Originally posted by Reilly
There have actually been a number of studies that suggest sexual orientation is biological. For instance:
1. A study by Simon LeVay, a neuroanatomist at the Salk Institute, in 1991, found brain structural differences between gay and straight people dealing with the size of the hypothalamus gland.
2. Psychologist Michael Bailey of Northwestern University conducted sibling studies that found siblings are more likely to share a sexual orientation.
3. Studies by 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.
4. 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.
You can find more information about these studies on the internet.