Genetic explanations for Schizophrenia | AQA A-Level Psychology Revision

For 7182 specification, first teach in September 2025

AQA A-Level Psychology | Free Revision Notes

Estimated study time: 60 minutes

Genetic explanations propose that inherited differences in DNA contribute to a person’s vulnerability to schizophrenia. These Genetic explanations A-Level Psychology revision notes examine familial patterns, twin and adoption research, concordance rates, candidate genes and the polygenic nature of schizophrenia.

Genes increase susceptibility rather than guaranteeing that schizophrenia will develop. This makes genetic vulnerability an important foundation for the later interaction between predisposition and environmental stress. AQA requires genetic explanations as part of the biological account of schizophrenia.

Learning Objectives 🎯

By the end of this revision page, you should be able to:

• Explain how inherited genetic differences may contribute to schizophrenia.

• Define genetic vulnerability, heritability and concordance.

• Explain evidence from family, twin and adoption studies.

• Explain why schizophrenia is considered polygenic.

• Apply genetic explanations to unfamiliar family-history scenarios.

• Evaluate the genetic explanation using evidence, methodological issues and the diathesis-stress model.

Revision Notes 📚

Genetic explanations of schizophrenia overview

A genetic explanation proposes that variation in a person’s genes contributes to their likelihood of developing schizophrenia.

Genes are sections of DNA inherited from biological parents. They contain instructions that contribute to the development and functioning of the body and nervous system.

The explanation does not claim that a person directly inherits:

• A hallucination.

• A delusion.

• Speech poverty.

• Avolition.

Instead, they may inherit biological characteristics that increase vulnerability to developing the positive and negative symptoms associated with schizophrenia.

The inherited characteristics might affect:

• Brain development.

• Neurotransmitter systems.

• Sensitivity to environmental stress.

• Cognitive processing.

• The way several biological systems interact.

Detailed neurotransmitter and brain-based mechanisms are covered in dopamine activity and neural correlates.

The core genetic argument

The basic explanation can be represented as:

inherited genetic variation → biological vulnerability → increased probability of schizophrenia

The word probability is essential.

Genetic explanations describe an increased risk, not a guaranteed outcome.

Genes and inheritance

What is a gene?

A gene is a section of DNA containing instructions that contribute to a biological characteristic.

People inherit genes from their biological parents.

An individual’s complete genetic information is known as their genotype.

The observable characteristics produced through the interaction of the genotype and environment form the phenotype.

Schizophrenia itself is not a simple characteristic produced by one gene. It is a complex psychological condition influenced by:

• Many genetic variants.

• Biological development.

• Environmental experiences.

• Interactions between these factors.

Alleles

An allele is a particular version of a gene.

Some alleles may contribute to greater vulnerability than others.

An allele associated with increased probability is often described as a risk allele.

Possessing a risk allele does not mean that a person:

• Has schizophrenia.

• Will inevitably develop schizophrenia.

• Will show a particular symptom.

• Can be diagnosed using that allele alone.

Risk alleles normally have small effects and operate in combination with many other influences.

Genetic transmission

Genetic transmission is the passing of genetic material from biological parents to their children.

If genetic factors contribute to schizophrenia, the condition should tend to occur more frequently among biological relatives than among unrelated people.

Researchers therefore investigate whether risk increases with genetic relatedness.

Genetic vulnerability

What is genetic vulnerability?

Genetic vulnerability is an inherited predisposition that increases the likelihood of developing a condition.

It is also called:

• Genetic susceptibility.

• Genetic predisposition.

• Genetic diathesis.

Vulnerability means that the person possesses a greater risk than someone without the same inherited factors.

It does not mean that the condition is already present.

Vulnerability rather than inevitability

Consider two people who inherit different combinations of risk alleles.

• Person A may possess relatively low genetic vulnerability.

• Person B may possess greater genetic vulnerability.

Person B is more likely to develop schizophrenia, but this is not certain.

Environmental and developmental influences may affect whether the vulnerability is expressed.

The accurate relationship is:

genetic vulnerability increases risk

It is not:

genetic vulnerability automatically causes schizophrenia

Applying genetic vulnerability

Mina’s biological father and one of her biological grandparents have been diagnosed with schizophrenia. Mina has never experienced any symptoms.

A genetic explanation would suggest that:

• Mina may have inherited some risk alleles.

• Her family history could indicate increased genetic vulnerability.

• She has not inherited schizophrenia as a certain outcome.

• She may never develop the condition.

Do not conclude that Mina has schizophrenia simply because it appears among her biological relatives.

Different levels of vulnerability

People may inherit:

• Different numbers of risk alleles.

• Different combinations of alleles.

• Variants affecting different biological systems.

This helps explain why people with schizophrenia may differ in:

• The symptoms they experience.

• The age at which symptoms appear.

• Severity.

• Response to treatment.

• The environmental events that trigger difficulties.

Familial patterns

Familial aggregation

Familial aggregation means that a characteristic or condition occurs more frequently within families than in the wider population.

If schizophrenia shows familial aggregation, the relatives of a person diagnosed with schizophrenia should have a greater probability of receiving the diagnosis themselves.

The closer the biological relationship, the more genetic material is usually shared.

Relationship	Approximate genetic relatedness
Identical or monozygotic twins	Approximately 100%
Biological parent and child	Approximately 50%
Non-identical or dizygotic twins	Approximately 50%
Full biological siblings	Approximately 50%
Grandparent and grandchild	Approximately 25%
Unrelated individuals	Approximately 0% beyond population similarity

A pattern in which schizophrenia risk increases with genetic relatedness is consistent with genetic influence.

