You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Biological explanations of schizophrenia locate the origins of the disorder in physiological processes: genetic inheritance, neurochemical activity (principally dopamine) and structural abnormalities of the brain (neural correlates). These explanations are supported by a substantial and methodologically diverse body of research, and they underpin the drug treatments examined later in this option. They have also attracted sustained criticism for being reductionist and deterministic, and for the difficulty of establishing cause rather than correlation. This lesson examines the genetic basis, the dopamine hypothesis (original and revised) and the principal neural correlates, all discussed in the measured, scientific register appropriate to clinical psychology.
Key Definition: Biological explanations propose that schizophrenia arises primarily from physiological factors — inherited genes, neurotransmitter activity (especially dopamine) and brain structure — rather than from psychological or social factors alone.
This lesson covers the biological explanations for schizophrenia named in the AQA 7182 Paper 3 option: genetics and neural correlates, and the dopamine hypothesis. You must be able to describe the genetic evidence (family, twin and adoption studies, and the polygenic picture from genome-wide association studies), the original and the revised dopamine hypothesis (hyperdopaminergia in the subcortex; hypodopaminergia in the prefrontal cortex; the role of D2 receptors), and neural correlates such as enlarged ventricles. You must then evaluate these explanations as AO3, including the correlation-versus-causation problem, reductionism and determinism, and the way the biological account is best integrated with psychological factors through the diathesis-stress model (developed fully in the next lesson). The recurring examiner theme is that strong biological evidence does not, on its own, establish that biology is a complete cause.
Schizophrenia tends to run in families, which prompted researchers to investigate its genetic component using family studies, twin studies, adoption studies and, more recently, molecular genetics. Each design is intended to separate the contribution of shared genes from that of shared environment.
Family studies compare the rate of schizophrenia among the biological relatives of a diagnosed individual (the proband). The general finding, summarised by Gottesman (1991) in a classic pooled analysis, is that risk rises with genetic closeness:
| Relationship | Shared Genes (approx.) | Approximate Lifetime Risk |
|---|---|---|
| General population | — | ~1% |
| Sibling of a person with schizophrenia | 50% | ~9% |
| Child of one affected parent | 50% | ~13% |
| Dizygotic (DZ) twin | 50% | ~17% |
| Monozygotic (MZ) twin | 100% | ~48% |
| Child of two affected parents | 50% from each | ~46% |
The pattern is clear: the more genes an individual shares with an affected relative, the higher their risk. The crucial limitation is equally clear: relatives who share genes also typically share environments, so family studies on their own cannot disentangle nature from nurture.
Twin studies exploit the fact that monozygotic (MZ, identical) twins share essentially 100% of their DNA, whereas dizygotic (DZ, non-identical) twins share on average about 50% — the same as ordinary siblings. If genes matter, MZ concordance should exceed DZ concordance. Gottesman and Shields (1966) reviewed hospital records of 57 twin pairs in which at least one twin had a diagnosis of schizophrenia and found concordance of approximately 42% for MZ twins and 9% for DZ twins. The substantially higher MZ figure strongly implies a genetic contribution.
Two features of these data deserve emphasis. First, the gap between MZ and DZ concordance is the evidence for genetic influence. Second, the fact that MZ concordance is far below 100% is decisive evidence that genes are not the whole story: if schizophrenia were wholly genetic, MZ twins (who are genetically identical) would always be concordant. They are not, so environmental factors must also contribute — which is exactly the logic that motivates the diathesis-stress model.
Exam Tip: Always note the equal environments assumption when evaluating twin studies. MZ twins may be treated more alike than DZ twins (dressed identically, given the same expectations), so part of their higher concordance could reflect a more similar environment, not only more similar genes. This is precisely why adoption studies are valuable.
Adoption studies separate genes from environment by studying children raised apart from their biological parents. Tienari et al. (2004) followed 155 adopted-away offspring of Finnish mothers diagnosed with schizophrenia and compared them with 186 adopted-away offspring of mothers without the diagnosis. Approximately 6.7% of the high-genetic-risk adoptees developed schizophrenia, compared with about 2% of the controls — evidence for a genetic contribution, since the elevated risk travelled with the high-risk children even though they were raised in unrelated families. Critically, the elevated risk was concentrated in those raised in dysfunctional adoptive family environments, with low-risk adoptees in such families largely unaffected. Tienari's study is therefore cited as evidence both for genetic vulnerability and for the diathesis-stress model: genes raised susceptibility, but a stressful family environment was needed to express it.
Modern genomic methods have searched directly for the genes involved. Ripke et al. (2014) conducted a genome-wide association study (GWAS) pooling data from over 36,000 patients and more than 100,000 controls, and identified 108 genetic loci associated with schizophrenia, including genes involved in dopamine signalling (notably DRD2, which codes for the D2 receptor), glutamate signalling and immune function. No single gene "causes" schizophrenia; the disorder is polygenic, arising from the combined small effects of very many genetic variants. The involvement of DRD2 is a notable convergence, because it links the genetic evidence directly to the dopamine hypothesis discussed next.
Key Definition: Polygenic means a characteristic is influenced by the combined action of many genes, each contributing a small effect. Schizophrenia is highly polygenic, which is one reason no simple genetic test exists.
