the most frequently used methods for studying the causes of Schizophrenia is of twin
studies, where monozygotic (identical) twins are compared with dizygotic (non-identical) twins to
look for concordance rates of a particular characteristic or disorder to determine whether there is a
biological or environmental link for those characteristics or disorders
This essay will investigate what these studies have revealed about the possible causes of
Schizophrenia and whether these findings fall on the nature or the nurture side of the debate.
Some will argue that the disorder has a clear genetic link and therefore is pre-determined, while
others would say that our environment is what shapes us and may cause Schizophrenia. A final
argument would say that is a combination of both environmental and biological factors that cause
the disorder; a multifactorial system.
A definition of Schizophrenia
Although there are some disparities in the definition of Schizophrenia globally, there are some
elements of the disorder which are frequently noted. ‘Schizophrenia is a disorder of thinking where
a person’s ability to recognise reality, his or her emotional responses, thinking processes,
judgement and ability to communicate deteriorates so much that his or her functioning is seriously
impaired.’ (Birchwood & Jackson, 2001) Therefore Schizophrenia is considered a psychotic illness
under the psychosis heading, rather than the neurosis heading, based on the fact that for a
disorder to be a form of psychosis, a person can no longer distinguish what is reality and the
symptoms that they are experiencing.
Some of the major symptoms of Schizophrenia can be divided between positive and negative
symptoms. Positive symptoms include behaviours which are in addition to normal function, with
common examples being auditory or visual hallucinations, disorganised speech, delusions ,
otherwise known as ‘false beliefs whereby the sufferer believes they are someone they are clearly
not (such as a member of the royal family’ (Workman, 2012), as well as also movement disorders,
for example, repetitive movements or catatonia, which ‘ is a state in which a person does not
move and does not respond to others.’ (National Institute of Mental Health, 2009)In comparison
negative symptoms are when a behaviour associated with normal functioning is no longer present.
Symptoms include: social withdrawal, little or no speech, ‘flat affect’ or general apathy.
Schizophrenia is a complex disorder which can be divided into sub categories. This is relevant to
the study of twins as a concordant set of twins may not be diagnosed with the same type of
Schizophrenia. Some of these sub types include paranoid schizophrenia, catatonic schizophrenia
and disorganised schizophrenia.
Furthermore, Schizophrenia usually develops in stages; the first of these stages is the Prodromal
phase, when a person will begin to show signs of Schizophrenia, which typically will include
isolation, lack of motivation and willingness to participate in activities. However, it is clear that
these behaviours may be due to other emotional factors, general teenage behaviour or may relate
to other disorders such as depression, ‘Until a patient experiences psychotic symptoms, a physician
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cannot diagnose schizophrenia.’ (Veague, 2009) This is why Schizophrenia is difficult to diagnose
and highlights how twins’ studies could useful; knowing how one twin experiences the prodromal
stage may make it easier to spot the onset of the disorder in the other twin. Furthermore,
comparisons on the effect of the environment on how they experience the start of the disorder can
be investigated. The stages after the prodromal period include the acute stage and the residual
stage.
Schizophrenia is not necessarily a long term disorder, yet many people who have suffered with the
disorder suggest the stigma and distress of Schizophrenia will affect them throughout their lives. It
is believed that the chance that Schizophrenia will be a lifelong condition is 1% and ‘between 20%
and 25% will have one episode with full remission.’ (Birchwood & Jackson, 2001)This means that
these peoples experience of the illness can most valuable as it can show the effectiveness of the
treatment they were given. Schizophrenia is typically treated using antipsychotic medication and /
or psychosocial treatments such as Cognitive Behavioural Therapy (CBT) or rehabilitation.
DSM IV definition of schizophrenia
A. Characteristic symptoms: Two (or more) of the following each present for a significant portion of
time during a 1-month period (or less if successfully treated):
(1) delusions
(2) hallucinations
(3) disorganised speech (e.g., frequent derailment or incoherence)
(4) grossly disorganised or catatonic behaviour
(5) negative symptoms, i.e., affective flattening, alogia, or avolition.
Note: only one Criterion A symptom is required if delusions are bizarre of hallucinations consist of a
voice keeping up a running commentary on the person’s behaviour or thoughts, or two or more voices
conversing with each other.
