Yellow, Gender Neutral

Gender roles and steroeotypes have been imprinted on us from an early age. In class we talked about how gender roles have changed, and I would just like to note that they have improved, but still exist. New parents immediately know how to dress or decorate a nursery based on their newborns gender. Girls like pink, princesses, ponies, and purple. Boys, on the other hand like blue, baseball, football, cowboys, trucks and cars. When did these common associations become established?
I guess my main question is whether gender roles are the product of nurture or nature?
Yes, it is statistically proven that boys have a tendency to have trouble sitting still and enjoy action. This biological trait makes the sports stereotype seem to be caused by nature. However,  why is it true that most boys can sit for hours to watch a football game, whereas many girls (apparently with a higher affinity to focus and sit still for long periods of time) become bored and restless?

But then again, I am a girl. And I love watching and playing sports, as do many of my female as well as male friends do as well. Are these stereotypes all subjective? They still exist today, but they seem to be broken daily. My favorite color is yellow, not pink or girly purple. I prefer to watch a baseball game vs. the new episode of Hannah Montana, and enjoy participating in sports as well. Does this make me any less of a female?
I think not. I am very much a teenage girl. But then why do these stereotypes exist if they are so inaccurate? This makes me question the validity of any stereotype.
 Are african americans natural athletes? Are Asians technological geniuses? Some are. But the majority are not. When did these stereotypes become ingrained into society. Why is a woman expected to be the primary caregiver to her children? Why is a male expected to be tough, strong and emotionless?