Unfortunately, her research was... a little flawed.
Adults may define a smack as something a lot gentler than a hit, but children were clear that a smack is a hard hit that hurts both emotionally and physically.
Fear and pain may sometimes achieve short-term obedience, but in the long term these emotions are unlikely to contribute to positive behavioural outcomes or promote children’s effective learning.
Many of the children believed smacking did not work as a disciplinary tool. They said that the use of time out, having privileges removed or being grounded were far more effective means of discipline.
The children’s responses render many adults’ claims and justifications highly suspect. It is also concerning that quite large numbers of children reported adult behaviour that was in fact abusive.
You can read about this, and other research over at Halfdone (my main blog) - check out the booklet "by Fear and Fallacy" where the above quote comes from. That booklet goes into considerable detain over the flaws in this and other research, far to extensively to quote here.
The book repeatedly claims that Dobbs’ research found physical punishment could lead to harm or abuse, yet all her data established that not one of the 80 children interviewed had reported such harm or abuse. Rather than suggesting smacking leads to abuse, the data suggest exactly the opposite: most of the children she interviewed had been smacked, none of them had been abused. Clearly, if Dobbs’ data indicates anything, it is that smacking does not lead to abuse.
Dobbs argues that “children are, in many senses, the experts on family discipline.” At first this appears just loopy, but examined in the context of the book it becomes apparent Dobbs is not simply saying that because children are disciplined they are experts, but that they have the moral maturity to make judgements about the discipline given them, whereas the adults giving the discipline don’t have that moral maturity. The whole reason children need special care and moral training is that they are not experts, they are not mature, and they are not morally astute. If they are such experts they don’t need “reward” either to establish the pattern of behaviour that indicates maturity and expertise.
Claims by anti-smackers that smacking doesn't work should always be treated with suspicion, if not contempt. Yes, smacking can be done badly and hence ineffectively, but if done correctly it is a useful tool of parenting.
But the biggest evidence is the case that started all this off.
The riding crop smack only came to the attention of authorities because the kid suddenly started behaving at school and social workers asked what wonderful think the parent had done.
The fact that that (very effective) discipline was regarded by a jury of 12 men and women as "reasonable in the circumstances*" set off a media fire-storm stoked by liberal "child protection" groups.
And here we are today.
*Oh, and it's always omitted that "the circumstances" included swinging a weapon at an adult's head. I guess there's an irony in that - a law to stop reasonable actions in response to an attempted serious assault called a law against assault.