Nevertheless, Serbin’s group found that children’s toy preferences are firmly in place by this age, especially among boys. When the experimenters offered boys a truck or a doll, most boys chose the truck. In fact, boys preferred trucks over dolls more strongly than girls preferred dolls over trucks. That ought to be surprising if you buy into gender schema theory because 18-month-old girls were more likely than boys to be able to classify themselves and other children by gender. If gender schema theory is correct, the girls should show a stronger preference for gender-typical toys because girls this age are more likely to know that they are, in fact, girls. But the reality is just the opposite.

Another group, led by child psychologist Anne Campbell, looked at toddlers at nine months of age—and they found similar results. Nine-month-old boys strongly preferred “boy toys,” such as balls, trains, and cars. Nine-month-old girls preferred “girl toys,” such as dolls and baby carriages, although the girls’ preference was (again) not as strong as the boys’ preference. Campbell’s study is especially striking because she showed clearly that nine-month-old infants have no clue what gender they belong to. Boys and girls show gender-typical toy preferences long before they understand gender. Dr. Campbell concluded, politely, that “the impact of cognitive variables may have been overestimated.” In other words, most 18-month-old boys don’t choose to play with trucks rather than dolls because they know they’re “supposed” to. They choose trucks because they’d rather play with trucks.