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Summary of Studies on the Origins of Sexual Orientation

Inner Ear Difference In Lesbians

Researchers at the University of Texas, Austin found that the cochlea (a structure of the inner ear) in lesbians differs from the cochlea of heterosexual women. The findings were published in the Proceedings of the National Academy of Sciences (March 1998). The difference was detected using a test that measures a very slight sound that the cochlea makes when responding to a soft clicking sound. Results indicate that lesbians have click responses that are significantly weaker than those of heterosexual women, and are more similar to those of men. Generally, the cochlea in women is more sensitive than that of men. Dennis McFadden, the lead author of the study, believes the cochlea of lesbians may be affected by hormone exposure before birth. It is presumed that an unknown site or sites in the brain that influence sexual orientation may be similarly affected.

While this study has yet to be replicated, it does suggest a biological component may be involved in the determination of sexual orientation.

Twin Studies

Studies of identical twins who were separated at birth and raised in different homes have been performed and replicated. It has been found that in many cases of identical twins, that if one twin is homosexual, the other twin is also homosexual. This lends credence to the theory that sexuality has a very strong genetic component, and is not purely determined by life experiences.

Anatomical Brain Differences and Sexual Orientation

In 1991 Simon LeVay, a neuroanatomist at the Salk Institute, examined the brain tissue from 41 people as follows:

  • 19 homosexual men who had died from complications of AIDS
  • 16 heterosexual men (6 had died from complications of AIDS, and 10 from other causes)
  • 6 heterosexual women (none were reported to have AIDS)
  • No samples from lesbians were available.

He found the INAH3 (a structure within the hypothalamus) was twice as large in heterosexual men (even those who had died form AIDS) as in homosexual men. He also found a similar difference between heterosexual men and heterosexual women. This study suggests that sexual orientation has a biological substrate.

Sibling Studies

Psychologist Michael Bailey of Northwestern University and Psychiatrist Richard Pillard of Boston University studied the sexual orientation of siblings raised together. They found that if one sibling is homosexual the chance of another sibling being homosexual is as follows:

  • 52% for an Identical Twin
  • 22% for a Fraternal Twin (non-identical twins)
  • 10%(approx.) for adopted or non-genetic siblings

Again, this study suggests that sexual orientation has a genetic component.

Atypical Gender Behavior in Children as an Indicator of Sexual Orientation

Richard Greene of the University of California at Los Angeles reports that children who manifest aspects of gender-atypical play indicates a homosexual orientation 75% of the time. Richard Greene’s observations suggest that sexual orientation is in place early in the life cycle.

Finger Print Studies

J.A.Y. Hall and D. Kumura at the University of Western Ontario at London ON Canada compared the number of ridges(finger prints) on the index finger and thumb of the left hand with corresponding digits on the right hand. They found that 30% of homosexuals had excess ridges on the left hand digits, while only 14% of heterosexuals showed the same characteristic. Because fingerprints are fully developed in the fetus before the 17th week and do not change thereafter, this study may suggest a genetic link to sexual orientation that is determined before birth, perhaps at conception.

DNA Study

Research led by Dean Hamer at the National Cancer Institute compared the DNA of 40 pairs of homosexual brothers. They found that almost all shared a genetic marker in the Xq28 region of the X chromosome (one of the two sex chromosomes). While this study hasn’t precisely isolated a gay gene, it suggests that sexual orientation may have a genetic component.  

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