Nowadays, we do actually have published research that backs this up, but with this as in many other cases, experienced teachers at the chalk face didn’t need research to tell them what commonsense already had.
Most of the published research claims CP is harmful for both boys and girls.
Because the research is clear, and it doesn’t take any real effort at all.
You claimed that commonsense had already told the teachers what you claim the research subsequently showed. The researchers do not make the claims you do ( in general they claim CP is harmful to both sexes) so I’d like to know why you think the policy of not caning girls was based on “commonsense”.