Twin studies

The logic of twin studies

• Monozygotic (MZ) twins come from one fertilised egg and share 100% of their genes.
• Dizygotic (DZ) twins come from two eggs and share about 50%, like ordinary siblings.
• Both types usually grow up in the same family, so they share much of their environment.

If MZ twins are more similar for a behaviour than DZ twins, the extra similarity is put down to genes.

Concordance and correlation

Concordance is the percentage of twin pairs where, if one twin has a characteristic, the other does too. For scores on a scale (such as aggression ratings), researchers compare the correlation between twins' scores for MZ and DZ pairs.

Researchers split variation into genetic, shared environment (what makes twins alike, such as family) and non-shared environment (what makes them different, such as different friends or teachers).

Twin studies of aggression

Brendgen et al. (2005) studied 6-year-old twins in Montreal using peer and teacher ratings. For physical aggression the MZ correlation was almost twice the DZ correlation, suggesting a strong genetic influence; for social aggression MZ and DZ correlations were similar, suggesting environmental influence.

Evaluation

• Strength: a natural way to separate genes and environment that would be unethical to manipulate.
• Weakness: the equal environments assumption may be wrong; MZ twins may be treated more alike.
• Weakness: zygosity must be correctly identified (Brendgen used physical resemblance, checked with DNA for some).
• Weakness: twins may not represent the general population, limiting generalisation.
• Concordance below 100% in MZ twins shows environment also matters.

Worked example

In a study, 42 out of 60 MZ pairs and 18 out of 60 DZ pairs are concordant for a behaviour. Calculate the concordance rates and interpret them.

1. MZ: 42 / 60 x 100 = 70.0%.
2. DZ: 18 / 60 x 100 = 30.0%.
3. MZ concordance is much higher, suggesting a genetic influence; but it is below 100%, so environment matters too.

Answer: MZ 70.0%, DZ 30.0%: genetic influence plus environmental influence.

Check yourself

1. What proportion of genes do monozygotic twins share?
   • 50%
   • 25%
   • 75%
   • 100%
   Show answer

   100%. MZ twins come from one egg.

2. What proportion of genes do dizygotic twins share on average?
   • 100%
   • 50%
   • 25%
   • 0%
   Show answer

   50%. DZ twins are like ordinary siblings.

3. 28 out of 40 MZ twin pairs are concordant for high aggression. What is the concordance rate?
   Show answer

   70 %. 28 / 40 x 100 = 70%.

4. MZ concordance for aggression is 70%. What does the 30% discordance suggest?
   • Environmental factors also influence aggression
   • Aggression is entirely genetic
   • The twins were really DZ
   • Concordance cannot be measured
   Show answer

   Environmental factors also influence aggression. If it were purely genetic, MZ concordance would be 100%.

5. Which is a weakness of twin studies?
   • Twins have no genes in common
   • Twin studies always use animals
   • MZ twins may share more similar environments than DZ twins
   • They can only study physical illness
   Show answer

   MZ twins may share more similar environments than DZ twins. This is the equal environments assumption problem.

6. Explain how twin studies are used to investigate genetic influences on aggression.
   Show answer

   Researchers compare how similar MZ twins, who share 100% of their genes, and DZ twins, who share about 50%, are for aggression; if MZ twins are more similar (higher concordance or correlation) this suggests aggression is partly genetic.. Compare MZ and DZ similarity.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification, Issue 5 (February 2021)
• Pearson IAL Psychology component guide: WPS02 (Issue 2, November 2024)
• Pearson IAL Psychology summary of studies: WPS02

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