There is no denying that boys and girls are different, but why? In trying to explain sex differences, the pendulum swings regularly from nature to nurture. Are the differences inborn, or do they arise predominantly from environmental influences?
Behavioural differences between girls and boys are apparent from early infancy, and the subject has been studied extensively over the years. But in recent years, the focus was mainly on identifying the effects of socialisation, parenting practices and physical differences between the sexes. Clearly such factors do influence how children develop, and are also responsible for some of the characteristics that mark out each sex. However, new evidence also points to influences from pre-natal exposure to hormones on later sex-typed choices and gender-differentiated motor skills. Here, we review this research with the aim of challenging some of the traditional explanations of why girls and boys develop differently.
THE EYES OF THE BEHOLDER
The ways in which adults treat children, even newborns, differ as a function of the child's sex. In one experiment, differences were detected in the muscular pressure of the hands of adults holding boy or girl newborn infants.
In another experiment, adult participants were shown a video of a baby kicking and rolling about in a cot, but one group of adults saw the baby dressed in pink and the other group saw exactly the same baby dressed in blue. The baby's movements were identical for the two groups. Yet, when asked to describe what they saw, the group of adults who watched the baby dressed in pink described the infant as delicate and graceful, while those who saw the baby dressed in blue described the same movements as more vigorous, with some even going as far as to suggest that the baby would become a champion footballer!
Other research has confirmed that the way we talk to children, play with them and nurture them is strongly influenced by our knowledge of their sex. Countless studies of parent-child interaction have examined how parents differ in their approaches to boys and girls, and how this in turn contributes to shaping a child's behaviour. Similarly, a large body of data exists comparing the differences in motor abilities, growth rate and cognitive development of the sexes. However, until recently, the exact causes of these differences remained either unexplained or were attributed to the environment, both social and physical.
While environmental factors do have some influence, new research is beginning to reveal that differential pre-natal exposure to androgens (the hormones responsible for the masculinisation of the foetus) may make a significant contribution to subsequent sex-related variations in humans.
SCIENTIFIC EXPLANATION
From around seven weeks gestation onwards, a male foetus' testes will produce hormones. This results in significant differences in the levels of testosterone to which boy and girl foetuses are exposed. Androgen receptors exist in certain parts of the brain, and these receptors have an effect on how the brain functions and becomes progressively structured.
Variations in exposure to androgens during prenatal life plays a role in brain development, thereby accounting in part for some of the general differences between males - who are exposed to large concentrations of androgens - and females, who are not.
In order to understand how prenatal hormonal exposure affects development, scientists have examined gender-related play behaviour and toy preference. We know that boys and girls engage in markedly different types of play, with boys preferring the more physical, rough and tumble activities, and girls leaning more toward creative, imaginative and nurturing play.
Toy choices are also well documented: boys gravitate towards construction toys, and vehicles, while girls usually prefer soft animals and dolls. This all sounds highly stereotypical - and of course there are exceptions - but has been found the world over.
In order to gauge the role of hormone exposure on early play behaviour and toy choices, researchers have studied atypical female children who were exposed to unusually high levels of androgens before birth, due to genetic disorders. It turns out that girls suffering from congenital adrenal hyperplasia (CAH), and who as a result received high levels of testosterone in the womb, have a strong tendency to opt for male-typical play and show less female-typical behaviour in their choices of toys and games.
Is this due to parental influences because their CAH daughters look more masculine? The answer is no. Research shows that parents of girls with CAH actually encourage feminine behaviour to compensate for the condition. So the differences must, in part, be due to the early hormonal over-exposure.
Other research has assessed testosterone concentrations in pregnant mothers' blood and amniotic samples to see whether correlations exist between levels of testosterone in normally developing foetuses and later behaviour. The findings reveal that higher levels of androgens in utero are linked to increased male-typical behaviour in childhood in both girls and boys.
Without a doubt, parental behaviour, nurture and social influences play a significant role in how children develop. But there are clearly fundamental prenatal influences at work too.
In order to take human parental behaviour out of the equation, researchers turned to non-human primates and studied primate play, offering different types of toys to male and female vervet and rhesus monkeys. The findings support the theory that there are forces other than nurture at work in sex-related toy choices: just like human children, male monkeys tended to prefer wheeled toys, while the female monkeys generally opted for soft, cuddly toys. Since monkeys are not socialised into gender-specific choices of human toys, there must be other, more basic reasons for these preferences.
