As a response to the argument: "I smack my kids so they won't end up as criminals" - it would be interesting to do a comparative study to see if the prison population (particularly violent offenders) were more or less likely to have been smacked [as children] than the average person.

I've had occasion to meet more than my share of nasty types (for reasons I won't go into) and my gut feeling is that you'd be hard pressed to find an ex-convict that wasn't physically disciplined during their childhood.

I realise that any results of such a study could be challenged by the 'chicken or the egg' argument (i.e. convicts were only smacked more often as children because they demonstrated early criminal tendencies) but if my hunch is right, it would suggest that smacking is unnecessary, and if it turns out that convicts were actually less likely to have been smacked, it would suggest that smacking really does work.