B. Social/occupational dysfunctions: For a significant portion of the time since the onset of the
disturbance, one or more major areas of functioning such as work, interpersonal relationships, or selfcare are markedly below the level of achieved prior to the onset (or when the onset is in childhood or
adolescence, failure to achieve expected interpersonal, academic, or occupational achievement).
C. Duration: Continuous signs of the disturbance persist for at least 6 months. This 6-month period must
include at least 1 month of symptoms (or less if successfully treated) that meet Criterion A (i.e.,
active – phase symptoms) and may include periods of prodromal or residual periods, the signs of the
disturbance may be manifested by only negative symptoms or two or more symptoms listed in
Criterion A present I an attenuated form (e.g., odd beliefs, unusual perceptual experiences).
D. Schizoaffective and mood disorder exclusion: Schizoaffective disorder and mood disorder with
psychotic features have been ruled out with (1) no major depressive, manic, or mixed episode have
occurred concurrently with the active-phase symptoms; or (2) if mood episode have occurred during
active-phase symptoms, their total duration has been brief relative to the duration of the active and
residual periods.
E. Substance/general medical condition exclusion: The disturbance is not due to the direct physiological
effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition.
F. Relationship to a pervasive development disorder: If there is a history of autistic disorder or another
pervasive development disorder, the additional diagnosis of schizophrenia is made only if prominent
delusions or hallucinations are also present for at least a month (or less if successfully treated).
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ICD diagnostic criteria for schizophrenia
A. One of the symptoms under A1
Or two of the symptoms under A2
Must be present for most of the episode lasting at least a month
A1a. Thought echo, insertion, withdrawal, broadcasting
A1b. Delusions of control, influence, passivity, delusional perception
A1c. Verbal hallucination, with running commentary or discussing patients or coming from a part of
the body
A1d. Delusions that are persistent and culturally implausible
A2e. Persistent hallucinations with half-formed delusions with clear affective content.
A2f. Breaks in train of thought, giving rise to incoherent or irrelevant speech, neologism
A2g. Catatonic behaviour
A2h. Negative – apathy, paucity of speech, blunted or incongruent affect, not due to depression or
medication
B. If manic or depressive episode, Criterion A must be met before mood disturbance developed.
C. Not attributable to organic brain disease (FO) or substance abuse (F1).
All three conditions, A, B and C must be satisfied.
Figure 1: the DSM IV and the ICD 10 definitions of schizophrenia, which highlights the complex components of the
disorders as well as the number of factors that can be considered in diagnosis (Birchwood & Jackson, 2001, pp. 11-12)
Historical context
Twin studies themselves have been used for many years. In the early 19th Century when the
psychiatrist Benjamin Rush commented on the usefulness of twins in study mental illness. Since
then many researchers have used twins as participants for studies on development, mental illness,
language, cognition and learning difficulties. One of the most infamous researchers of twins was
Josef Mengele, who worked in Auschwitz during the holocaust and who was particularly interested
in discovering information on how twins are different to the rest of the population. His procedures
on twins ranged from attention and memory where a twin would be required to do ‘reading aloud
and remembering the sentence read ’ as well as looking at twins ‘spiritual development’.
(Strzelecka, 2011) However, his work could often be distressing and cause much pain to his
victims, with one set of twin ‘so much blood was drawn that they died.’ (Strzelecka, 2011) During
his time at Auschwitz Mengele, ‘identified approximately 1,500 twin pairs,’ (Fuller Torrey, et al.,
1994) This highlights the scale of his work, which demonstrated to others the benefits of using
twins in research. Furthermore, studies were completed on twins in Germany in the 1920’s to test
for Schizophrenia; Luxenburger reported higher concordance rate of schizophrenia for identical
twins (58%) than for fraternal twins (0%). This was the start of many studies of twins with
findings showing various concordance rates.
The historical context of twin studies of Schizophrenia shows the development in methods from
the use of diagnostic details by psychologists and researchers through to modern methods such as
fMRI (functional Magnetic resonance imaging) and EEG (Electroencephalography) scans which are
used to investigate brain volume and activity. Studies conducted in the 1950s-60s have been
documented on Mittler’s book ‘The Study of twins.’ (Mittler, 1971)
Mittler argues for and against the use of the twin method in studying the causes of Schizophrenia.
He suggests that twin studies are limited as they do not account for the psychological differences
between twins and the rest of the population, which is investigated further in the essay.
