Business of LifeFrom The Science of Sexual Orientation
There are few issues as hotly contested — and as poorly understood — as the question of what makes a person gay or straight. It's not only a political, social, and religious question but also a scientific question, one that might someday have an actual, provable answer.
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While biologists look at hormones for answers about human sexuality, other scientists are looking for patterns in statistics. And hard as this is to believe, they have found something they call "the older brother effect."
"The more older brothers a man has, the greater that man's chance of being gay," says Bailey.
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If this comes as a shock to you, you’re not alone. But it turns out, it’s one of the most solid findings in this field, demonstrated in study after study.
And the numbers are significant: for every older brother a man has, his chances of being gay increase by one third. Older sisters make no difference, and there's no corresponding effect for lesbians. A first-born son has about a 2 percent chance of being gay, and the numbers rise from there. The theory is it happens in the womb.
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"One of the things we've only found out lately is that older brothers affect a boy only if the boy is right-handed," Breedlove said. "If the boy is left-handed, if his brain is organized in a left-handed fashion, it doesn't matter how many older brothers he has, his probability of being gay is just like the rest of the population."