AGE REPORTED CONCORDANCE
STUDY DATE RANGE MALE MZ TWINS MALE DZ TWINS SAMPLE SOURCE
Kallmann 1952 >20 37/37 (100%) 3*/26 (12%) Psychiatric, correctional and charitable agencies, plus direct contacts
Heston & Shields 1968 20-52 3/7 (43%) 1/7 (14%) Hospital Twin Register
Bailey & Pillard 1991 19-65 29/56 (52%) 12/54 (22%) Homophile publications
* Concordance rate varies from 3/26 (Kinsey scale 3-6) to 11/26 (Kinsey scale 1-6)
N.B. The data in this table represent only those cases where the sexual orientation of each subject is 'fairly certain', and is either predominantly homosexual or predominantly heterosexual.
MZ stands for monozygotic twins, and DZ dizygotic twins. (the former shares 100% of genetic material, the latter, 50%)
The lack of 100% accordance in genetically identical twins is true for many other biological phenomenoms ( cancer, diabetes etc.) because we now know epigenetic factors play a role (something not commonly known to non bioligists)
More on physical differences:
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.
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.
Although none are conclusive, these are the the latest in a growing scientific literature suggesting that sexual preferences may not be simply a matter of personal preference but part of our ingrained biology. The causes may be mutiple, ie: genetic, hormonal, epigenetic (I favor epigenetic and hormonal) and there may not be one single determinant, but the data continues to accrue that many homosexual behavior has a biological basis. Personally, the best data comes from the hormonal influences such as the congenital adrenal hyperplasia since large numbers and epidimeological data can be harvested. The twin studies, etc. all are still limited mainly by the size (, as in numbers,) of the study subjects.
Physicians and pediatricians are privey to this knowledge that not even family members realize.....most homosexuals know they are homosexuals long before puberty or around puberty but keep it hidden from society. My link also addresses a study by a psychologists that show how by analyzing toddlers at play, and their atypical behavior predicts homosexuality later in life. So in fact, this may be ingrained by the time they are toddlers. All my homosexual patients were raised in heterosexual family structures, many from bible thumping, southern households with very macho dads who have since disowned them. I see adults and teenager raised in homosexual households who are cleary heterosexual, so there is no correlation between "upbringing" . I keep careful demographic data on all 14,000 active patients due to clinical studies I have to do and such social parameters are kept track of in detail so I can corroborate what psychologists have been saying for a long time.
http://www.youdebate.com/DEBATES/gay_adoption.HTM
http://www.rtis.com/reg/bcs/pol/tou...mmer97/mueh.htm