Sex-related variation in human behavior and the brain

Abstract

Male and female fetuses differ in testosterone concentrations beginning as early as week 8 of gestation. This early hormone difference exerts permanent influences on brain development and behavior. Contemporary research shows that hormones are particularly important for the development of sex-typical childhood behavior, including toy choices, which until recently were thought to result solely from sociocultural influences. Prenatal testosterone exposure also appears to influence sexual orientation and gender identity, as well as some, but not all, sex-related cognitive, motor and personality characteristics. Neural mechanisms responsible for these hormone-induced behavioral outcomes are beginning to be identified, and current evidence suggests involvement of the hypothalamus and amygdala, as well as interhemispheric connectivity, and cortical areas involved in visual processing.

Figure 1

Influences of prenatal androgen exposure in females, caused by congenital adrenal hyperplasia (CAH), on preferences for sex-typed toys and on a broad measure of sex-typical activity and interest preferences. Girls with CAH are exposed to high levels of androgens prenatally, similar to levels experienced by unaffected boys and boys with CAH. For toy preferences, values are the mean (+SD) percentages of time spent playing with girls’ toys (a) or boys’ toys (b) in a playroom setting where a variety of girls’ toys (e.g., dolls, tea set, cosmetics) and boys’ toys (e.g., cars, trucks, guns) are available, along with neutral toys (e.g., books, puzzles, crayons and paper). For sex-typed activities and interests, values are mean (+SD) scores on the Pre-School Activites Inventory (PSAI), a 24-item, standardized measure . Scoring of the PSAI involves subtracting scores on girl-typical items from scores on boy-typical items. Consequently, higher scores represent more male-typical behavior. In all three panels (a – c), means for girls and boys differ significantly, as do means for girls with and without CAH. Data in panels a and b are adapted from and data in panel c are adapted from .

Figure 2

Examples of a male and a female vervet monkey contacting human children’s sex-typed toys. The female animal (left) appears to be inspecting the doll, in a manner similar to that in which vervet monkeys inspect infant vervets. The male animal (right) appears to be moving the car along the ground as a child might do. Reproduced with permission from .