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.
Even in girls schools where corporal punishment was in use, or co-educational schools where it was used with girls, it was far more likely that it was seen as a last resort to be tried only in cases where other methods weren’t working. When they worked (as they normally did) you didn’t need to try it. In boys schools, the cane was often used much earlier on, and because it worked so often, schools tried other methods only when it had failed. Mid level (or sometimes first response) strategy with boys – last resort with girls.