However, related people may also share environments, so family patterns cannot prove a genetic cause on their own.

Family studies

What is a family study?

A family study examines whether a condition occurs more frequently among the biological relatives of a diagnosed person.

The diagnosed person from whom the investigation begins may be called the index case or proband.

Researchers may compare risk among:

• Parents.

• Children.

• Siblings.

• More distant biological relatives.

• Unrelated comparison participants.

Expected genetic pattern

A genetic explanation predicts:

closer biological relationship → greater average risk of schizophrenia

For example, a sibling would usually be expected to have a higher risk than a distant cousin because siblings share more genetic material on average.

What family studies can show

Family studies can demonstrate:

• Familial clustering.

• A relationship between biological relatedness and risk.

• The possibility of inherited vulnerability.

They cannot determine with certainty whether the similarity is caused by:

• Shared genes.

• Shared home environments.

• Similar stressful experiences.

• Similar treatment by other people.

• A combination of genes and environment.

Worked application

Three of Jordan’s close biological relatives have experienced schizophrenia, whereas no cases are known among unrelated members of Jordan’s household.

This pattern is consistent with genetic transmission because:

• The condition occurs among biological relatives.

• The family history may indicate inherited vulnerability.

• Greater biological similarity could be associated with greater risk.

It does not prove that Jordan will develop schizophrenia.

Twin studies

Why study twins?

Twin studies compare the similarity of:

• Monozygotic twins, usually abbreviated to MZ.

• Dizygotic twins, usually abbreviated to DZ.

MZ twins develop from the same fertilised egg and share almost all their genetic material.

DZ twins develop from two separate fertilised eggs and share approximately half their segregating genetic material, like ordinary biological siblings.

If genes contribute to schizophrenia, MZ twins should show greater similarity than DZ twins.

Concordance

Concordance occurs when both members of a pair possess the same characteristic or diagnosis.

A concordance rate is the percentage of relevant twin pairs in which both twins have the condition.

A simplified calculation is:

Concordance rate=pairs where at least one twin has schizophreniapairs where both twins have schizophrenia​×100

Hypothetical example

Researchers identify 40 MZ twin pairs in which at least one twin has schizophrenia.

In 18 pairs, both twins have the condition.

MZ concordance=4018​×100=45%

They also identify 40 DZ pairs, of whom 6 pairs are concordant.

DZ concordance=406​×100=15%

The MZ rate is:

45−15=30

percentage points higher.

Interpreting the result

The higher MZ concordance rate supports a genetic explanation because MZ twins share more genes than DZ twins.

However, the MZ concordance rate is not 100%.

This suggests that:

• Genes are not sufficient by themselves.

• Environmental influences also matter.

• Different developmental experiences may affect the twins.

• Genetic variants produce vulnerability rather than certainty.

AQA mark schemes explicitly recognise MZ-DZ comparisons, concordance research and family studies as evidence relevant to biological explanations of schizophrenia.

Discordance

Discordance occurs when one twin has the condition and the other does not.

Discordant MZ twins are particularly important.

If genetically identical twins can have different outcomes, then non-genetic factors must influence development.

Possible differences include:

• Prenatal conditions.

• Birth complications.

• Life events.

• Relationships.

• Substance use.

• Exposure to stress.

The equal-environments assumption

Twin research often assumes that environmental similarity is broadly comparable for MZ and DZ pairs.

This is called the equal-environments assumption.

However, MZ twins may:

• Be dressed more similarly.

• Spend more time together.

• Be treated as more alike.

• Share more friends.

• Experience stronger identification with one another.

Their higher concordance might therefore be partly environmental.

The direction of the problem

If MZ twins experience more similar environments than DZ twins, this could exaggerate the apparent genetic effect.

A well-developed evaluation should therefore state:

Higher MZ concordance supports genetic influence, but MZ twins also tend to experience more similar environments, so the relative contribution of genes cannot be isolated completely.

Adoption studies

Why use adoption studies?

Adoption studies help separate:

• Genetic relationships.

• Rearing environments.

Researchers may examine children who:

• Have a biological parent with schizophrenia.

• Were adopted and raised by biologically unrelated parents.

If these children have an elevated risk despite growing up apart from the affected biological parent, this supports inherited vulnerability.

Biological and adoptive relatives

A genetic explanation predicts that the child’s outcome should be more closely related to the mental-health history of biological relatives than adoptive relatives.

Finding	Possible interpretation
Similarity to biological relatives	Supports genetic transmission
Similarity to adoptive relatives	Supports environmental influence
Effect of both biological and adoptive backgrounds	Supports interaction between vulnerability and environment

Strength of adoption evidence

Adoption studies reduce the shared-rearing problem found in ordinary family studies.

The child does not grow up in the same household as the affected biological parent.

This makes genetic and environmental influences easier to distinguish.

Limitations of adoption studies

Adoption does not create a perfect separation of nature and nurture.

Possible problems include:

• The prenatal environment is shared with the biological mother.

• Children may spend some time with biological relatives before adoption.

• Adoptive placements may not be random.

• Adoption agencies may use selective placement.

• Information about biological family history may affect treatment by adoptive parents.

• Adopted samples may not represent everyone.

Selective placement

Selective placement occurs when an adopted child is placed with a family resembling the biological family in some way.

For example, the families might have similar:

• Social backgrounds.

• Education.

• Cultural values.

• Living environments.

This could make genetic and environmental influences harder to separate.

Molecular genetics

Moving from family patterns to DNA

Family, twin and adoption studies investigate whether schizophrenia is heritable.

Molecular genetics investigates which genetic variants may be associated with the vulnerability.

