Research from the 1940s until recently that I am aware of. I’m not sure what you mean by ‘those policies’ though.
The policies of not punishing girls.
Policies of not using corporal punishment were in place a lot earlier than the 1960s in some places, and weren’t in place until after that (if at all) in others. I don’t know why you’ve picked on the 1960s here as some point of relevance to this issue. Policies of not punishing girls, in my view, if they existed, were rare enough as to have very little relevance to anything.
I know some did – with very good results.
Not according to most people involved in such practices, and definitely not according to research.