Hormones

Sex Differentiation

Hormones first become relevant to sex and gender as a zygote (fertilized egg) starts to develop into a fetus. The default sex is female, and several things have to [Page 439]occur before a male can develop normal reproductive organs. Humans have 46 chromosomes, 2 of which are sex chromosomes (XX in females; XY in males). A first critical step for sex differentiation is the chromosome contributed by the male (X or Y). If the zygote has an XY chromosome pair, a gene on the Y chromosome called SRY codes for the development of testes. In absence of this gene, testes do not develop.

Second, as the testes develop in the normal male, they begin to produce the hormone testosterone. In the developing female embryo, the lack of testes results in a lack of testosterone. Third, the presence or absence of testosterone affects the development of internal sex organs. Testosterone causes the Wolffian ducts to develop into the vas deferens and seminal vesicles. In absence of testosterone, the Miillerian ducts develop into a uterus and fallopian tubes. Finally, a testosterone by-product called DHT triggers the development of external male genitalia. The absence of testosterone causes the sex organs to remain inside the body.

Early in development, sex hormones have an influence on parts of the developing brain, creating structures that respond to these hormones later in life. Experiments with rats have shown that sexual behavior is entirely dependent on these organizing hormones. Administering female hormones to a developing male rat will lead to an inhibition of normal male sexual behavior. Likewise, administering male hormones to a developing female rat will lead to an increase in male-typical sexual behavior. In humans, the links to behavior are more complicated, but sexual dimorphism in brain structures can be seen as early as 2 years of age.

Hormones and Behavior

Hormones have also been implicated in a number of behaviors that tend to show gender differences. Aggression is defined as behavior intended to harm another person who does not want to be harmed. It was once accepted that females were far less aggressive than males. However, more recent research has broadened the definition of aggression to include more indirect forms, such as ostracism and spreading rumors. When these forms of aggression are included, the gender difference in aggression is considerably smaller.

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