Researchers may use:

• Gene mapping.

• Candidate-gene studies.

• Genome-wide investigations.

• Comparisons of genetic variants in diagnosed and non-diagnosed groups.

Past AQA mark schemes have recognised gene mapping, candidate genes, interactions between genes and polygenic effects as relevant genetic content.

Candidate genes

What is a candidate gene?

A candidate gene is a gene selected for investigation because its biological function might plausibly relate to a condition.

Researchers compare how often particular variants occur among:

• People diagnosed with schizophrenia.

• People without the diagnosis.

If a variant is more frequent in the diagnosed group, it may contribute to vulnerability.

Examples accepted in past AQA materials

Past mark schemes have accepted examples such as:

• PCM1

• PPP3CC

• Interactions involving NRG1, NRG3 and ERBB4

These examples show that several biological pathways have been investigated.

Students do not need to memorise a long list of gene names to explain the central theory effectively.

The more important point is:

no single candidate gene provides a complete explanation

Limitations of candidate-gene research

A particular variant may be associated with only:

• A small increase in risk.

• One subgroup.

• One symptom pattern.

• Particular environmental conditions.

Findings may also fail to replicate because schizophrenia is extremely complex.

This helped move research towards a polygenic account.

Polygenic inheritance

What does polygenic mean?

Polygenic means that a characteristic is influenced by many genes.

Schizophrenia is not thought to be controlled by one “schizophrenia gene”.

Instead, vulnerability results from:

• Many genetic variants.

• Each making a small contribution.

• Different combinations appearing in different people.

• Interactions between genetic variants.

AQA’s 2025 mark scheme contrasts an early single-gene view with the modern polygenic account and identifies Ripke’s research as relevant evidence.

Risk is cumulative

A simplified model is:

few risk variants → lower genetic vulnerabilitymore or particularly influential variants → greater genetic vulnerability

This model is simplified because:

• Variants do not all have equal effects.

• Some variants may interact.

• Protective genetic factors may exist.

• Environmental conditions affect expression.

Different combinations

Two people diagnosed with schizophrenia may not possess exactly the same collection of risk alleles.

For example:

• Person A may have one combination affecting neurotransmitter development.

• Person B may have another combination affecting cognitive or neural functioning.

Different genetic routes can contribute to similar diagnostic outcomes.

This helps explain the considerable variation among people diagnosed with schizophrenia.

Genome-wide research

A genome-wide association study, often abbreviated to GWAS, compares genetic variation across the genome in large groups.

Researchers look for variants that occur more frequently among people with a diagnosis.

Large genome-wide investigations have identified many areas of genetic variation associated with schizophrenia, supporting a polygenic explanation.

However:

• Association does not mean that a variant directly causes schizophrenia.

• Each individual effect is generally small.

• Many people carrying risk variants do not develop the condition.

• Not every diagnosed person has the same variants.

Genetic vulnerability and neural functioning

Genes influence biological development.

Risk variants may affect systems involved in:

• Brain structure.

• Neural communication.

• Neurotransmitter production.

• Receptor development.

• Cognitive functioning.

Genetic and neural explanations should therefore not be treated as completely separate.

A possible pathway is:

genetic variants → altered biological development → neural or cognitive vulnerability → increased risk of symptoms

The specific role of dopamine and identified brain regions is covered in neural correlates and the dopamine hypothesis.

Genes do not code directly for complex symptoms

It is inaccurate to say:

“One gene causes hallucinations.”

A more appropriate explanation is:

“A combination of genetic variants may affect the development of neural systems, increasing vulnerability to the processes associated with schizophrenia.”

Genetic vulnerability and the diathesis-stress model

Genetic vulnerability as a diathesis

A diathesis is an underlying vulnerability.

Within the diathesis-stress model, the vulnerability may be genetic.

Environmental stress may then contribute to the development of symptoms.

The relationship is:

genetic diathesis + sufficient environmental stress → increased probability of schizophrenia

The AQA specimen material applies this process to a person with a family history of schizophrenia who later experiences major stressful life events.

Vulnerability may remain unexpressed

A person can possess genetic vulnerability without developing schizophrenia.

For example:

Two siblings inherit overlapping genetic risk. One experiences severe and persistent environmental stress, while the other does not. Only the first develops symptoms.

The difference does not disprove genetic influence.

It suggests that genes are one component within an interaction.

Genetic loading

Genetic loading refers to the degree of inherited vulnerability.

A person with several close biological relatives diagnosed with schizophrenia may be described as having greater genetic loading than a person with no known family history.

This does not allow an exact prediction for an individual.

Threshold idea

Vulnerability may be imagined as building towards a threshold.

• Genetic factors contribute to vulnerability.

• Environmental factors may add further risk.

• Protective factors may reduce risk.

• Symptoms appear only if the combined influences become sufficient.

This is a simplified way of understanding why the same genetic vulnerability can lead to different outcomes.

The full interactionist account is covered in biological vulnerability and environmental stress.

Worked application: family history

Luke’s mother and biological uncle have both been diagnosed with schizophrenia. Luke has recently begun experiencing hallucinations.

A genetic explanation would state:

• Schizophrenia occurs among Luke’s biological relatives.

• Luke may have inherited a combination of risk alleles.

• This creates genetic vulnerability.

• His symptoms are consistent with the vulnerability being expressed.

• The family history does not establish that genes acted alone.

A weak answer would say:

“Luke inherited schizophrenia from his mother.”

Schizophrenia is not transmitted as a simple, certain package.

Worked application: unaffected relative

Yasmin’s identical twin has schizophrenia, but Yasmin has never experienced symptoms.

