Hormones at work
It is not only sex chromosomes but also sex hormones that that determine whether we grow up as males or females
Thanks to the SRY gene – the ‘maleness’ gene on the Y chromosome – males develop testes. These produce the hormone testosterone, as well as anti-Müllerian hormone, which suppresses the development of the female reproductive system.
In females the sex hormones oestrogen and progesterone drive the development of the female body during puberty. Females also make testosterone, but it is converted into oestrogen.
The most obvious effects of the sex hormones are on the reproductive organs, but in fact they act throughout the body, including on the brain. When the levels of sex hormones drop back after birth, the brain retains this male- or female-specific ‘wiring’, which then plays a role in individual behaviour.
The period we spend in the womb, exposed to sex hormones, is critical to our later development. From about 1–2 years onwards children show sex-specific differences in behaviour. Girls, for example, typically go for more ‘feminine’ toys – dolls rather than trucks – when given a free choice (this has been seen in monkeys, too). Boys tend to go in for more ‘rough-and-tumble’ games.
But is this because sex hormones have programmed their behaviour, or are children conforming to ‘typical’ boys’ and girls’ behaviour or being influenced by their parents? It is hard to be certain – evidence exists for all of these effects.
Animal experiments suggest that sex-specific characteristics are at least partly dependent on sex hormones. For example, blocking the action of testosterone prevents some monkeys’ genitals from developing properly, although it does not necessarily affect their sexual behaviour.
The production of sex hormones undergoes a second major boost at puberty. Puberty has been likened to human metamorphosis: it converts us from being a juvenile to an adult, able to survive independently of our parents. It is a period of extraordinary biological change, involving more obvious physical changes as well as extensive rewiring of the brain. At the end of it, formerly similar boys and girls will have become quite different men and women.
The timing of puberty depends on interactions between genes, hormones and other internal factors – like body weight – or external factors. For example, puberty may start earlier in females from disrupted family backgrounds.Lead image:
Marcus Hansson/Flickr CC BY
- Information on the sex-determining region Y protein
- Sex steroids and the organization of the human brain (2012)
- Infants’ preferences for toys, colors, and shapes: sex differences and similarities (2010)
- Psychology Today: Sex-specific toy preferences – learned or innate?
- The Conversation: There’s no good reason to push pink toys on girls
- Guardian: Pink v blue – are children born with gender preferences?
- Hormonal influences on sexually differentiated behavior in nonhuman primates (2005)
- ABC Science: Puberty genes linked to body fat
- Birthweight and paternal involvement predict early reproduction in British women (2009)