Likelihood Of Being Gay Linked To Birth Of Older Brothers

Having older brothers increases the chances that a boy will be gay.

According to the Los Angeles Times, a report in the Proceedings of the National Academy of Sciences claims an analysis of 905 men and their siblings found the only factor that was significantly related to whether a boy becomes gay was the number of times a mother had previously given birth to boys:

The so-called fraternal birth order effect is small: Each older brother increases the chances by 33%. Assuming the base rate of homosexuality among men is 2%, it would take 11 older brothers to give the next son about a 50-50 chance of being gay.

The analysis found no evidence that social interactions among family members played a role in determining whether a man was gay or straight.

Scientific data showing that sexual orientation is the result of biology and not determined by social factors such as upbringing or a merely the result lifestyle choice should increase acceptance of gay rights:

Polls show that people who believe sexual orientation is governed by biology tend to support gay rights, whereas those who consider it a choice don't, said Dr. Jack Drescher, who chaired the American Psychiatric Assn.'s Committee on Gay, Lesbian and Bisexual Issues for six years.

A 2003 survey found that 30% of Americans believed sexual orientation was innate and 14% said it was determined by upbringing, besides the 42% who considered it a lifestyle choice. That survey was conducted by the Pew Research Center for the People & the Press and the Pew Forum on Religion & Public Life.

The Pew survey [pdf file] is a treasure trove of data on American attitudes towards homosexuals.

UPDATE: At PoliPundit, Oak Leaf poses two perplexing questions prompted by this finding:

1. Just as we treat other pre-natal conditions, should we not treat this condition as any other illness?
2. For those that are pro-choice, should we not offer screening for pregnant woman so that they can make a choice like they do with many other fetal conditions?

Any thoughts?

Comments

[spungen]

It's disconcerting to think that at least some gayness may be caused, basically, by brain damage. The theory is that the mother's body has an immune response to the male fetus, thereby resulting in this condition. I'd think the more likely result of screening would be treatment for this response, rather than abortion -- because you wouldn't know if the response had necessarily caused gayness. They think this immune response may also cause autism. So, even if you had no problem with gayness, you'd probably want to prevent autism, and would take the therapy. And, regardless of the autism prospect, it's upsetting to think one's pregnant body is attacking one's boy fetus.

If every case of this was treated and prevented, there'd be one-seventh fewer gay men out there (as the study estimated). An unfortunate side effect is that those who remained would be an even smaller minority and would have an even harder time in society. If women had abortions for this, it would skew the male-female ratio.

I would want to know if this response has always happened at the same rate, or if some environmental or evolutionary factor is causing it to happen more often. I could more easily accept it if it was the former.