This scenario shows:

• Yasmin and her twin share almost all their genes.

• Yasmin may share substantial genetic vulnerability.

• Their outcome is discordant.

• Genetic factors cannot determine the condition completely.

• Different environmental or developmental experiences may explain the difference.

Do not say that the scenario disproves genetics.

It challenges a completely deterministic genetic explanation.

Worked application: adoption

Theo was adopted shortly after birth. His biological father had schizophrenia, but his adoptive parents did not. Theo later developed schizophrenia.

This is consistent with a genetic explanation because:

• Theo did not grow up with the affected biological parent.

• He may nevertheless have inherited genetic vulnerability.

• The outcome is more closely connected with his biological family history than shared upbringing.

However, one case cannot establish causation, and prenatal or later environmental factors must also be considered.

Worked application: polygenic vulnerability

Genetic testing in a research study finds that two people with schizophrenia possess different combinations of associated genetic variants.

This finding is consistent with a polygenic explanation because:

• Several genes contribute to vulnerability.

• There is no requirement for every diagnosed person to share one variant.

• Different combinations may produce similar clinical outcomes.

Evaluating genetic explanations

Strength: family patterns support heritability

Schizophrenia occurs more frequently among close biological relatives of diagnosed people than among unrelated individuals.

Risk generally increases with genetic relatedness.

This supports the genetic explanation because a predictable relationship exists between:

• The amount of DNA shared.

• The probability of the diagnosis.

However, families share environments as well as genes.

Family evidence demonstrates familial aggregation, but cannot by itself establish genetic transmission.

Strength: higher MZ than DZ concordance

MZ twins share more genetic material than DZ twins.

Their generally higher concordance is therefore consistent with genetic influence.

The logic is:

1. Both twin types often grow up in similar family environments.

2. MZ twins share more genetic material.

3. MZ concordance is higher.

4. Genetic similarity is therefore associated with greater diagnostic similarity.

The June 2023 examiner report observed that evidence was particularly effective when students analysed twin research rather than merely naming it.

Limitation: MZ concordance is below 100%

If schizophrenia were completely determined by genes, genetically identical twins would always be concordant.

They are not.

This means:

• Genes are not a sufficient cause.

• Non-genetic influences must contribute.

• Vulnerability may be expressed only under certain environmental conditions.

Rather than rejecting genetics, this supports a probabilistic and interactionist explanation.

Limitation: twins share environments

MZ twins may experience more similar environments than DZ twins.

They may be:

• Treated more alike.

• Encouraged to share activities.

• Placed in the same classes.

• More likely to share friendship groups.

Higher concordance could therefore result partly from environmental similarity.

Twin studies cannot isolate genes with complete confidence.

Strength: adoption studies separate genes and rearing environment

If adopted children show greater similarity to biological relatives than adoptive relatives, a genetic explanation is supported.

This evidence is particularly valuable because:

• The biological relationship remains.

• The ordinary shared family environment is reduced.

The design improves on ordinary family studies.

Limitation: adoption does not remove all environmental influence

Adopted children share a prenatal environment with their biological mother.

They may also experience:

• Conditions before adoption.

• Selective placement.

• Knowledge of their biological background.

• Stress associated with adoption.

• Environments correlated with biological-family characteristics.

Adoption research separates genes and environment imperfectly.

Strength: molecular-genetic evidence supports a biological contribution

Genome studies have identified numerous genetic variants associated with schizophrenia.

This supports the idea that inherited biological variation contributes to vulnerability.

It is also consistent with the polygenic account because:

• Many areas of variation are involved.

• No single variant explains all cases.

• Different combinations can increase risk.

Limitation: associated genes have small effects

Finding an association does not mean that a variant is a powerful or direct cause.

A risk allele may:

• Slightly alter probability.

• Operate only with other variants.

• Operate only under particular environmental conditions.

• Be linked with another genuinely relevant variant.

Genetic data cannot currently provide a certain individual prediction.

Strength: converging evidence

The genetic explanation is supported by several different approaches:

• Family studies.

• Twin studies.

• Adoption studies.

• Gene mapping.

• Genome-wide research.

When different methods point towards inherited vulnerability, confidence in a genetic contribution increases.

This is known as converging evidence.

Each method has limitations, but the same overall pattern is not dependent on one research design.

Limitation: problems with diagnosis affect the evidence

Genetic studies depend on identifying who does and does not have schizophrenia.

However, co-morbidity, symptom overlap, culture bias and gender bias can affect classification.

If participants are diagnosed inconsistently:

• Concordance rates may be inaccurate.

• Genetic associations may involve mixed conditions.

• Samples may contain people with different symptom profiles.

• Replication may become difficult.

The validity of a genetic study cannot be greater than the validity of the diagnosis it uses.

Limitation: schizophrenia is heterogeneous

Heterogeneity means variation within a group.

People diagnosed with schizophrenia can differ in:

• Symptoms.

• Severity.

• Onset.

• Course.

• Treatment response.

A genetic variant might be associated with:

• One symptom.

• One subgroup.

• One biological pathway.

rather than schizophrenia as a single uniform condition.

This makes the search for universal genes more difficult.

Limitation: causal conclusions are difficult

Researchers cannot ethically:

• Assign people particular genes.

• Manipulate genetic vulnerability.

• Randomly create family histories.

Most genetic evidence is therefore:

• Correlational.

• Quasi-experimental.

• Based on naturally occurring differences.

The evidence shows relationships but cannot demonstrate simple cause and effect.

Better causal timing

Genes are present before the symptoms of schizophrenia emerge.

This helps establish that genetic variation cannot be a consequence of receiving the diagnosis.

