Let's say you're a male, and you have three older brothers. All of them are stepbrothers or adopted. Your chances of being gay are not affected. But if those same three brothers (all older than you, remember) are your blood relatives, your chances of turning out gay increase perceptibly with each older brother you have.
This suggests prenatal influences, since the family socialization in this case has been shown to be not a factor.
See this article in the London Times: Younger brothers more likely to be born gay.
The effect was found even where the older brothers were raised elsewhere.
What I was thinking of was the socialization; this kind of article is often couched in terms of what is called the "nature versus nurture" debate. In utero is said to be "nature" whereas your brothers beating up on you (or not) is said to be "nurture."
But when wolf cubs chew on each other's ears, or follow their mother to hunt snowshoe hares, we, watching this on the BBC or PBS, easily think of that nurturing as "nature."
And we're right. All nurture takes place in a natural setting and is an instance of nature: we're animals. There is nothing we can do that's not a manifestation of nature.
So, a question. Is sin [ahem] nature?