http://www.webmd.com/content/article/100/105486.htm
"The genetic scans showed a clustering of the same genetic pattern among the gay men on three chromosomes -- chromosomes 7, 8, and 10. These common genetic patterns were shared by 60% of the gay men in the study. This is slightly more than the 50% expected by chance alone.
The regions on chromosome 7 and 8 were associated with male sexual orientation regardless of whether the man got them from his mother or father. The regions on chromosome 10 were only associated with male sexual orientation if they were inherited from the mother."
I was probably wrong in saying "it's pretty solid" - actually the field of study is quite in its infancy, but anything accurately called instinctual behavior is due to genetics. I'd bet on it.
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