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Furthermore, Mittler noted an important limitation of the study of twins in Schizophrenia is that
there is difficulty in ‘obtaining a consensus of agreement on the nature of Schizophrenia,’ (Mittler,
1971, p. 124) meaning that there was not a universal definition of Schizophrenia and even two
colleagues in the same hospital, using the same diagnosis manual, may not come to the same
diagnosis. This could imply that some of the studies conducted by individual researchers may lack
credibility, as there is the possibility of experimenter bias, with either an intentional diagnosis of
Schizophrenia if they are of the view that it is a genetic condition, or basing their diagnosis on a
particularly limited definition. To reduce the levels of researcher bias diagnoses could have been
gained from other clinical psychologists or professionals in this field. This would have enhanced
the inter-rater reliability and would strengthen the credibility of the results.
In summary, based on increased knowledge and research it could be fair to suggest that current
definitions of Schizophrenia are the most accurate, which means that some of the earlier pieces of
research may be less credible than the work conducted nowadays. Yet, the work of Gottesman
and Shields is well renowned and is frequently referenced by psychologists, meaning it is felt it still
is valid. Their work also included an additional meta-analysis, which is a review of similar studies
and analysis of the collective data, in this case of results from twins’ studies of schizophrenia,
including their own.
A Significant Study
Gottesman and Shields conducted a study to investigate whether there is genetic basis of
Schizophrenia and to present a critical analysis of 11 major twin studies to date. Records were
reviewed from 1948-1964. Participants were selected from 392 twin patients, with 62 twins used,
57 pairs of which were identified as being schizophrenic. The sample included both male and
female Monozygotic (MZ) and Dizygotic (DZ) twins concordant or discordant with schizophrenia.
Because the study was completed before DNA testing ‘zygosity was determined by a combination
of blood grouping, finger-print analysis and resemblance in appearance.’ (Gottesman & Shields,
1966) Knowing this information about MZ and DZ is vital as concordance rates must be valid, if a
twin is wrongly labelled the results will contribute to data which will be incorrect. For the rarity of
the situation this sample size is respectable and reflects the target population. Furthermore the
sample used in the study compared results from twins between the ages 19-64, showing how the
researchers were conscious of needing a representative cross sectional sample of the population,
as this age range is inclusive of the group of people who are most likely to be diagnosed with
schizophrenia. This makes the study valid as the results can be generalised to other people.
These 16 years of records included the case histories of the 57 twin pair patients, in a short stay
hospital for psychiatric patients, as well as qualitative data collected from semi structured
interviews and personality profiles, completed both for the twins and for the families. Gottesman
and Shields found that there was a 42% concordance rate for monozygotic twins and a 9%
concordance rate for dizygotic twins. This evidence indicates a link between schizophrenia and
genetics because the concordance rate is so contrasting between the MZ and DZ twins. As MZ
twins have identical DNA and there is concordance in schizophrenia, it seems the evidence points
to a genetic basis in the disorder. However, in both cases the chance of developing Schizophrenia
is less than the chance of not developing the disorder; 58 is higher than 42, and 91 is higher than
9, which may suggests that genetics do not predetermine schizophrenia, rather that there may be
a combination of environmental cues that triggers the disorder. As a single study, this may lack
reliability, yet it has been supported by evidence from other studies.
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Below is a compilation of data some of which was used in Gottesman, Shields and Slaters (1967)
review of 11 studies of schizophrenia. They found that in the studies they compared there was an
average MZ concordance rate of 53% and an average DZ concordance rate of 11%. By finding this
average it increased the reliability of the results and a common trend was found to support the
role of genetics as the cause for schizophrenia. (Mittler, 1971)
On the other hand, all these studies have been conducted in Western nations, which may suggest
the perspective that a genetic link to schizophrenia may be ethnocentric as there is not as much
evidence from other nations around the world. From this evidence many have presumed that there
was a genetic link, but as we don’t know about concordance rates in other countries it may just be
that these nations environments or systems of diagnosis are different, which could impact the
disorders prevalence.
Researcher (s)
Kallmann
Adult
Children
Slater
Resident
Consecutive
Essen-Möller
Rosanoff et al.
