Empirically, the authors explain, there are two ways to investigate this hypothesis, with either a prospective or retrospective study.
Using the prospective method, young children displaying sex-atypical patterns are followed into adolescence and early adulthood so that their sexual orientation can be assessed at maturity. This method is not terribly practical for several reasons.
Given that a small proportion of the population is homosexual, prospective studies require a large number of children. This approach also takes a long time, around 16 years.
