Upbringing versus biology
By about 18 months, most children begin to display gender-specific behaviours. Boys will tend to choose vehicles or construction toys to play with; girls go for dolls. These behaviours appear so early that some suggest that they must be innate, while others argue that they simply reflect parental influences or the child's desire to conform.
These possible causes are difficult to study. With human subjects, controlled studies cannot be carried out to compare the impact of, say, prenatal hormones or particular styles of parenting.
Observational studies suggest that parents tend to treat male and female children differently from birth, even without realising it.
On the other hand, girls with congenital adrenal hyperplasia – who have relatively high testosterone levels – are typically more 'tomboyish' than other girls. Early in life, they also show greater preference for 'male' toys, even when parents encourage them to play with gender-appropriate toys. Such studies argue in favour of a biological effect mediated through hormones.
Studies on animals support this idea. For example, rats show sex-specific behaviours, but these disappear if the effect of sex hormones before birth is blocked. Some primates have recently been shown to demonstrate sex-specific choices – boy monkeys preferring trucks and girl monkeys preferring dolls. Although primates may have social structures, this is more likely to reflect innate preferences.
One way to tease apart cultural/environmental and genetic/biological influences is through twin studies. By comparing characteristics in identical twins (which share all their genes) and non-identical twins (which share half their genes), researchers can estimate the relative contributions of genes and environment (though such studies do not say anything about what these influences actually are).
A recent study of 4000 pre-school children confirmed that sex-typed behaviours were influenced by both environmental and genetic factors (the latter affecting girls particularly strongly). It seems likely that the genetic effects are mediated through the action of hormones in the developing baby.