Professor of Psychology at Northwestern University
Not Clearly Pro or Con to the question "Is Sexual Orientation Determined at Birth?"
"It is important not to equate 'inborn' with 'genetic.' Environment can matter yet affect sexual orientation before birth. One of the most interesting hypotheses right now concerns fraternal birth order, or the number of older brothers a man has. The more older brothers a man has, the more likely the man’s orientation will be homosexual. Furthermore, it appears it's the number of older brothers that were inside his mother’s uterus, and not the number of older brothers that were in his house–that is, it's an innate and not a social influence. Gay men with identical twins usually have straight identical twins. So environment matters. I think, however, it's not the social environment, but more likely the early womb environment.
Also, I would separate men and women. I am much less confident that women's sexual orientation is inborn."
[Editor's Note: In an Aug. 21, 2017 email in reply to our question whether a categorization of 'not clearly pro or con' would accurately fit his position, Bailey stated, "Sure, or you could say Males: 'Yes' Females: 'Uncertain.'"]