No. 74: Mar-Apr 1991
Past studies of identical twins separated at birth have documented remarkable similarities between them, despite the fact that they were reared under radically different circumstances. Their physical appearances, habits, vocations, health histories, and other factors are often eerily the same. For example, two female identical twins, who had never seen each other, each wore eight rings! The upshot of such investigations is that most of a person's characteristics are genetic in origin; that is, Nature dominates nurture.
But what about identical twins who are remarkably different? They can, for instance, differ appreciably in size, intellect, and behavior. In such cases, does nurture dominate nature? No! Identical twins may diverge even in the womb, where one may receive more oxygen and nutrients than the other. One also may be assailed in by viruses, bacteria, or drugs, while the other escapes. Even more drastic is the possi bility that one twin may pick up an extra chromosome soon after the original egg has split. Also, mutations may doom one twin to Down's syndrome or some other genetic affliction, while the other is unscathed. Identical twins may even be of different sex! Of course, such twins are genetically different, but they are still monozygotic (from the same egg). Blood tests will show them to be identical.
It used to be thought that the small differences that did exist between identical twins separated at birth were surely due to nurture, not nature. But, considering all the differences that can accrue in, it seems that the role of nurture in shaping individuals is much smaller than thought, possibly negligible. (Horgan, John; "Double Trouble," Scientific American, 263:25, December 1990.)
Comment. Such conclusions have profound philosophical and political implications that are beyond the pale of the Sourcebook Project.
Reference. Discordances between socalled "identical" twins are cataloged in BHA8 and BHB1 in Biological Anomalies: Humans I. To order, see: here.