However, the relationship between a particular variant and symptoms may still involve:

• Neural development.

• Environmental exposure.

• Other genes.

• Diagnostic practices.

Genes have temporal priority, but their precise causal pathway remains complex.

Limitation: genetic reductionism

A genetic explanation may be biologically reductionist if it reduces schizophrenia to inherited DNA differences.

This risks ignoring:

• Life events.

• Family relationships.

• Social disadvantage.

• Trauma.

• Substance use.

• Cognitive interpretations.

Schizophrenia occurs within a social and psychological context.

Breaking it down to genes may produce measurable variables, but it may lose the meaning of the person’s experiences.

This connects with levels of explanation and biological reductionism.

Value of reductionism

Reductionism is not wholly negative.

Studying genes can:

• Produce objective biological evidence.

• Identify mechanisms.

• Generate testable predictions.

• Support research into biological treatment.

• Improve understanding of vulnerability.

The limitation appears when genetic factors are presented as a complete explanation.

Limitation: biological determinism

A strong genetic account may imply biological determinism, the view that inherited biology controls behaviour.

This could encourage the belief that:

• Schizophrenia is unavoidable.

• Recovery is impossible.

• Environmental support cannot help.

• People with a family history have little control over their future.

The incomplete concordance of MZ twins directly challenges this hard deterministic position.

A softer account recognises genetic influence without treating outcomes as inevitable.

This links with biological determinism and human agency.

Strength: reduced family blame

Genetic explanations may reduce blame placed on:

• The diagnosed person.

• Parents.

• Family communication.

If schizophrenia involves inherited biological vulnerability, it cannot simply be attributed to deliberate choices or “bad parenting”.

Past AQA mark schemes recognise that biological and family-dysfunction explanations can create different attitudes towards family responsibility.

Limitation: genetic stigma

Biological explanations can also increase stigma.

People may wrongly assume that schizophrenia is:

• Permanent.

• Untreatable.

• Inevitably passed to children.

• A sign of biological defect.

• A reason to avoid relationships or employment.

Genetic explanations may reduce blame while increasing beliefs about permanence and difference.

Social sensitivity

Research into inherited vulnerability is socially sensitive because findings may affect:

• People diagnosed with schizophrenia.

• Their relatives.

• Decisions about parenthood.

• Insurance or employment attitudes.

• Genetic counselling.

• Public stigma.

Researchers should explain clearly that:

• Risk is probabilistic.

• Many relatives do not develop schizophrenia.

• No single genetic test determines the diagnosis.

• Environment and support remain important.

Practical application: early support

Understanding genetic vulnerability may help identify people who could benefit from:

• Monitoring.

• Information.

• Early support when difficulties emerge.

• Reduction of avoidable stress.

• Rapid access to assessment.

However, identifying someone as “high risk” also creates possible problems:

• Anxiety.

• Labelling.

• Self-fulfilling expectations.

• Unnecessary surveillance.

• Stigma despite the person never developing symptoms.

Practical application: treatment research

Genetic research may contribute to understanding biological pathways involved in schizophrenia.

This may inform the development of more targeted biological treatments.

However, a gene associated with risk does not automatically identify:

• The exact neural mechanism.

• A suitable medicine.

• Which person will respond.

• Whether psychological treatment is unnecessary.

Biological and psychological support may still need to be combined.

Genetic explanations compared with family dysfunction

Genetic explanation	Family-dysfunction explanation
Biological level of explanation	Social and psychological level
Family contributes through inherited DNA	Family contributes through communication and interaction
Family’s role is largely passive	Family behaviour is treated as an active influence
Nature is emphasised	Nurture is emphasised
May reduce parental blame	May create ethical concerns about blaming families
Leads towards biological treatment research	Leads towards family-based intervention
Biologically deterministic if taken too far	Environmentally deterministic if taken too far
Supported by family, twin and adoption evidence	Supported by research into family relationships and relapse

The explanations may interact.

A person may inherit vulnerability and then experience environmental stress within or outside the family.

This is more complete than assuming that either genes or family behaviour operates alone.

The genetic explanation and the nature-nurture debate

Genetic explanations emphasise nature.

They propose that inherited biology contributes to schizophrenia.

Evidence that:

• MZ concordance is below 100%.

• Adoptive environments matter.

• Stress can trigger symptoms.

shows that nurture is also important.

The best-supported conclusion is interactionist:

genes affect sensitivity to environments, and environments affect whether vulnerability is expressed

This illustrates the interaction between heredity and environment.

Gene-environment interaction

A gene-environment interaction occurs when the effect of genetic vulnerability depends on environmental conditions.

For example:

• High genetic vulnerability may produce symptoms under relatively moderate stress.

• Lower genetic vulnerability might require more severe stress.

• A supportive environment may reduce the expression of vulnerability.

This is not the same as simply adding separate genetic and environmental effects.

The effect of one depends on the presence of the other.

Gene-environment correlation

Genes and environments may also become correlated.

For example, inherited characteristics could influence:

• How other people respond to someone.

• Which environments the person selects.

• How stressful an event feels.

• The relationships they experience.

This makes it difficult to separate nature and nurture cleanly.

An apparent environmental effect may partly reflect genetically influenced characteristics, while a genetic effect may depend on environmental exposure.

Research methods in genetic investigations

Quasi-experiments

Twin and adoption studies use naturally occurring groups.

Researchers do not decide whether a participant is:

• An MZ twin.

• A DZ twin.

• Adopted.

• Genetically related to someone with schizophrenia.

These are therefore quasi-experimental comparisons.

Strength

The studies investigate variables that could not ethically be manipulated.

