Schizophrenia: genetic explanation

The genetic explanation

Schizophrenia runs in families. The genetic explanation says people inherit a vulnerability: many genes, each with a small effect, raise the risk (it is polygenic). It is not caused by a single gene.

The diathesis-stress idea adds that inherited vulnerability (diathesis) may only lead to schizophrenia if combined with environmental stress.

Family and twin evidence

Risk rises with genetic closeness (Gottesman, 1991; Kendler, 1983):

Gottesman and Shields (1966)

A twin study using records from the Maudsley and Bethlem Royal Joint Hospital, London: 57 same-sex twin pairs, with zygosity checked by blood tests and physical resemblance. Concordance for a schizophrenia diagnosis was 42% for MZ twins and 9% for DZ twins; including close variants it was 54% and 18%.

MZ twins share all their genes, DZ about half, so higher MZ concordance supports a genetic role. But MZ concordance is far below 100%, so environment must matter too.

Evaluation

• Strong, consistent evidence that risk follows genetic relatedness.
• Nature and nurture are confounded: families share environments as well as genes, and MZ twins may be treated more alike than DZ twins.
• Not 100% concordance: genes give vulnerability, not certainty.
• Reductionist but useful: it can identify at-risk people, but raises socially sensitive issues (9.3.11), for example the hypothetical discovery of a 'schizophrenia gene'.
• Compare with dopamine: genes may work partly by affecting neurotransmitter systems, so the two biological explanations can be linked (9.3.4).

Worked example

Gottesman and Shields found 42% concordance for MZ twins and 9% for DZ twins. What do these figures suggest?

1. MZ twins share 100% of their genes and DZ twins about 50%.
2. MZ concordance is 4.67 times higher in ratio terms (42 / 9), so greater genetic similarity goes with greater risk: evidence for genes.
3. But 58% of MZ co-twins did not have schizophrenia, so genes are not the whole story: environment matters too.

Answer: Genes raise vulnerability, but environmental factors are also needed.

Check yourself

1. In Gottesman and Shields (1966), concordance was 42% for MZ twins and 9% for DZ twins. How many times higher was MZ concordance? Give your answer to 2 decimal places.
   Show answer

   4.67. 42 / 9 = 4.67.

2. What percentage of MZ co-twins in that study did NOT have a schizophrenia diagnosis?
   Show answer

   58 %. 100 - 42 = 58%.

3. Why do researchers compare MZ and DZ twins?
   • MZ twins are always raised apart
   • MZ twins share all their genes and DZ about half, while each pair usually shares an environment
   • DZ twins have identical genes
   • MZ twins never share an environment
   Show answer

   MZ twins share all their genes and DZ about half, while each pair usually shares an environment. The genetic difference with roughly equal environments lets researchers estimate genetic influence.

4. What does it mean to say schizophrenia is polygenic?
   • One gene causes schizophrenia
   • It is caused by a virus
   • Many genes each add a small amount of risk
   • It is only caused by the environment
   Show answer

   Many genes each add a small amount of risk. Poly = many.

5. According to Gottesman (1991), what is the approximate risk of schizophrenia for someone with one parent with schizophrenia?
   • 13%
   • 46%
   • 2%
   • 100%
   Show answer

   13%. One parent: about 13%; two parents: about 46%.

6. Explain one weakness of twin studies as evidence for a genetic explanation of schizophrenia.
   Show answer

   Twins usually grow up in the same family, so they share their environment as well as their genes, and MZ twins may be treated more alike, so higher concordance could be due to environment rather than genes.. Shared environments confound genetic conclusions.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification (issue January 2026)
• Pearson IAL Psychology component guide: WPS04 (Issue 2, November 2024)
• Gottesman and Shields (1966), Schizophrenia in twins: 16 years' consecutive admissions to a psychiatric clinic, British Journal of Psychiatry 112
• Macmillan Learning: table of schizophrenia risk by genetic relationship (Gottesman, 1991; Kendler, 1983)
• Pearson IAL Psychology component guide: WPS04 (Issue 2, November 2024)

Last updated 8 November 2026. Spotted a mistake? Tell us.