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Why do researchers compare identical twins with fraternal twins in mental health studies?
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mental disorder
A discussion of the advances made in psychiatric research and treatment.
Researchers compare identical twins (monozygotic) with fraternal twins (dizygotic) to disentangle the relative influences of heredity and environment on mental disorders and other complex traits.1
This method serves as a quasi-experimental design because genes and environments cannot be manipulated in humans. The logic is as follows:1
- Genetic Similarity: Identical twins are genetically identical, whereas fraternal twins share only about half of their genes, similar to ordinary siblings.1
- Environmental Control: Because both types of twins are typically reared by the same parents in the same home at the same time, they share similar environmental influences.21
- Inference: If a trait is influenced by genes, identical twins should show a higher degree of concordance (similarity) for that trait than fraternal twins. If behavioral variability were caused solely by environmental factors, both types of twins would be expected to show similar levels of resemblance.1
By comparing these groups, researchers can estimate the extent to which heredity contributes to conditions like schizophrenia or depression.21