We assume a teacher must be female, we assume a nurse must be a woman, we assume a mechanic to be male. These are all common stereotypes. Some of it comes from experience some comes from what we were taught (heck my primary school primer taught me to read with such D.ick and Jane biases). Regardless, this comes from imprinting. Change starts with us adults though and how we choose to imprint such things on our children. I made a point when my children were young to refer to such things for instance as genderless. To my male child, "One day when you grow up to be a nurse," to my daughter, "one day when you are a firefighter," sorts of statements. I would hope as roles are changing that youth are growing up less and less imprinted with stereotypes whether based on gender, ethnicity, or whatever we adults were led to believe. This comes though straight from parents' mouths and own actions.