Limitation

Participant variables cannot be controlled completely, so causal conclusions are limited.

Correlational evidence

Genetic relatedness is associated with diagnostic similarity.

A correlation between these variables supports genetic influence.

It does not prove that genes alone caused schizophrenia.

Large samples

Genome-wide research can use very large samples.

Large samples:

• Increase statistical power.

• Reduce the effect of unusual individual cases.

• Allow the detection of small genetic associations.

However, very large samples do not correct:

• Invalid diagnoses.

• Population differences.

• Confounding environmental variables.

• Misinterpretation of correlation as causation.

Retrospective data

Family studies may depend on:

• Medical records.

• Interviews about relatives.

• Historical diagnoses.

Older diagnoses may have been made using:

• Different criteria.

• Different cultural assumptions.

• Less reliable assessment.

This could reduce consistency across generations.

Concordance and significance

A higher concordance rate is not enough by itself.

Researchers should examine whether the difference is statistically significant.

A statistically significant result suggests that the difference is unlikely to have occurred by chance under the null hypothesis.

Statistical significance still does not establish that genetics caused the difference.

Applying the explanation in examinations

Step 1: Identify evidence of biological relatedness

Look for references to:

• Biological parents.

• Identical or non-identical twins.

• Siblings.

• Grandparents.

• An adopted person’s biological family.

Step 2: Use cautious terminology

Write:

“This suggests inherited genetic vulnerability.”

Do not write:

“This proves that schizophrenia was inherited.”

Step 3: Explain the mechanism

State that:

• Risk alleles may be transmitted.

• Schizophrenia is polygenic.

• The combination increases susceptibility.

• The outcome is probabilistic.

Step 4: Consider the environment

Where the scenario includes stress, explain that genetic vulnerability may interact with it.

Do not describe stress as a gene.

Step 5: Link evidence to the theory

A strong application sentence is:

“Because both Sam’s father and biological aunt have experienced schizophrenia, Sam may have inherited a combination of risk alleles. This creates genetic vulnerability rather than certainty, so additional environmental influences may determine whether symptoms develop.”

Exam paragraph structure

For an evidence paragraph, use:

Point: Twin evidence supports genetic influence.Evidence: MZ twins usually show higher concordance than DZ twins.Explanation: MZ twins share more genetic material, so greater diagnostic similarity is consistent with heritability.Counterpoint: MZ concordance is below 100%, and MZ twins may share more similar environments.Conclusion: Genes contribute to vulnerability but do not provide a complete explanation.

This is stronger than simply listing a concordance statistic.

Overall conclusion

Genetic explanations propose that schizophrenia is partly heritable.

Evidence from:

• Families.

• Twins.

• Adoptions.

• Molecular genetics.

supports an inherited contribution.

Modern accounts describe schizophrenia as polygenic, meaning that many genetic variants combine to create different levels of vulnerability.

Genes do not determine schizophrenia. The strongest evidence for this is that:

• MZ concordance is below 100%.

• People with substantial family histories may never develop symptoms.

• Environmental stress affects whether vulnerability is expressed.

The most defensible conclusion is therefore:

Schizophrenia involves genetic vulnerability operating within a wider biological, psychological and social context.

Hints from the Examiner Reports 💡

Examiner hint: Do not write that a person “inherits schizophrenia”. Write that they may inherit a genetic vulnerability or combination of risk alleles.

Examiner hint: Analyse twin evidence rather than merely naming a study. The June 2023 report noted that successful use of biological evidence involved analysis of twin findings and their implications.

A useful analysis is:

Higher MZ than DZ concordance supports genetic influence because MZ twins share more genetic material. Concordance below 100% shows that genes are not sufficient.

Examiner hint: Do not confuse concordance with genetic relatedness.

• Genetic relatedness concerns the proportion of genes shared.

• Concordance concerns whether both people have the diagnosis.

Examiner hint: In application questions, connect family history directly with inherited vulnerability. The June 2023 mark scheme credited linking a father’s mental-health history with possible genetic inheritance.

Examiner hint: Use the word polygenic accurately. Schizophrenia is associated with the combined influence of many genes, not one dominant “schizophrenia gene”.

Examiner hint: Evaluation should target the explanation. A generic statement that a twin study used a small sample becomes relevant only when you explain how this weakens confidence in the claimed genetic influence.

Examiner hint: Extended responses need developed analysis. AQA mark schemes recognise:

• Twin, family and adoption evidence.

• The diathesis-stress approach.

• Determinism.

• Reductionism.

• Implications for early identification and treatment.

Examiner hint: When a question asks for comparison, compare throughout. The November 2020 examiner report noted that knowledge was often sound but explicit comparison was limited.

Use connected statements such as:

“Whereas the genetic explanation presents the family as a source of inherited vulnerability, the family-dysfunction explanation focuses on stressful communication within the family.”

Examiner hint: Do not treat genes and neural correlates as interchangeable. Genes may contribute to the development of neural differences, but the dopamine hypothesis and specific brain correlates require their own explanation.

Examiner hint: Use evidence to reach a conclusion, not as decoration. After describing a result, explain whether it supports:

• Heritability.

• Polygenic inheritance.

• Environmental influence.

• An interactionist account.

Common Mistakes ⚠️

Mistake: Saying there is one schizophrenia gene

Why this is incorrect:

Schizophrenia is considered polygenic.

How to improve:

Explain that many variants combine to create vulnerability.

Mistake: Saying a risk allele guarantees schizophrenia

Why this is incorrect:

Risk alleles alter probability rather than determining the outcome.

How to improve:

Use terms such as “predisposition”, “susceptibility” and “increased risk”.

