PINK BRAIN,BLUE BRAIN

In another study,mothers estimated how steep a slope their 11-month-olds could crawl down. Moms of boys got it right to within one degree; moms of girls underestimated what their daughters could do by nine degrees,even though there are no differences in the motor skills of infant boys and girls. But that prejudice may cause parents to unconsciously limit their daughters physical activity. How we perceive childrensociable or remote,physically bold or reticentshapes how we treat them and therefore what experiences we give them. Since life leaves footprints on the very structure and function of the brain,these various experiences produce sex differences in adult behaviour and brainsthe result not of innate and inborn nature but of nurture.

For her new book,Pink Brain,Blue Brain: How Small Differences Grow Into Troublesome GapsAnd What We Can Do About It,Eliot immersed herself in hundreds of scientific papers her bibliography runs into 46 pages. Marching through the claims,she explains that assertions of innate sex differences in the brain are either blatantly false,cherry-picked from single studies,or extrapolated from rodent research without being confirmed in people. For instance,the idea that the band of fibers connecting the right and left brain is larger in women,supposedly supporting their more holistic thinking,is based on a single 1982 study of only 14 brains.

Fifty other studies,taken together,found no such sex differencenot in adults,not in newborns. Other baseless claims: that women are hard-wired to read faces and tone of voice,to defuse conflict,and to form deep friendships; and that girls brains are wired for communication and boys for aggression. Eliots inescapable conclusion: there is little solid evidence of sex differences in childrens brains.

Yet there are differences in adults brains,and here Eliot is at her most original and persuasive: explaining how they arise from tiny sex differences in infancy. For instance,baby boys are more irritable than girls. That makes parents likely to interact less with their nonsocial sons,which could cause the sexes developmental pathways to diverge. By four months of age,boys and girls differ in how much eye contact they make,and differences in sociability,emotional expressivity,and verbal abilityall of which depend on interactions with parentsgrow throughout childhood. The message that sons are wired to be nonverbal and emotionally distant thus becomes a self-fulfilling prophecy. The sexes start out a little bit different in fussiness,says Eliot,and parents react differently to them,producing the differences seen in adults.

Those differences also arise from gender conformity. You often see the claim that toy preferencestrucks or dollsappear so early,they must be innate. But as Eliot points out,six- and 12-month-olds of both sexes prefer dolls to trucks,according to a host of studies. Children settle into sex-based play preferences only around age one,which is when they grasp which sex they are,identify strongly with it,and conform to how they see other,usually older,boys or girls behaving. Preschoolers are already aware of whats acceptable to their peers and whats not, writes Eliot. Those play preferences then snowball,producing brains with different talents.

The belief in blue brains and pink brains has real-world consequences,which is why Eliot goes after them with such vigour. It encourages parents to treat children in ways that make the claims come true,denying boys and girls their full potential. Kids rise or fall according to what we believe about them, she notes. And the belief fuels the drive for single-sex schools,which is based in part on the false claim that boy brains and girl brains process sensory information and think differently. Again,Eliot takes no prisoners in eviscerating this patently absurd claim.