Another possibility is that market responds to demand. I have young children (of both genders) and other parents I talk to often lament the "gendered" nature of toys. "Why are the girls toys pink with changeable clothes and the boys ones all carrying weapons?" My response is that if there was a market for a toy like that then there probably would be a toy like that. The memo in question postulates that the biological differences may extend beyond just physical size and reproductive organs and manifest themselves in our career choices as well. It's not that big of a stretch. I don't think anyone's out there suggesting that the elementary education field is inherently sexist but it's dominated by one gender as well.