Gender roles

There is no doubt that gender roles are influenced by society, however I believe there is a biological aspect behind them as well. As I was reading my biopsych book the other day, I couldn't help but related what I was learning to out topic in AMS about gender roles. One interesting example I came across in favor of a biological explanation for gender roles involved playing behavior in primates. When given monkeys the choice to play with toys such as dolls or trucks, it was shown that female monkeys chose to play with toys such as dolls, where as the males chose to play with more active toys such as trucks. This shows gender roles to have some type of biological influence, because monkeys do not have the same socially constructed gender roles as we do. There was also another study done with male and female infants which concluded that female infants look at faces longer, such as showing a preference to look at their mother's face, where as male infants looked at mobiles longer, preferring to look at something more active. I do believe there are biological differences between males and females, and there are most definitely differences in hormones in male and female brains, as well as sexually dimorphic characteristics of the brain. But this leads me to kind of wonder what came first, the chicken or the egg. Did these biological differences in males and females cause our society to cater to differences that already existed, or did the gender roles come first which we then identified with due to our biological differences of preference?