While an anti-smacking bill was passed in New Zealand in 2007, a new study has revealed the long term damage that could be caused by smacking a child. The study's lead researcher says "smacking is never necessary".
Elizabeth Gershoff was part of a team made up of researchers from the University of Texas and the University of Michigan who looked into the risks associated with physically disciplining a child.
They used five decades of research involving more than 160,000 children to form what they call a "meta-analysis" - an average of all the research to date.
Their conclusions, published in the Journal of Family Psychology, show that smacking is associated with several unintended outcomes. They include mental health problems, lower cognitive ability and a risk of accepting physical abuse as a norm later in life.
Dr Gershoff told news.com.au roughly 80 per cent of parents around the world use smacking to discipline their children. She said they do so for the right reasons but often the outcomes are negative.