Inouye
Luxenburger
Gottesman and Shields
Fischer, Harvard and Hauge
Harvard and Hauge
Kringlen
Tienari
Pollin et al
Year
Country
%
MZ %
DZ
Concordance
Concordance
1946
1956
USA
USA
69
88
11
23
1953
1953
1941
1934
1961
1928
1966
1969
1965
1964
1968
1969
UK
UK
Sweden
USA
Japan
Germany
UK
Denmark
Denmark
Norway
Finland
USA
65
64
64
61
60
58
42
24
29
25
31
14
11
16
15
15
18
0
9
10
6
17
5
4
Table 1: A compilation of data collected by Gottesman, Shields and Slaters (1967)
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Concordance rates %
A graph showing the concordance rates for schizophrenia in
Monozygotic and Dyzgotic twins found in studies from 1928-1969
100
90
80
70
60
50
40
30
20
10
0
MZ concordance %
DZ concordance %
Researchers
Figure 2: A line graph showing a convergence of concordance rates over 40
Concordance rate
A bar graph showing the difference in concordance
rates between MZ and DZ twins
100
90
80
70
60
50
40
30
20
10
0
MZ concordance %
DZ concordance %
Figure 3: A bar graph showing the significant contrast in concordance rates for MZ and DZ twins
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An argument proposed by Joseph is that as time has passed the connection between genetics as
the cause of schizophrenia has weakened, as the concordance rates for MZ twins decrease
(Joseph, 2003). He suggests that this is due to improved knowledge of the mental disorder and
ability to accurately diagnose it, believing that more recent studies are, ‘generally considered more
methodologically sound.’ (Leo, 2003) This appears to have some credibility as the general trend in
the concordance rates is a decline in the percentage of MZ twins being diagnosed with
schizophrenia. This may suggest that the studies lack validity and the diagnoses’ made were not
accurate in some of the earlier studies, implying a lack of predictive validity, where if a diagnosis
was made now with the same case history it would not be the same as the previous judgment.
It is clear that the outlier in this data is of Kallmann's study of children which as a MZ concordance
rate of 88% this is particularly high. It has been suggested by Mittler that Kallmann intentionally
diagnosed some of his sample with schizophrenia to make his results more significant. It could be
said that this is particularly relevant in this instance as it is very difficult to diagnose children with
schizophrenia as they are not in the age range when schizophrenia is most prevalent.
Furthermore, as the only person to diagnose the sample it cannot be said that the data is
particularly reliable, as such a high concordance as that is rarely or never found.
Yet in every one of these studies the concordance rates for MZ twins are higher than for DZ and
this must be considered as it shows there is a distinction in the prevalence between the two
groups which may imply a genetic basis to the disorder. To investigate this suggestion a statistical
test can show the significance of the data as to whether the results are a coincidence or if they
hold much importance. Because there are two sample groups of data; MZ and DZ, the test needs
to investigate the difference, therefore a Mann – Whitney U check is necessary.
A statistical analysis of this data, using a Mann-Whitney U shows that the figures are significant at
the 1% level which means that there is a clear distinction between the two sets of results,
indicating a solid pattern in the difference between MZ and DZ concordance rates for
schizophrenia. This means we can be 99% sure that the results are not due to chance and they
are in fact significant. However, the sample only includes data from between 1928- 196. Thus it
cannot be assumed that the significance of the studies post -1969 would be similar, which, if
Joseph’s argument is to be believed, would decrease the level of significance to some degree and
therefore lower its relevance to the Psychological field. Nevertheless, at this point due to the
significance of the results from this sample, it could be presumed that the explanation is genetic,
as the result of the Mann-Whitney U confirms that there may be a link, as has been suggested
between identical DNA and the likelihood of the disorder.
A Study using a range of techniques
The study was conducted between 1987-1992 in Washington, with 64 identical twin pairs as the
participants, comparing identical twins that were concordant and discordant for the disorder. The
aim of the study was to investigate both genetic and non-genetic influences, such as stress or
cerebral trauma, on the development of Schizophrenia. The twins underwent thorough
examination using MRI scans, fingerprinting, blood tests, eye tracking tests, EEGs
(Electroencephalography), as well as, observations, clinical interviews and parent questionnaires.
Information was also collected on substance abuse or dependence, family history, medication
records and a SANS assessment.