So, what is it about cars versus dolls that gives rise to these gender-related inclinations? Perhaps shape or colour play an important role: boys' toys tend to be angular and often have primary colours like blue, whereas girls' toys tend to be rounded, often in pale shades like pink. An investigation of these hypotheses examined 12- to 24-month-olds' attention biases towards different toys, colours and shapes. The findings were clear-cut: the infants showed the anticipated sex differences in object preferences, with girls looking longer than boys at dolls and boys looking longer than girls at cars. However, there were no sex differences in colour or shape preferences: both boys and girls looked equally long at round or angular toys, coloured pink or blue. Thus, sex-typed colour/shape preferences do not seem to underlie sex-typed toy preferences.
Another possible explanation is that boys react to toys that move in space. If prenatal androgen exposure affects the structure and function of different areas of the brain, it may be that high levels of testosterone in utero lead to differences in the development of the visual system in boys, making them particularly attracted to objects that move. This would account for the differences in toy choice that occur across all cultures.
MOTOR AND COGNITIVE ABILITIES
Prenatal testosterone levels have also been related to motor abilities that show sex differences. One study found that females with CAH had greater male-typical accuracy in throwing balls and darts at targets, a result that was not explicable in terms of increased muscular strength. However, testosterone levels have not been found to have a profound influence on cognition. It thus seems that prenatal testosterone exposure has a stronger impact on motor abilities that show sex differences (targeting, fine motor performance) than on cognitive abilities such as the verbal fluency in females and the spatial proficiencies of males. But clearly, further research is needed.
DIFFERENTIATION IN BRAINS OVER TIME
Human behaviour is controlled by the neurons in the brain, so the mechanisms responsible for sex-specific behavioural tendencies must at least in part reflect dissimilarities in the brains of the two sexes. Extensive research already documents many differences in male and female brains, in terms of size and volume, as well as how certain neural networks respond to stimuli.
Whereas the brains of males are larger than those of females, female brains ultimately have greater cortical thickness. But these differences are not easy to relate directly to sex-differentiated behaviour. One area that has received a lot of attention is the human visual cortex.
As mentioned above, research shows that baby boys are particularly drawn to toys that move. This may be the result of early exposure to androgens affecting the development of the networks in the brain responsible for motion detection. More studies are obviously required to substantiate such links, but they point to the existence of interesting prenatal foundations of behaviours that have, up to now, been regarded as socially rather than biologically mediated.
Mother and daughter
Annette Karmiloff-Smith studied in Geneva with Jean Piaget, where she completed her doctorate, and is now a Professorial Research Fellow at the Centre for Brain and Cognitive Development, Birkbeck, University of London
Kyra Karmiloff has a BSc in Anthro-pology and an MSc in Psychological Research Methods 
from University College London and was a research assistant at the Centre for Studies in Language at Cambridge University.
She is a novelist and co-author with her mother of three books on child development and on language acquisition
FURTHER READING
- - Auyeung, B et al (2009) 'Fetal testosterone predicts sexually differentiated childhood behavior in girls and in boys', Psychological Science, 20, 144-148
- - Fagot, BI (1978) 'The influence of sex of child on parental reactions to toddler children', Child Development, 49, 459-465
- - Golombok, S et al (2008) 'Developmental trajectories of sex-typed behavior in boys and girls: a longitudinal general population study aged 2.5-8 years', Child Development, 79, 1583-1593
- - Hassett, JM (2008) 'Sex differences in rhesus monkey toy preferences parallel those of children', Hormonal Behavior, 54, 359-364
- - Hines, M (2010) 'Sex-related variation in human behavior and the brain', Trends in Cognitive Sciences, 14(10), 448-456
- - Martin, CL et al (2002) 'Cognitive theories of early gender development', Psychological Bulletin, 128, 903-933
- - Pasterski, VL et al (2005) 'Prenatal hormones and postnatal socialization by parents as determinants of male-typical toy play in girls with congenital adrenal hyperplasia', Child Development, 76, 264-278
- - Reinius, B & Jazin, E (2009) 'Prenatal sex differences in the human brain', Molecular Psychiatry, 14, 988-989