Prospective studies that systematically build sex into the design to study the longitudinal evolution of sex differences in males and females are needed. For example, in a prospective study of the course of young children at the initial diagnosis of ASD, Szatmari and colleagues 11 showed that the sex of the affected child with ASD was the only significant predictor of differential trajectories of symptoms of ASD over time. They found that boys had more stable, severe symptoms over time, whereas girls exhibited less severe symptoms and improvement over time. In fact, some girls no longer manifest the cognitive and language problems at follow up. This illustrates that the age or developmental stage at ascertainment of a condition may influence the sex ratio. Developmental studies of sex differences across the life course will also aid in our understanding of the factors underlying these differences. Biologic and social factors in early life development, such as parenting style or social environment, have long been seen as clinically relevant to later adult psychopathology, 133 but there remains a dearth of information on how these early factors may differentially impact the sexes.
