<P> An example of a positive MZ discordant effect is shown below on the left . The twin who scores higher on trait 1 also scores higher on trait 2 . This is compatible with a "dose" of trait 1 causing an increase in trait 2 . Of course, trait 2 might also be affecting trait 1 . Disentangling these two possibilities requires a different design (see below for an example). A null result is incompatible with a causal hypothesis . </P> <Table> <Tr> <Td> A depiction of MZ - discordance data </Td> <Td> MZ discordant test of hypothesis that exercise protects against depression </Td> </Tr> </Table> <Tr> <Td> A depiction of MZ - discordance data </Td> <Td> MZ discordant test of hypothesis that exercise protects against depression </Td> </Tr> <P> Take for instance the case of an observed link between depression and exercise (See Figure above on right). People who are depressed also reporting doing little physical activity . One might hypothesise that this is a causal link: that "dosing" patients with exercise would raise their mood and protect against depression . The next figure shows what empirical tests of this hypothesis have found: a null result . </P>

Which set of subjects would share the most similar genetic heredity