Studies In Nonhuman Mammals

Experimental studies in mammals convincingly demonstrate that androgens and estrogens affect behaviors that show sex differences. Hormones affect behavior in two ways, depending on when they are present.

In early life, when the brain is developing, hormones produce permanent changes in brain structure that affect behavior (organizational effects). High levels of androgen during critical developmental periods are associated with high levels of male-typical behavior (higher in level or frequency in males than in females) and low levels of female-typical behavior (higher in level or frequency in females than in males). Female rodents and primates who are treated with high doses of androgen during prenatal and neonatal periods when the brain develops show sexual behavior more typical of males than of other females. Compared to typical females, they also engage in more rough play, are more aggressive, and perform better in mazes in which males excel. Conversely, males deprived of androgen during these sensitive periods behave in ways more typical of females than of other males. Excess or reduced androgen also produce changes in the hypothalamus, which is involved in sexual behavior, and hippocampus, involved in spatial learning.

Behavior is also affected by hormones circulating in the body throughout adolescence and adulthood (activational effects). Studies in animals show that both sexual and nonsexual behaviors are affected by circulating hormones acting on neural activity. For example, changes in estrogen across the estrous cycle in female rats are associated with variations in motor function, perception, and learning and memory. These effects are mediated by estrogen action on underlying brain regions, including striatum, cerebellum, and hippocampus.

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