Twin study

A twin study describes a kind of genetic study done to determine heritability. The premise is that since identical twins (especially identical twins raised apart) have identical genotypes, differences between them are solely due to environmental factors. By examining the degree to which twins are differentiated, a study may determine the extent to which a particular trait is influenced by genes or the environment.

Twin studies have been called into question because of the difficulty of isolating environmental factors from genetic factors. For example, twins that are "raised apart" are still often raised in similar environments, even by different parts of the same family. Inadequately controlling for such similarities may bias studies towards overestimating the influence of genetic factors.