The study found that of 13 concordant twin pairs for schizophrenia, within 4 pairs the twin pairs
suffered from the same diagnosis sub-type, whether it was paranoid schizophrenia or
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undifferentiated schizophrenia; whilst 9 pairs suffered from different diagnosis sub-types. This
suggests that there may be an environmental difference which affects how the disorder is
categorised for each twin. Yet it was also found that the living situation of the twin pairs was often
similar: both twins living with family, or being hospitalised or living with a partner, although there
were some disparities with this rule. It could be suggested that identical twins have similar ways of
recovering from treatment post schizophrenia and that this may be pre-determined. (Fuller
Torrey, et al., 1994, pp. 152-154) These results emphasise the importance of the concept of
‘individual differences’, which suggests that even though people may be genetically different their
environment and upbringing will mean that no two people are the same. (Fuller Torrey, et al.,
1994)
Findings from MRI scans
It is not only diagnosis statistics that are looked at when studying twins and Schizophrenia. The
use of brain scans is important as they can show the physiological differences between those
monozygotic twins who are concordant or discordant with Schizophrenia. MRI scans conducted in
a study by Haren et al. (2004) have revealed that ‘Schizophrenic twins, whether from concordant
or discordant pairs, had smaller whole brain volumes than control twins.’ (Haren, et al., 2004) This
suggests that there is genetic role in causing Schizophrenia as brain size itself is actually different.
However, in the same study it was found that there was the potential for environment to influence
changes in the brain, which link to Schizophrenia. They discovered that in discordant twins there
were more ‘abnormalities in hippocampal, third and lateral ventricular volumes than concordant
twins’ (Haren, et al., 2004) and they suggested that these differences in volumes sizes of the brain
may be caused by non-genetic influences. This is important as genetic factors have often been
shown as highly important, yet this research suggests that the environment may have an impact
on out brain’s development. As the study had standardised procedures it could be assumed that
the results are reliable and if replicated consistent results would be found in brain volumes.
Figure 4: An image showing the difference in brain structure between a schizophrenic and nonschizophrenic twin, as described above
Corroborating evidence has found similar results with the ill twin typically having a ‘smaller left and
right hippocampi, a larger left lateral ventricle, and a larger total ventricle size than the mentally
well co-twin’ (McNeil, et al., 2000). This study of 22 monozygotic twins also interestingly found a
correlation between obstetric complications and prolonged labour and differing sizes in areas of
the brain for discordant monozygotic schizophrenic twins, with typically the schizophrenic twin
having ‘relatively large right ventricle size and relatively large total ventricle size.’ (McNeil, et al.,
2000) This suggests that there are environmental factors which impact on the development in
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Schizophrenia. Many researchers have supported the view that perinatal and birth complications
(PBCs) are associated with Schizophrenia, although a study by (Done, et al., 1991)of 49
schizophrenic patients ‘failed to find any difference,’ (Birchwood & Jackson, 2001) in brain size,
suggesting that there findings ‘can be taken as
providing evidence against a role of early brain
injury as an aetiological factor in schizophrenia.’
(Done, et al., 1991)
However, McNeil et al (2000) investigated more
areas of the twins’ lives by conducting surveys
with the parents on the birth of the child,
therefore improving the validity of the study.
Haren et al (2004) on the other hand were
limited, as they had no information on birth
complications or medical records both of which
could influence or cause Schizophrenia.
Although this research may focus on the
potential for new developments it may be
limited in that it isn’t only a snapshot of the
brains structure and does not show any changes or progression in brain volumes, hence it cannot
be confirmed that it is a cause and effect relationship only that there is a correlation between the
two factors. This evidence would be stronger if it was a longitudinal study to show the brain over
time and how it is affected by more advanced levels of schizophrenia versus a milder form,
perhaps in the prodromal stage and again compared against a control group of discordant twins
A Case Study - Genain Quadruplets
Although this is not a study of twins it is a fascinating case study which can help to develop our
understanding of the nature/nurture debate. All four of the Genain Quadruplets, named Nora, Iris,
Myra and Hester, were born in the USA in the 1930’s and were professionally diagnosed with
Schizophrenia by the age of 25. The rarity of the situation is increased as ‘blood studies have
shown them to be identical,’ (Fuller Torrey, et al., 1994). The sisters were of much interest to the
psychology world and to the media, which meant they were part of research and experiments,
including neuropsychological examinations, interviews and EEG’s (Electroencephalography), for
much of their adult life. It is said that ‘the chance of all four developing the disorder independent
of sharing the same genes is 1 in 2 billion.’ (Workman, 2012)It may therefore be presumed that
the cause of the disorder was genetic. However, the sister’s upbringing was unusual, which may
have been a contributing factor.