The genetic studies are frequently summarised in a single statistic: the heritability of schizophrenia is often estimated at around 80%. It is essential to interpret this figure correctly, because it is widely misunderstood. Heritability is a population statistic: it describes the proportion of the variation in a trait, within a particular population, that can be statistically attributed to genetic variation. It does not mean that any individual's schizophrenia is "80% caused by genes", nor that 80% of people with the relevant genes will develop the disorder. A high heritability estimate is entirely compatible with the environment playing a decisive role, because heritability says nothing about the causes of the disorder in a single person — only about the sources of differences across a group. This distinction is examinable and is a reliable way to demonstrate sophisticated understanding: it allows you to accept the strong genetic evidence while still insisting, correctly, that environment matters.
A further interpretive point concerns gene-environment interaction. The genetic and environmental causes of schizophrenia are not simply additive; they interact, so that the same genotype may produce very different outcomes in different environments (as Tienari's adoption study directly showed). Where this is the case, trying to partition causation into a fixed "percentage genetic" and "percentage environmental" is conceptually misleading, because the contribution of the genes depends on the environment and vice versa. This reinforces the conclusion, returned to throughout this lesson, that an interactionist account is more defensible than either a purely genetic or a purely environmental one.
The dopamine hypothesis is the most influential neurochemical explanation of schizophrenia. Dopamine is a neurotransmitter — a chemical messenger that crosses the synapse to influence the next neuron — and the hypothesis proposes that abnormal dopaminergic transmission underlies the symptoms. It exists in an original and a revised form.
The original dopamine hypothesis proposed that schizophrenia results from excessive dopamine activity (hyperdopaminergia), particularly at D2 receptors in subcortical regions such as the mesolimbic pathway. Three independent lines of evidence supported it:
Davis and Kahn (1991) proposed a more sophisticated, two-part version. They argued that schizophrenia involves:
This revision is important because it explains an otherwise puzzling clinical fact: typical antipsychotics, which block dopamine, relieve positive symptoms but do little for — and may even worsen — negative symptoms. If positive and negative symptoms reflect opposite dopamine abnormalities in different brain regions, a drug that uniformly reduces dopamine cannot fix both.
graph LR
A[Dopaminergic origin<br/>Ventral Tegmental Area] -->|Mesolimbic pathway| B[Subcortex / Limbic system]
A -->|Mesocortical pathway| C[Prefrontal Cortex]
B -->|Hyperdopaminergia<br/>excess dopamine at D2| D["Positive symptoms<br/>hallucinations, delusions"]
C -->|Hypodopaminergia<br/>too little dopamine| E["Negative symptoms<br/>avolition, cognitive deficits"]
Exam Tip: The diagram shows the two pathways of Davis and Kahn's (1991) revised hypothesis. The key examinable point is that different dopamine abnormalities in different regions are proposed for positive versus negative symptoms. State this distinction explicitly — it is what marks out a developed answer.
The early support for the dopamine hypothesis was indirect — it relied on the effects of drugs rather than on direct measurement of dopamine in patients. More recently, PET (positron emission tomography) imaging has allowed dopamine activity to be estimated in living people. Studies in this tradition (for example, the programme of work reviewed by Howes et al., 2012) report elevated dopamine synthesis capacity in the striatum of patients, particularly in those experiencing acute positive symptoms, and there is evidence that this elevation can be detected even in individuals at high clinical risk before a full disorder develops. This is an important strengthening of the hypothesis, because it moves the evidence from inference-from-drug-action towards direct, biological measurement, and because finding the abnormality in the prodromal (pre-onset) phase makes it less likely that the dopamine change is merely a consequence of having the illness or of taking medication.
The dopamine hypothesis is also linked to the genetic evidence in a way that gives the whole biological account coherence. Several of the loci identified by Ripke et al. (2014) — notably DRD2, which codes for the D2 receptor that antipsychotics block — are part of the dopamine system. This convergence, where independent genetic and pharmacological lines of evidence both implicate dopamine, is more persuasive than either line alone, and is a point worth making explicitly in an essay.
Neuroimaging (MRI, fMRI and PET) has identified structural and functional brain features that are statistically associated with schizophrenia. These are termed neural correlates — patterns that co-occur with the disorder, which is not the same as having caused it.
The lateral ventricles are fluid-filled cavities in the brain; in many people with schizophrenia they are enlarged relative to controls, first reported using imaging by Johnstone et al. (1976). Enlarged ventricles imply a loss of surrounding brain tissue (reduced grey matter), and the finding has been replicated widely. However, two qualifications limit its value: enlarged ventricles are not specific to schizophrenia (they appear in other conditions, including some dementias), and not every patient shows them. Enlarged ventricles are therefore associated with the disorder, especially with negative symptoms, but are neither necessary nor sufficient for it.
The prefrontal cortex supports executive functions such as planning, working memory and the regulation of behaviour. Many patients show reduced blood flow and metabolic activity here — termed hypofrontality. Weinberger et al. (1986) combined PET imaging with the Wisconsin Card Sorting Test and found that patients showed less activation of the dorsolateral prefrontal cortex than controls during the task. Hypofrontality fits the revised dopamine hypothesis well, since reduced prefrontal dopamine (hypodopaminergia) would be expected to impair prefrontal function and produce negative and cognitive symptoms.
The superior temporal gyrus, part of the temporal lobe involved in language, has been linked to auditory hallucinations. Allen et al. (2008) found increased activation in language-related regions (including Broca's area) during hallucinatory experiences, consistent with the idea — also central to the cognitive explanation in the next lesson — that hallucinations may involve self-generated inner speech being misattributed to an external source.
Key Definition: Neural correlates are measurable patterns of brain structure or activity associated with a mental state or disorder. Because they are correlational, they cannot by themselves establish that the brain feature caused the disorder.
The biological explanation of schizophrenia connects to several wider areas of the specification:
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.