In one, scientists dressed newborns in gender-neutral clothes and misled adults about their sex. The adults described the "boys" (actually girls) as angry or distressed more often than did adults who thought they were observing girls, and described the "girls" (actually boys) as happy and socially engaged more than adults who knew the babies were boys. Dozens of such disguised-gender experiments have shown that adults perceive baby boys and girls differently, seeing identical behavior through a gender-tinted lens. 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 daughter's physical activity. How we perceive children-sociable or remote, physically bold or reticent-shapes 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 behavior and brains-the result not of innate and inborn nature but of nurture.

Table 1 Statistical significance of differences between right and left cerebral cortical thickness in male and female rats ( S=statistically significant; NS=nonstatistically significant)           Cortical Areas           Age (days) N Frontal   Parietal     Occipital         10 4 3 2 18 17 18A   6 13 S S S NS S S S   14 17 S S S NS S S S Males 20 15 S S S NS S S NS   90 15 NS S S NS NS S NS   185 15 S NS S S S S S   400 15 S NS S S S S NS   900