Spanner in the works for Sue Bradford

Fresh New Zealand research, unlike other studies, has compared outcomes for children who were smacked only with the open hand as opposed to having experienced other forms of physical discipline.

The project appeared to be the world's first long-term study to separate people who had merely been smacked with an open hand, lead author of the physical punishment part of the Dunedin study psychologist Jane Millichamp said.

"Study members in the 'smacking only' category of punishment appeared to be particularly high-functioning and achieving members of society," she said.

In terms of aggression, substance abuse, adult convictions and school achievement, this group had "similar or even slightly better outcomes" than those who were not smacked.

Dr Millichamp said the problem with a lot of studies was that they lumped a range of physical punishments together. She said she had not found any evidence that an occasional mild smack with an open hand on the clothed behind or the leg or hand was harmful or instilled violence in children.

Dr Millichamp acknowledged this was not a popular thing to say.