Sex (male or female) pertains to biological parameters: chromosomes, xx (biological female)or xy (biological male, hormone profile, eg: testosterone, males typically produce more of this, progesterone and estrogen. Show the child a picture of either a boy or a girl in the centre of the screen. In the congruent condition: they need to decide if the picture matches either picture on left or right (male typical toy or female typical toy pictures: measure avg response time. Sex and gender development: evolution, genetics, endocrinology. Summarizes known sex/gender differences: cites approx 18 00 studies, 30 000 citations (10% on human animal, divided into 6 developmental periods: Infant: toddler, early childhood, adolescent, adult, divided into 7 world regions. Complexities: sex / gender differences are often contentious: claims of sex differences are often contentious, such claims might be viewed as challenging political momentum toward gender equality.