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In a few twin studies, use has been made of MZ twins brought up apart in different families, so they should have the same IQ. Those arguing that genetic factors are of most importance would expect such twins to have very similar IQ's. In contrast, those favouring an environmentalist position would argue that placing twins in different environments should ensure they are not similar in intelligence. Bouchard and McGue (81) found the mean correlation coefficient for MZ twins brought up apart is +0.72. The problem with theses findings is that MZ twins brought up apart were brought up in different branches of the same family. Going on the evidence found from these studies by Bouchard and McGue it is convincing to say that genetics do play big part in the development of measured intelligence. .
Another method of assessing the role of genetic and environmental factors in intelligence is by means of adoption studies. The measured intelligence of adopted children might depend more on genetic factors. Horn (83) discussed the finding from the Texas Adoption project, which involved almost 500 adopted children. The correlation between adopted children and biological mothers for intelligence was +0.38 which is only a moderate degree of similarity. There was a +0.15 correlation between children and their adoptive mothers. Both of these correlation's are so low that it is hard to make any definite statements about roles of heredity and environment, but it is clear that the role of heredity is greater. The findings of a study by Capron and Duyne (89) are consistent of those from twin studies suggesting that about 50% of the variance in intelligence scores is due to genetic factors. However, less clear findings have emerged from adoption studies, in part because of the problems of interpretation posed by selective placement. In many studies, some of the correlation between the children and their biological parent is due to selective placement rather than genetic factors.