If your principal research interest is to study the extent to which environmental factors and genetic variance interact to give rise to a given trait, you should use a:

Question

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Twin studies are ideal for investigating the extent to which genetics and environment interact to produce a given trait. By studying identical twins who share 100% of their genes and fraternal twins who share only 50% of their genes, researchers can assess the relative contribution of genetics and environment to the trait of interest. Twin studies can also help control for certain environmental variables since twins raised together are likely to have more similar environments than twins raised apart.

If Your Principal Research Interest Is to Study the Extent