Evolution selects for child-rearing practices that ensure that offspring will live to reproduce -- not necessarily child practices that are optimal for the offsprings mental well being.
Furthermore, there may have been a time in the past where evolutionarily selected child-rearing practices that today we would consider abhorrent were actually necessarily to ensure survival. In a brutal world you need to be brutal to survive and one way to ensure that your offspring is brutal is to well... brutalize them.
Our western world is far less brutal a place than it was even fifty years ago and that may necessitate developing child-rearing methods that are counter to our evolutionary ingrained instincts.