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Foot binding.
The side effect of massively mistreating/torturing the majority of parents (who were actually doing parenting) had all kinds of psychological fallout that totally retarded the national psyche's interest in advancement. Something similar happened to Islam with its increasingly brutal treatment of women from the 1400s onward.
Check out the work's of Lloyd DeMause at psychohistory.com: It's decidely not for the squeamish, but it does present much convincing evidence that societal advancement flows from improvements in childrearing and not the other way around. My signature connects to a particularly relevant article.