ABSTRACT

A large number of studies over the past twelve years have shown that gay men have more older brothers than do heterosexual men (see Blanchard, 1997, 2001, and 2004 for reviews). Many of these studies were conducted using data collected in Canada, the Netherlands, England, and the United States. In contrast, older sisters do not influence the sexual orientation of their younger brothers. Furthermore, neither older brothers nor older sisters affect the sexual orientation of younger sisters; that is, lesbians do not have more older brothers and/or older sisters. This effect has been called the “fraternal birth order effect” (Blanchard, 1997). Results of a combined statistical analysis of 14 studies representing 10,143 male subjects convincingly show that gay men do, on average, have higher birth orders; that is, they are born later to the same mother than heterosexual men (Blanchard, 2004). This finding implies that the number of older siblings, or some factor related to this, must affect a newborn boy’s sexual orientation.