twin study
research utilizing twins. The purpose of such research is usually to assess the relative contributions of heredity and environment to some attribute. Specifically, twin studies often involve comparing the characteristics of identical and fraternal twins and comparing twins of both types who have been reared together or reared apart. For example, two types of study have been used to investigate intelligence in twins: (a) identical twins reared apart—here, the genotypes (genetic makeups) are identical, but as there is no shared environment, any disparity in intelligence must result from the different environments; and (b) comparisons between identical twins reared together and fraternal twins reared together—here, one can assume that each pair of twins shares the same environment, but whereas the identical twins have 100% of their genes in common, the fraternal twins share only 50% of their genes, suggesting that if the identical twins show a greater similarity
in intelligence than do the fraternal twins, the genes must play a role in that trait. The assumptions made in these studies are, however, never completely fulfilled. For example, the identical twins reared apart have had some common environment, if only their intrauterine experiences. Moreover, identical twins reared together usually have more similar environments than fraternal twins raised together. These differences can make the estimations of heritability of intelligence open to some doubts.