What do you mean, cultural norms?
It's true, the culture you grow up in and live in influences your posture. Your parents influence you most of all, but seeing everyone around you day in and day out living the same way is something that molds us. Also the way a culture sleeps, works and rests are body moulding. Do you sleep in a hammock? Or do you sleep on a bed? Do you sleep on tatami mats on the floor? Or some other way? If you sleep on a mattress, did you know that many cultures consider different types of mattresses preferable? Do you sit on a chair, or kneel? Are you bending over in a rice paddy, or sitting on a tractor seat? Do you have motor transportation in your area, or is everything transported by human porter? Do you work in front of a computer, or with paper files? You can see how all of these things would change your posture. It's important to pay attention to what your cultural norm is, and to look carefully at some of it's assumptions.