Although biological findings currently dominate the research literature on the determinants of sexual orientation, biological theorizing has not yet spelled out a developmental path by which any of the various biological correlates so far identified might lead to a particular sexual orientation. The Exotic-Becomes-Erotic (EBE) theory of sexual orientation (Bem, 1996) attempts to do just that, by suggesting how biological variables might interact with experiential and sociocultural factors to influence an individual's sexual orientation. Evidence for the theory is reviewed, and a path analysis of data from a large sample of twins is presented which yields preliminary support for the theory's claim that correlations between genetic variables and sexual orientation are mediated by childhood gender nonconformity.