Physical punishment does nothing to improve children's behaviour, a new review has concluded - the opposite is true, it turns out.
Yet most countries around the world still allow it, despite smacking being "increasingly seen as a form of violence".
"Parents hit their children because they think doing so will improve their behavior," said Elizabeth Gershoff of the University of Texas, senior author of the new paper, published in prestigious medical journal The Lancet.
"Unfortunately for parents who hit, our research found clear and compelling evidence that physical punishment does not improve children's behavior and instead makes it worse."
The international team of researchers looked at 69 previous studies into physical punishment - excluding violence bad enough to be considered child abuse such as punching, choking, using weapons or burning them - and found no long-term association with better behaviour.
The more frequently physical punishment was used, the worse their behaviour got - increasing the risk of tantrums and defiant behaviour, as well as increasing the risk they'd be subjected to more severe violence.
Even parents who were usually warm and kind to their children reported the same outcomes.
"There is no evidence that physical punishment is good for children," said Dr Gershoff. "All the evidence indicates that physical punishment is harmful to children's development and wellbeing."
So far 62 countries have anti-smacking laws on the books. New Zealand did so in 2008, by removing 'correction' as a form of defence in child abuse prosecutions. The first was Sweden in 1979 according to the NGO End Violence Against Children, followed by Finland, Norway and a couple of dozen others before New Zealand.
The United Nations 2006 Convention on the Right of the Child says children have the right to freedom and protection from corporal punishment.
End Violence Against Children says only 13 percent of the world's children live in countries where they're fully legally protected from physical violence. In 31 countries, kids can still be legally whipped, flogged and caned. Even in the US. corporal punishment is legal in schools in 19 states.
"Given the strength of the evidence that physical punishment has the potential to cause harm to children, policymakers have a responsibility to protect children and legislate to end the use of physical punishment in all settings," said Anja Heilmann of the University College London, lead author of the paper.
A New Zealand study published in April found smacking is still "fairly common", particularly against toddlers, despite the law and growing evidence against it.