Introduction to twin and adoption research methods

Psychologists use twin and adoption studies to explore whether behaviours are influenced more by genetic factors (nature) or environmental factors (nurture). These methods help separate the effects of genes from those of upbringing and surroundings. By comparing similarities in traits between related individuals, researchers can infer the relative contributions of biology and experience.

Key terms in genetic research

• Genes - Units of heredity passed from parents to children that determine physical and behavioural traits.
• Nature - The influence of genetic inheritance on behaviour.
• Nurture - The influence of environmental factors, such as upbringing and experiences, on behaviour.
• Traits - Characteristics or behaviours that can be observed and measured, such as personality or mental health conditions.

These studies assume that higher similarities in traits among those who share more genes point to genetic influences, while similarities despite genetic differences suggest environmental effects.

Twin studies and their role in psychology

Twin studies compare pairs of twins to assess genetic influences on behaviour. Twins provide a natural experiment because they can share genes to varying degrees while often sharing environments.

Types of twins

• Monozygotic (MZ) twins - Also known as identical twins; they develop from a single fertilised egg and share 100% of their genes.
• Dizygotic (DZ) twins - Also known as non-identical or fraternal twins; they develop from two separate fertilised eggs and share about 50% of their genes, similar to regular siblings.

Researchers compare MZ and DZ twins because MZ twins are genetically identical, while DZ twins are not. If MZ twins show more similarities in a trait than DZ twins, this suggests a genetic basis for that trait.

Concordance rates

Concordance rates measure the likelihood that both twins in a pair share a specific trait or behaviour. A higher concordance rate indicates stronger similarity.

How concordance rates work:

• These rates are calculated as percentages; for example, if 35% of MZ twin pairs both have schizophrenia, the concordance rate is 35%.
• Psychologists compare concordance rates between MZ and DZ twins to infer genetic influence.
• However, most twins share similar environments, so similarities might result from nurture rather than nature. This is a key limitation.

Twins reared apart

To address the shared environment issue, researchers study MZ twins who were separated at birth and raised in different homes. These twins have identical genes but different environments.

Benefits of studying twins reared apart:

• If they still show high similarities in traits, this strengthens the evidence for genetic influences, as environmental factors are minimised.
• This approach helps overcome the confound of shared upbringing in standard twin studies.

Adoption studies and their role in psychology

Adoption studies examine children who are raised by adoptive parents but have biological parents elsewhere. This method helps disentangle genetic and environmental influences because adopted children share genes with their biological parents but environments with their adoptive ones.

How adoption studies work

Adopted children share 50% of their genes with each biological parent but none with their adoptive parents. They share the environment (home, upbringing) with adoptive parents but not with biological parents after adoption.

Researchers look for similarities in traits:

• Similarities with adoptive parents - Suggest environmental influences, as there are no shared genes, implying the child learned the behaviour through nurture.
• Similarities with biological parents - Suggest genetic influences, as shared genes but no shared environment point to nature.

This comparison helps determine if a behaviour is more likely biologically determined or shaped by surroundings.

Strengths of adoption studies

• Provides clear separation of nature and nurture.
• Useful for studying traits like intelligence or mental health.

Limitations of adoption studies

• Early environmental influences before adoption might affect results.
• Selective placement (adoptive homes matching biological backgrounds) can complicate findings.

Gottesman and Shields (1966) twin study on schizophrenia

Schizophrenia is a mental health disorder characterised by symptoms such as hallucinations, delusions, and disordered thinking. Gottesman and Shields conducted a twin study to investigate whether schizophrenia has a genetic basis by comparing MZ and DZ twins.

Aims

The study aimed to examine the relative importance of genetic and environmental factors in schizophrenia. It sought to replicate previous findings of a genetic link and analyse twins over a 16-year period from a London hospital.

Participants

The researchers selected 57 twin pairs (aged 19-64 years, average 37 years) from hospital records of psychiatric patients at the Maudsley and Bethlem Royal Joint Hospital. All pairs were same-sex and born between 1893 and 1945, having survived to age 15.

Blood and visual tests confirmed whether twins were MZ or DZ.

Procedure

Secondary data was gathered from hospital records of 392 patients with same-sex twins (from about 45,000 psychiatric patients total).

Additional data included:

• Case histories from self-report questionnaires and interviews with twins and parents to record verbal behaviour.
• A personality test.
• A test for disordered thinking administered to twins and parents.

Concordance for schizophrenia was assessed in three grades:

1. Both twins hospitalised and diagnosed with schizophrenia.
2. Both hospitalised, but co-twin has a different diagnosis.
3. Co-twin shows some psychiatric abnormality (e.g., outpatient care, GP treatment, or abnormal profile).

Results

The study found higher concordance rates for schizophrenia in MZ twins compared to DZ twins, suggesting a genetic component.

Concordance rates for schizophrenia:

Conclusions

The higher concordance rates in MZ twins indicate that genes play a significant role in schizophrenia. MZ twins were at least 48 times more likely to develop schizophrenia than the general population. However, rates were not 100%, suggesting environmental factors also contribute.

Evaluation

Strengths:

• Large sample and multiple data sources (e.g., tests and interviews) provide robust evidence.
• Comparison of MZ and DZ twins isolates genetic effects.

Limitations:

• Reliance on secondary data may introduce inaccuracies.
• Shared environments for many twins could confound results.
• The study does not fully account for twins reared apart.