Mistake: Saying a person inherits hallucinations

Why this is incorrect:

Genes contribute to biological vulnerability, not to one fully formed experience.

How to improve:

Explain how genes may influence neural or cognitive development.

Mistake: Treating family history as a diagnosis

Why this is incorrect:

A relative may possess greater vulnerability without experiencing symptoms.

How to improve:

State that family history indicates possible risk, not the presence of schizophrenia.

Mistake: Defining concordance as the percentage of genes twins share

Why this is incorrect:

That is genetic relatedness.

How to improve:

Define concordance as both twins showing the same diagnosis or characteristic.

Mistake: Saying MZ twins share 100% concordance

Why this is incorrect:

They share almost all their genetic material, but they are not always concordant for schizophrenia.

How to improve:

Keep genetic similarity and diagnostic similarity separate.

Mistake: Saying DZ twins share no genes

Why this is incorrect:

DZ twins share approximately 50% of their segregating genes on average.

How to improve:

Compare them with ordinary biological siblings.

Mistake: Claiming higher MZ concordance proves genetic causation

Why this is too strong:

MZ twins may experience more similar environments.

How to improve:

State that the finding supports, but does not prove, genetic influence.

Mistake: Ignoring MZ discordance

Why this weakens evaluation:

Concordance below 100% is strong evidence that genes are not sufficient.

How to improve:

Use discordance to support an interactionist conclusion.

Mistake: Saying family studies separate genes and environment

Why this is incorrect:

Biological relatives often share both.

How to improve:

Use adoption studies as an attempt to separate these influences.

Mistake: Saying adoption removes every environmental influence

Why this is incorrect:

Prenatal conditions and selective placement may remain.

How to improve:

Explain that adoption improves separation without making it perfect.

Mistake: Treating every candidate gene as a confirmed cause

Why this is incorrect:

Associations may be small, inconsistent or dependent on other factors.

How to improve:

Place candidate genes within a polygenic account.

Mistake: Listing gene names without explaining the theory

Why this is incomplete:

A list does not show understanding of genetic vulnerability.

How to improve:

Explain inheritance, combined effects and increased probability.

Mistake: Treating polygenic as meaning everyone has identical risk genes

Why this is incorrect:

Different combinations may create vulnerability in different individuals.

How to improve:

Explain variation in genetic pathways.

Mistake: Saying environmental evidence disproves genetics

Why this is incorrect:

Genes and environments may interact.

How to improve:

Conclude that genetic vulnerability is incomplete rather than irrelevant.

Mistake: Describing the full diathesis-stress model instead of genetics

Why this loses focus:

The question may ask specifically for a genetic explanation.

How to improve:

Use interaction as evaluation while keeping the central description genetic.

Mistake: Giving a methodological criticism with no link

Why this is incomplete:

“Twin studies are correlational” does not explain the consequence.

How to improve:

State that causality cannot be inferred, so higher concordance cannot be attributed entirely to genes.

Mistake: Calling the explanation fully deterministic

Why this is too simplistic:

Modern genetic explanations describe vulnerability and probability.

How to improve:

Distinguish genetic influence from genetic inevitability.

Exam-Style Questions ✍️

Questions

1. What is meant by genetic vulnerability to schizophrenia?[2 marks]

2. Explain what is meant by saying that schizophrenia is polygenic.[3 marks]

3. What is meant by a concordance rate in twin research?[2 marks]

4. Explain why researchers compare monozygotic and dizygotic twins when investigating

schizophrenia.[4 marks]

5. Researchers identify the following results:

Twin type	Number of pairs in which at least one twin has schizophrenia	Number of pairs in which both twins have schizophrenia
MZ	50	20
DZ	50	8

a) Calculate the concordance rate for MZ twins.[2 marks]

b) Calculate the concordance rate for DZ twins.[2 marks]

c) Explain one conclusion that can be drawn from the results.[3 marks]

6. Hana’s biological mother and grandfather have experienced schizophrenia. Hana was adopted as a baby and has recently begun experiencing delusions.

Explain Hana’s experience using a genetic explanation.[4 marks]

7. One member of an identical twin pair has schizophrenia, but the other does not.

Explain why this finding is important when evaluating genetic explanations.[4 marks]

8. Explain one strength and one limitation of using adoption studies to investigate genetic vulnerability to schizophrenia.[6 marks]

9. Explain one strength and one limitation of genetic explanations of schizophrenia.[6 marks]

10. Compare genetic explanations with the family-dysfunction explanation of schizophrenia.[8 marks]

11. Explain why finding genes associated with schizophrenia does not mean that schizophrenia is genetically determined.[6 marks]

12. Discuss genetic explanations of schizophrenia.[16 marks]

Answers and Mark Scheme

Question 1

Award up to two marks:

• Genetic vulnerability is an inherited predisposition or susceptibility to schizophrenia.

• It increases the probability of developing the condition but does not make the outcome inevitable.

Question 2

Award up to three marks:

• Polygenic means that schizophrenia is influenced by many genes.

• Each genetic variant may make a relatively small contribution.

• Different combinations of risk alleles may create different levels of vulnerability.

• There is no single gene that determines every case.

Question 3

Award up to two marks:

• A concordance rate is the percentage of relevant twin pairs in which both twins show the same diagnosis or characteristic.

• In schizophrenia research, it indicates how often both twins have schizophrenia when at least one twin has it.

Question 4

Award up to four marks:

• MZ twins share almost all their genetic material.

• DZ twins share approximately half on average.

• The twin types may otherwise experience broadly similar family environments.

• If MZ concordance is higher, the greater genetic similarity is consistent with inherited influence.

