There has been a lot of research into homosexuality of late, and not just in humans, all of which increasingly shows the trait to be biological—across hundreds of species studied thus far.  Now we have more research indicating homosexuality is not a choice:

WASHINGTON – Men who have several older brothers have an increased chance of being gay — whether they were raised together or not — a finding researchers say adds weight to the idea that sexual orientation is based in biology.

The increase was seen in men with older brothers from the same mother, but not those who had stepbrothers or adopted brothers who were older.

“It’s likely to be a prenatal effect,” said Anthony F. Bogaert of Brock University in St. Catharines, Canada, who did the research. “This and other studies suggest that there is probably a biological basis” for homosexuality.

One might assume the effect is environmental insomuch as the older brothers somehow influence the sexuality of the younger sibling.  One would be wrong in making such an assumption.

“Anybody’s first guess would have been that the older brothers were having an effect socially, but this data doesn’t support that,” [S. Marc Breedlove, a professor in the neuroscience and psychology department of Michigan State University,] said in a telephone interview.

The only link between the brothers is the mother and so the effect has to be through the mother, especially since stepbrothers didn’t have the effect, said Breedlove, who was not part of the research.

And just how many brothers does it take?

Bogaert said the increase can be detected with one older brother and becomes stronger with three or four or more.

There is apparently a near-factorial influence based on the number of elder brothers.

But is it safe to assume this indicates a biological function rather than some other influential input that is as yet unidentified?  Unless you’re a hating religious zealot who simply can not see beyond your own prejudice on the gay issue, therefore always willing to stand in the face of hard facts and deny they exist, it’s difficult based on these findings alone to argue with the premise that homosexuality is biological.  In fact, these results are difficult to argue with except when blinded by faith-based bigotry.

Bogaert said he concluded the effect was biological by comparing men with biological brothers to those with brothers to whom they were not biologically related.

The increase in the likelihood of being gay was seen only in those whose brothers had the same mothers, whether they were raised together or not, he said.

Men raised with several older step- or adopted brothers do not have an increased chance of being gay.

While the study can not account for the incidence of homosexuality in those without older male siblings, it does establish a biological influence for those borne of a mother who has already given birth to at least one other male.

The implications of this study are manyfold.  First and most important is the obvious: this clearly indicates a prenatal tendency based on the number of male siblings.  Similarly intriguing is that the tendency was not influenced by whether or not the children were raised together (which negates the idea of older male siblings somehow causing homosexuality in others, whether directly or indirectly).  The point that clarifies this as a biological function rather than environomental is that the same influence is not seen with regards to stepbrothers or adoptees (having several non-biological older brothers does not cause the same impact).  Since the influence only presents with offspring from the same mother, the tendency appears based on a prenatal function.

I guess this means the psychiatrists have been right all along.  You really can blame it all on your mother!