Twin studies: opportunity for researchers to investigate the contributions of genetic and environmental influences. If MZ (identical) twins, who share 100% of their genes, are more concordant in terms of a trait like sz, than DZ (non-identical) twins who share 50% of their genes, then this suggests that the greater similarity is because of genetic factors.
Joseph - concordance rate for MZ twins 40%, and for DZ twins 7.4%.
More recent blind studies (researchers don't know whether the twins are MZ or DZ) have tended to report a lower concordance rate of MZ twins.
However still argued there is a genetic basis because of the still higher concordance rate in MZ twins than DZ twins
Evaluation
MZ twins also have a greater environmental similarity than DZ twins, they are treated more similarly. They are also more likely to experience more identity confusion. This shows that there is reason to believe that the differences in concordance rates between MZ and DZ twins reflect nothing more than the environmental differences
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