The twin method has been robustly defended as a rough screen for genetic influence on behaviour.
Although the assumptions of the twin method have survived indirect tests, it is now possible to test findings from the twin method directly using DNA alone in samples of unrelated individuals, without the assumptions of the twin method.
The twin method relies on the accident of nature that results in identical (monozygotic, MZ) twins or fraternal (dizygotic, DZ) twins.
The impact of this finding may have been blunted because it depends on the validity of the twin method.
By comparing sets of MZ twins to carefully matched control sets of DZ twins (the "twin method"), researchers have attempted to elucidate the relative importance of heredity versus environment in the development of certain diseases, in the formation of the personality, and in intelligence.
Behavioral genetics and the twin method are concerned with the genetic and environmental influences on individual differences in complex traits.
The twin method uses MZ (monozygotic, identical) and DZ (dizygotic, fraternal) twin intraclass correlations to dissect phenotypic variance into genetic and environmental sources [7].
In other words, multivariate twin analysis uses the twin method to estimate genetic and environmental contributions to the covariance of two or more traits as well as the variance of each trait [39].
Finding that the heritability of corrected-achievement is about 50 percent implies that about half of the reliable variance is due to environmental differences between children; the twin method is equally effective at demonstrating the effects of nurture as of nature.
The twin method provides substantial evidence for the important role of non-shared environmental influences on a range of cognitive and behavioural measures [7], but what are these non-shared environmental influences?
The twin method is able to carry the analysis of nurture a step further, and distinguish two types of environmental influence: shared environment that contributes to the similarity of siblings growing up in the same family and attending the same school and non-shared environment which does not [7].
