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No. 74: Mar-Apr 1991

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When Identical Twins Are Not Identical

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.

From Science Frontiers #74, MAR-APR 1991. 1991-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987