Professor of Psychiatry at the University of Toronto
Pro to the question "Is Sexual Orientation Determined at Birth?"
"The finding that a man's sexual orientation relates to his number of older brothers is based entirely on epidemiological rather than experimental data. The evidence for this relation arises from events in prenatal life rather than experiences in childhood or adolescence is scanty but consistent."
"Quantitative and Theoretical Analyses of the Relation Between Older Brothers and Homosexuality in Men," Journal of Theoretical Biology , 2004