I wonder if it has to do with too much sitting in chairs? Prof. Galen Cranz discusses some of the effects on posture of western sitting styles in her book, "The Chair: Rethinking Culture, Body and Design". Though not scientific, she compares the common slump shouldered, curved spine posture of many westerners to the (better) posture of those who grew up without frequent chair sitting. It may be there is also a connection to lifting posture.