scienceblogs.com/cognitivedaily/2009/06/10/six-month-olds-prefer-differen/
www.rps.psu.edu/probing/gender.html
For every opinion there is a polar opinion.
The differences are obvious, even at a young age - its due to hormones.
Nature will always triumph over nurture in the end.
The second article is on the CAH hormone, a study of girls born with enlarged clitoris and fused labia, surgery to correct at a young age and also given corrective hormones to bring the CAH levels down to a standard level. All showed male traits and were more aggressive, 'boy like' if you will. Those girls were gender stereotyped into female, but they were still exhibiting the sterotypical male behaviour of enhanced physical, spatial, mathematical skills.
Now why would that be? Hormones. Babies have hormones too. As shown by the girls in link 2, who had abnormally high CAH and were medically engineered and sterotyped into traditional females.
You can try to make your little boy like pink spangly things all you like, but by 18 months he'll have a natural gravitas towards trucks, and vice versa. Well, 99% of the time they will define their own gender - and I know there are going to be anecdotal stories about boys liking dolls and girls liking cars because there is always someone who is different.
I cant find the study - a Scandanvian one - where babies were given gender neutral toys and clothes for 2 years, then let loose in a room with gender specific toys. The girls gravitated towards dolls, houses, prams etc and the boys towards lego and cars.