>For the men, however, the results were more complex. Taking homosexual men as a whole, “the 2D:4D ratio of homosexual men was not significantly different from that of heterosexual men for either hand.”

However, when the researchers segregated eldest sons from younger sons (i.e. those with at least two older brothers), they found that younger sons, straight or gay, had a significantly more “masculine” finger ratio than eldest sons.
Now, previous studies had found that there was a higher proportion of homosexual men among younger sons. This study found support for that. The researchers found that homosexual men had twice as many older brothers than older sisters, when usually the chance of having an older brother would be nearly the same as having an older sister.
So, the general rule that came out of this study was: the more older brothers one had, the higher chance of being homosexual. At the same time, the more older brothers one had, the more “masculinised” one’s finger ratio was likely to be. Since prenatal androgens caused the masculinised finger ratio, they most likely were also a causative factor in homosexuality – among younger sons.

Now why would younger sons be so affected in the womb compared to other babies? This was the mystery posed by the study’s data. It would appear that a mother’s body could “remember” how many previous sons (but not daughters) she had carried, such that younger sons with older brothers got more androgens. At this point, no one knows how this comes about.
Moreover, still unexplained were homosexual eldest sons, whose finger ratio was no different from heterosexual men. It would suggest, the researchers said, that androgens were not the only cause for homosexuality; genes could be playing a part too.