From a recent study:
"In one experiment, mothers were asked to guess the steepness of a carpeted slope that their 11-month olds would be able to crawl. Then the children actually crawled the slope, and the difference between actual and mother-predicted angles was noted.
The results showed that both boys and girls were able to crawl the same degree of incline. However, the predictions of the mothers were correct within one degree for the boys and underestimated their daughter’s ability by nine degrees.
What this shows is that the presumption that boys are more physical causes parents to encourage their boys more in physical activities while cautioning their girls. This further translates into providing more opportunities for boys to be physical and fewer for girls.
Boys actually do develop stronger physical skills than girls. But not because of anything innate or biological, but rather because of the gender roles that the parents subconsciously projected onto their babies."
(It's a great read - there's more stuff in there about how parents unknowingly enforce the idea that boys shouldn't cry. And five great tips on simple things you can do to avoid enforcing gender roles on your kids.)