If you're referring to a twin study that claims that 50% of the identical twins who are themselves homosexual, have a homosexual twin, there are some serious problems with that study. The first is that the sample population was self-selected by responding to adds in the back of homosexual publications.
Other less-skewed twin studies have given numbers between 10% and 20%.
But there's a problem even calling those 10% to 20% evidence of a genetic link to homosexuality... because the nature vs. nurture debate is a false dilemma. There is a third possibility...
Pre-natal.
Numerous factors can affect pre-natal development. One particulary vulnerable phase occurs when that XY chromosome kicks in the hormones and turns an externally female fetus into the male it was genetically destined to be.
What if pre-natal conditions are somehow less than optimal during that hormone surge? What is the impact on the later sexual preference of that fetus?
We don't really know, but we know that identical twins share an identical pre-natal environment.
Basically, what the twin studies of homosexuals indicates is that the cause is most likely not genetic. Here's why...
Eye color is genetic. 100% of the identical twins with blue eyes have a blue eyed twin. But when twin studies of homosexuality give an identical correlation of 10% or 20% (heck, even 50%), then we know for a fact that homosexuality isn't genetic in 50% to 90% of the cases. The bottom line is that nurture and pre-natal probably account for the bulk of the rest.
After all, wouldn't a "gay gene" have a hard time perpetuating itself?
Hard to escape the fact that homosexuals are absolutely always procreated heterosexually.