• MZ concordance below 100% would indicate that genes do not operate alone.

Question 5a

5020​×100=40%

Award one mark for suitable working and one mark for:

40%

Question 5b

508​×100=16%

Award one mark for suitable working and one mark for:

16%

Question 5c

Award up to three marks:

• The MZ concordance rate is higher than the DZ concordance rate.

• MZ twins share more genetic material, so this supports a genetic contribution.

• The MZ rate is below 100%, so genes cannot provide a complete deterministic explanation.

• Environmental or developmental factors are also likely to influence the outcome.

Do not claim that the difference is statistically significant because no inferential-test result is provided.

Question 6

Award up to four marks:

• Schizophrenia occurs among Hana’s close biological relatives.

• She may have inherited a combination of risk alleles from her biological family.

• These alleles could create genetic vulnerability.

• Her adoption reduces the ordinary shared-rearing explanation involving her biological relatives.

• The later appearance of delusions is consistent with the vulnerability being expressed.

• The evidence does not establish that genes acted alone.

Question 7

Award up to four marks:

• Identical twins share almost all their genetic material.

• The pair is discordant for schizophrenia.

• If genes fully determined the condition, both twins should have developed it.

• The result therefore indicates that environmental or developmental factors also matter.

• Genetic explanations should describe vulnerability rather than inevitability.

Question 8

Award up to three marks for a developed strength and three for a developed limitation.

Possible strength:

Adoption studies separate inherited relationship from the ordinary rearing environment. If adopted children resemble affected biological relatives more than their adoptive families, this supports genetic transmission.

Possible limitation:

Adoption does not remove every environmental similarity. The child shares the prenatal environment with the biological mother, and selective placement may result in biologically and adoptively related environments being more similar than assumed.

Alternative limitations include:

• Time spent with biological relatives before adoption.

• Non-representative samples.

• Historical diagnostic inconsistencies.

• Knowledge of biological history influencing treatment.

Question 9

Award up to three marks for a developed strength and three for a developed limitation.

Possible strength:

Converging evidence from family, twin, adoption and molecular-genetic studies supports inherited vulnerability. In particular, higher MZ than DZ concordance links greater genetic similarity with greater diagnostic similarity.

Possible limitation:

MZ concordance is below 100%, showing that genetic similarity is insufficient. Twins also share environments, so the apparent genetic effect cannot be separated completely from environmental influence.

Alternative creditworthy points include:

• Polygenic genome evidence.

• Practical applications.

• Diagnostic validity problems.

• Genetic reductionism.

• Biological determinism.

• Social sensitivity.

• Inability to establish simple causality.

Question 10

A strong response may include:

• Genetic explanations operate at a biological level, whereas family dysfunction operates at a social and psychological level.

• Genetics presents the family as a source of inherited DNA.

• Family dysfunction presents family communication or emotional interaction as an environmental influence.

• The genetic family role is largely passive.

• The family-dysfunction role is more active.

• Genetics emphasises nature, whereas family dysfunction emphasises nurture.

• Genetic explanations may reduce parental blame.

• Family dysfunction may create ethical concerns by blaming parents.

• Both can be deterministic.

• Both are supported mainly by naturally occurring correlational evidence.

• Neither explanation alone establishes simple causation.

• The explanations may interact within a diathesis-stress account.

• They lead towards different treatments, biological treatment and family therapy respectively.

Higher marks require direct, connected comparisons rather than two independent descriptions.

Question 11

Award up to six marks:

• Associated genes are risk factors rather than certain causes.

• Schizophrenia is polygenic, with many variants making small contributions.

• Different diagnosed people may carry different combinations.

• Many people with risk variants do not develop schizophrenia.

• MZ concordance is below 100% despite near-identical genetic material.

• Environmental stress can affect whether vulnerability is expressed.

• Gene-environment interaction therefore provides a more appropriate explanation than hard genetic determinism.

Question 12

A strong response should include:

Knowledge and understanding

• Genetic transmission through DNA.

• Genetic vulnerability or predisposition.

• Risk alleles.

• Familial aggregation.

• Increased risk with closer biological relatedness.

• Family studies.

• MZ and DZ twins.

• Genetic relatedness.

• Concordance rates.

• Adoption studies.

• Separation of biological and adoptive relatives.

• Candidate genes or molecular-genetic research.

• Genome-wide research.

• Schizophrenia as polygenic.

• Different combinations of genes contributing to vulnerability.

• Genetic diathesis.

• Environmental stress influencing expression.

Evaluation

• Family evidence supports heritability.

• Shared family environments confound family studies.

• Higher MZ than DZ concordance supports genetic influence.

• MZ concordance below 100% challenges a deterministic explanation.

• The equal-environments assumption may be incorrect.

• Adoption research improves separation of genes and environment.

• Selective placement and prenatal influences limit adoption research.

• Molecular evidence supports polygenic vulnerability.

• Individual risk variants have small effects.

• Associated genes do not establish direct causation.

• Several methods provide converging evidence.

• Diagnosis and symptom overlap may weaken genetic samples.

• Schizophrenia is heterogeneous.

• Genetic research is largely correlational or quasi-experimental.

• Biological reductionism may ignore psychological and social context.

• Genetic explanations can become biologically deterministic.

• Genetic accounts may reduce blame but increase fatalism or stigma.

• Early identification has possible practical benefits and ethical risks.

• Gene-environment interaction provides a more complete account.

• The diathesis-stress model explains why vulnerability does not always lead to symptoms.

Higher-level responses will describe genes as creating probabilistic vulnerability, analyse rather than merely list evidence, and reach an interactionist conclusion.