In twin studies, it is assumed that heredity influences a characteristic if fraternal twins are more alike than identical twins.

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In twin studies, it is actually the opposite. If identical twins are more alike than fraternal twins, it is assumed that heredity influences a characteristic. The reason for this is that identical twins share 100% of the same genes, while fraternal twins share only about 50% of the same genes (the same as non-twin siblings). Therefore, if identical twins are more alike, it suggests that genetics have a stronger influence on the characteristic being studied.

In Twin Studies, It Is Assumed That Heredity Influences a Characteristic