I gather MM has not missed the target by much here, Chick or egg: if you go all the way to prebiotic soup, the Biological manu if you will, the prevailing means of increasing the number was self replication, in the chain of being able to adapt from one competing environment to the next brought about the diversity in life and sexual as well as asexual reproduction, therefore neither chicken came first nor egg, but both came at the same time, Kaboooom!!!. As far as culture and biology in creating sex roles is concerned, as MM has pointed out culture has a lot to do with it, the term MEME goes for characteristics transported/ reinforced via culture finally taking the form of a gene, as an example of it memes are seen as responsible carrier for flourishing of homosexuality in human culture, in congruent with MMs story above, In population during hunter gathers time, there used to be some men who rather stayed home with the women (because the population of women was less) to help them out in the house chores, and in due course of time as this happened generation after generation, this cultural memes adapted itself as a physical characteristics in human male population where some men with this gene opted to stay home by default because they were born with a GAY GENE, same with the gay female gene who went hunting with men earlier in time because in some population, the number of men was less. Of course for women giving birth is what forces them to adapt into gender roles but it�s the culture that can reinforce or cancel out these roles, so be careful when you say something like me run faster then women or women cook better then men. The word count/minute may not hold the same in all the cultures and is apt to change in due generation.