Researchers have also examined effects of birth order. Many gay men have been found to be born later than their siblings and have older brothers, but not older sisters (Blanchard, 2004; Camperio-Ciani et al., 2004; Ridley, 2003). Overall it has been esti­mated that 1 in 7 gay men’s sexual orientation was a result of fraternal birth order (the number of older brothers they have; Cantor et al., 2002).

Fraternal birth order could contribute to a homosexual orientation in two ways: first, the placenta cells of the uterine lining could influence later gestations; and children born later could develop an immune response. This immune response could influence the expression of key genes during brain development in a way that increases a boy’s at­traction to other boys (Ridley, 2003). This research is controversial, but nonetheless re­search in this direction continues to look for possible interactions. Interestingly, the re­lationship between sexual orientation and number of older brothers has been found to hold only for males (Blanchard, 2004).