Figure 5: An image of the Genain Quadruplets
Interviews and extensive studies have found
that there were no birth complications with the sisters, yet as the girls grew older their father was
‘irritable, abusive and intrusive,’ (Mirsky, et al., 2000) and the sisters had few opportunities to
socialise with other children as they were ‘restricted in their social activities by their strict parents.’
(Fuller Torrey, et al., 1994, pp. 185-188) This would have meant that the girls spent a lot of time
together and this may have meant that if one of the girls showed signs of schizophrenia at an
earlier age, as Hester did, this strange situation and being so different from other families may
have worsened their emotional situation. The sisters were said to be in the prodromal stage at
differing ages but were all diagnosed between the age of 22-24. Yet their experience of the
disorder from here is very different, Myra, for instance, was hospitalised for 3 weeks and then
went on ‘to work as a secretary for most of her life, was married and had two sons.’ (Mirsky, et al.,
2000) In contrast, Hester spent 12 years hospitalised and Iris 10 years. Interestingly both Iris and
Hester ‘underwent circumcision as adolescents, surely a traumatic experience for a young woman.’
(Fuller Torrey, et al., 1994, pp. 185-188)This could show how environmental influences may have
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prolonged the disorder and how the experience of Schizophrenia may not be pre-determined by
genetics. The study could support the concept of epigenetics as they may have all had the
phenotype set for the development Schizophrenia, which was triggered by their unusual
upbringing and situation, but was experienced differently by the quadruplets because of their
individual differences and life experiences.
It must be remembered that this is only a single case study and therefore it does lack some
reliability; it would be difficult to obtain similar results as the situation is so exceptional.
Furthermore, the Quadruplets were hospitalised during the late 1950’s, where the diagnosis made
would be from an earlier, less developed DSM. Yet the research conducted over the years typically
confirmed that the diagnoses were accurate, which increases the predictive validity of the findings.
Epigenetics
Epigenetics is a theory that suggests that humans have some genomes which can be affected by
the environment, in that the way they are expressed may be altered by changes in our
surroundings. ‘The genome dynamically responds to the environment. Stress, diet, behavior, toxins
and other factors activate chemical switches that regulate gene expression.’ (Genetic Science
Learning Center, 2012) The genome can be activated or deactivated through a process of gene
regulation. If certain conditions are not in place the genome will not be affected, therefore being
neither on the side of nature nor nurture, but suggesting they collaborate. The suggestion that
schizophrenia is caused by epigenetics is of great importance; if the environmental triggers and
the pre-set genomes could be determined it may become easier to recognise the disorder earlier
or find ways of limiting its onset. As epigenetics is a more recent concept some of the research has
not been replicated many times, which may mean these ideas lack some reliability. Yet the ideas in
development are interesting and supported by some evidence.
A study in 2011 which conducted MRI scans of 22 pairs of identical twins may have found potential
chemical markings which when activated by triggers from the environment could lead to
schizophrenia. The researchers found differences in the twins with ‘up to 20 per cent in the
amount of methylation, were in the promoter “switch” for a gene called ST6GALNAC1, which has
been linked to schizophrenia.’ (Coghlan, 2011) It has been suggested that these variations are as
a result of environmental triggers such as, ‘stressful events or diet.’ (Coghlan, 2011) These
hypotheses are supported by animal research, which found that these environmental influences
affected the genetic components of mice. Sweatt, a researcher in this field suggests that this is
important evidence which supports hypotheses of psychosis and epigenetics. If this is found to be
consistent it may hold crucial answers in the psychosis area, affecting those researching
schizophrenia as well as other mental disorders. It could also show that the distinction between
nature and nurture as the cause of disorders is not separate and that both genetics and
environment affect this.
Furthermore, the American study mentioned earlier, has suggested there may be a link between
viruses, particularly those present in the prenatal stage of development, and the onset of
schizophrenia. This would suggest that our environment may affect us if we catch viruses. Yet it
some people may be more susceptible to viruses, as determined by their DNA, suggesting a
potential genetic cause. (Fuller Torrey, et al., 1994) Evidence, if further replicated and found to be
significant, may support the role of epigenetic as the cause of schizophrenia.
A recent cohort study found a link between premature births and an increased risk for the
development of psychosis, as classified by the ICD; ‘Those born at less than 32 weeks' gestation
were 2.5 times more likely to have nonaffective psychosis.’ (Nosarti, et al., 2012)
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This study indicates an epigenetic role in schizophrenia and other psychotic disorders, as the
environmental cue was an early birth. Unlike the previous studies mentioned, this research was
completed with a large scale sample of non-twins from Sweden, which indicates that twin studies
are not always necessary in psychiatric research to find significant results.
