Physical genetic traits are one reason I believe Homosexuality is biological in origin. Homosexuality is linked to the genetically determined finger length ratio. In people with homosexual preference, there is a tendency for the ratio to be lower than in heterosexuals of the same sex. Marc Breedlove, currently the Barnett Rosenberg professor of neuroscience at Michigan State, found no difference between the straight and gay men 's finger-ratios until he separated the men by birth order. Then his team found that gay males who had two or more older brothers had slightly lower ratios than those with no older (Sigesmund). The probability that a man has homosexual preference increases with the number of older brothers he has. Each older brother increases the odds by 1/3 – 1/2 (Pease and Bull).This effect focuses on the mother progressively building antibodies against an unknown male protein, more so with each son. By this
Bibliography: Bagemihl, Bruce. Biological Exuberance: Animal Homosexuality and Natural Diversity. New York: St. Martin 's, 1999. Print. "Biological Correlates of Being Gay - Biological Determinism?" Home | The University of Texas at Austin. Ed. Craig M. Pease and James J. Bull. Web. 12 Nov. 2011. . Marmor, Judd. Homosexual Behaviour: A Modern Reappraisal. Print. Rana, Huma Q. "The Homosexual Brain?" Web. 12 Nov. 2011. . Sigesmund, B. J. "Homosexuality and Finger Length." University of Nebraska–Lincoln. Web. 13 Nov. 2011. .