A mental disorder reportedly suffered by 1% of the world population and most commonly diagnosed between the ages of 15 and 35. It can affect a sufferer's thought processes, physical functions and perception of reality.
Symptoms of schizophrenia
Positive symptoms
Negative symptoms
Positive symptoms
Symptoms in addition to normal functioning, including hallucinations and delusions
Hallucinations
Unusual perceptions of environment stimuli unique to the individual, which no one else can perceive. They are usually auditory, but may also be visual, olfactory or tactile.
Delusions
Irrational beliefs that seem real, but are not true in reality. Paranoid delusions involve believing they are being persecuted by others. Delusions of grandeur involve exaggerated beliefs about one's own abilities or importance.
Negative symptoms
Involve a reduction in normal functioning, including avolition and speech poverty
Speech poverty
A significant reduction in the amount or quality of what is spoken, involving reduced verbal fluency, reduced language complexity, or a notable delay in verbal responses during a conversation.
Avolition
The difficulty to begin and maintain goal-directed behaviour, involving significantly reduced self-motivation to take part in activities despite having the opportunity to do so.
Reliability
(in the context of diagnosis) The consistency of measuring the symptoms of schizophrenia, which can be assessed through test-retest reliability and inter-rater reliability.
Validity
(in the context of diagnosis) The accuracy of measuring symptoms and whether classification systems can distinguish schizophrenia from other disorders, which can be assessed through concurrent validity and predictive validity.
Culture bias concerns differential treatment towards ethnic groups, leading to a tendency for members of certain ethnic minorities to be over-diagnosed with schizophrenia.
Symptom overlap occurs when the characteristics of a particular disorder are shared with another, affecting the inter-rater reliability of diagnosis.
Co-morbidity
When a person has two or more disorders at the same time, affecting the validity of classification and diagnosis.
Gender bias refers to differential treatment or representation of males and females, based on stereotypes, leading to males being statistically more likely to be diagnosed with schizophrenia than females.
Genetic basis of schizophrenia
Biological characteristics are inherited from parents, with many candidate genes contributing to the risk of developing schizophrenia.
COMT gene
A candidate gene that controls an enzyme that breaks down dopamine, with a low activity variant leading to excessive dopamine activity linked to hallucinations.
Polygenetic explanation
Symptoms of schizophrenia are a result of a complex combination of many candidate genes, with no single gene responsible.
Twin studies
Investigate the genetic basis of schizophrenia by comparing concordance rates (the number of twins within a sample who both have the disorder) between monozygotic (identical) and dizygotic (non-identical) twins.
Gottesman (1991) reviewed over 40 twin studies and found the average concordance was 48% for MZ twins and 17% for DZ twins, suggesting schizophrenia is at least partly genetic.
Joseph (2004) conducted a meta-analysis of all twin study data prior to 2001, reporting a MZ twin concordance rate of 40%.
Monozygotic (identical) twins
Twins who share 100% of their genetic material
Dizygotic (non-identical) twins
Twins who share approximately 50% of their genetic material
Concordance rates
The number of twins within each sample who both have the disorder, usually represented as a percentage
If the concordance rate is significantly higher for MZ twins than for DZ twins, this suggests schizophrenia has a genetic basis
Gottesman (1991) reviewed over 40 twin studies and found the average concordance was 48% for MZ twins and 17% for DZ twins, which suggests schizophrenia is at least partly genetic
Joseph (2004) conducted a meta-analysis of all twin study data prior to 2001 and reported a MZ twin concordance rate of 40% compared to 7% for DZ twins
The concordance rate for MZ twins was not close to 100%, which suggests schizophrenia is not purely genetic
The findings indicate that although genetics are strongly involved in schizophrenia they are not the sole cause of the disorder and that other factors may be involved
A methodological issue of investigating concordance rates between family members living together is the fact that they are often affected by similar environmental influences
The regularly reported high concordance rates for schizophrenia between related individuals could therefore be due to exposure to family dysfunction and not genes
Tienari et al.'s (2000) Finnish adoption study found that 7% of the adopted children with schizophrenic mothers were diagnosed with the disorder, whereas only 2% of the adopted children with non-schizophrenic mothers received a diagnosis
The prevalence of schizophrenia is 3 times higher for the children with a biological mother with the schizophrenia
Genes may predispose an individual to develop the biology that makes them vulnerable to the disorder; however, they may demonstrate free will by making particular life choices that means they encounter environmental situations that mitigate any potential innate factors
Dopamine
A neurotransmitter which has many functions in the brain
An increase in dopamine activity (hyperdopaminergia) in the subcortex of the brain has been linked to positive symptoms of schizophrenia
A decrease in dopamine activity (hypodopaminergia) in the cortex has been linked to negative symptoms of schizophrenia
Typical antipsychotic drugs (such as chlorpromazine) have an antagonistic effect by blocking dopamine receptors
Thornley et al. (2003) reviewed 13 trials (1121 participants) and found that chlorpromazine was associated with better overall functioning and reduced symptom severity
The fact that other neurotransmitters may be involved in schizophrenia has led to the evidence for the role of dopamine being inconclusive
Noll (2009) claims antipsychotic drugs only help alleviate symptoms such as hallucinations in one-third of cases