Twins provide an invaluable tool for separating out the genetic and environmental causes of disease. All twin pairs--if they are not separated--share the same environment before and after birth. Whereas non-identical, or hereditary, twins are only genetically similar (like ordinary brothers), identical twins have matching sets of genes. Therefore, if a disease has a high genetic component, it is more likely to affect both twins of an identical rather than non-identical, pair. Twin studies show, for example, that depression has a very high genetic predisposition.


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