Limitations of the Twin Method
Within the twin method it could be said that twins are seen homogenously, with their individual
differences disregarded. Naturally, their appearance if they are identical will be similar however
this is an outward perspective, when twins themselves can be completely different in personality.
Furthermore twins may be atypical to the population which means that the results from the studies
mentioned previously could lack some validity. Within the twin method there is an EEA (Equal
Environment Assumption), strongly advocated by Jay Joseph, (Joseph, 2003), which presumes
that because their participants are twins they will be affected by the same external influences,
within areas such as upbringing and schooling.
‘The twin method is an environmentally confounded technique which is unable to disentangle
possible genetic and environmental factors.’ (Joseph, 2003, pp. 81-82) This implies that the twin
method is an ineffective way of investigating the nature/nurture debate as data cannot show
whether a brain abnormality or behaviour shown is caused by genetics or by environmental factors
such as stress.
Mittler argues that there is a bond between twins which is strong and which may mean that they
are not reflective of the target population who may not have this bond which is twin specific. If
monozygotic twins are found to have a higher concordance rate for a disorder than DZ twins then
it is assumed that this implies the disorder has a genetic cause. However, if MZ twins are more
likely to spend time together they may be influenced by each other’s development; they may see
each other’s behaviour as normal and imitate what they see in each other. Additionally, ‘Many
clinical studies, for example, report that one twin is dominant over the other,’ (Mittler, 1971, p.
51)which also have an emotional influence on behaviour. It has also been suggested that twin’s
experiences in the pre-natal period mean that they are already differences in the twin before they
are born. This would mean that their environment pre-birth pre-determines their behaviour,
limiting the importance of the
role of genetics.
Moreover, it has been found by
the World Health Organisation
that the age group at which
schizophrenia is most
prevalent in both males and
females is 24-29, when twins
could be assumed to no longer
have an ‘identical,’
environment then it may be
said that at this point the blur
between genetics and the
environment is too
confounding to draw precise
conclusions on the causation of
schizophrenia.
Figure 6: Chart showing the age distribution of a sample of people with
schizophrenia (Birchwood & Jackson, 2001, p. 29)
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A further limitation of twin studies of schizophrenia is that they are limited in their use in
researching alternative causes of the disorder. Although twin studies have some ability to show a
link between genetics or the environment in the development of schizophrenia, they are limited in
that they are less able to show how other influences affect it. For instance, they do not include the
impact of age or gender, the impact of substance misuse, excessive dopamine levels or the
correlation between social class and schizophrenia. These are all areas which have had multiple
studies and correlations have been found to some degree.
Conclusion - A return to the Nature / Nurture debate
The topic of this essay has been studied over many decades, with developments being made in
diagnosis and research methods. It could be said that there is strong and consistent evidence
which suggests that genetics are linked with schizophrenia. For many years the nature element of
the nature/nurture debate has held the role of genetics as the answer to psychosis. Yet in more
recent year the importance of our environment has been emphasised, with concordance rates and
the reliability of diagnosis being questioned. It now seems that a combination of both nature and
nature is a more appropriate answer to, ‘what have twin studies revealed about the causes of
schizophrenia?’ with an understanding that both the disorder and the method are complex. This is
a simplistic answer, but one that can be supported by much evidence. If this essay is written
again in 20 years’ time, it would be expected that the conclusion will be different and new studies
will emerge to support a multifactorial system.
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Table of Figures
Figure 1: the DSM IV and the ICD 10 definitions of schizophrenia, which highlights the complex components
of the disorders as well as the number of factors that can be considered in diagnosis (Birchwood & Jackson,
2001, pp. 11-12) ..................................................................................................................................... 3
Figure 2: A line graph showing a convergence of concordance rates over 40 ........................... 6
Figure 3: A bar graph showing the significant contrast in concordance rates for MZ and DZ twins6
Figure 4: An image showing the difference in brain structure between a schizophrenic and nonschizophrenic twin, as described above............................................................................................ 8
Figure 5: An image of the Genain Quadruplets ..................................................................................... 9
Figure 6: Chart showing the age distribution of a sample of people with schizophrenia (Birchwood &
Jackson, 2001, p. 29) .............................................................................................................................. 9
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