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Levels of Identity Confusion and Attachment among Reared-Together MZ and DZ Twin Pairs

When twin researchers attempt to assess twins’ environmental similarity, they usually ask questions such as whether, as children, twins shared the same bedroom, attended school together, dressed alike, played together, and so on. Answers to these questions reveal that reared-together MZ twin pairs (monozygotic, identical) grow up experiencing much more similar environments than experienced by same-sex DZ twin pairs (dizygotic, fraternal). Although these questions address some aspects of twins’ environmental similarity, they fail to adequately assess the nature of the attachment, conscious attempts to be alike, and the identity confusion experienced by MZ twin pairs to a far greater degree than DZ pairs.
In 1960, family therapy pioneer and twin study critic Don Jackson described “the intertwining of [MZ] twin identities, in the ego fusion that in one sense doubles the ego (because the other is felt as part of the self) and in another sense halves it (because t…