How could they know it “hardly ever did any harm”? Boys were more likely to be subject to CP; in adult life, men are more likely to suffer depression and commit suicide. The obvious (although possibly incorrect) conclusion is the CP is harmful to boys.
Well, no, it’s not an obvious conclusion, though it is definitely an incorrect one.
See, if this was the case, the suicide rate for men who’d experienced normative corporal punishment as children would be higher than the rate for those who had not. It isn’t. It’s lower – although not significantly enough lower, you could claim corporal punishment reduces suicide risk either.