JUST two smacks to your three-year-old can lead to a violent primary schooler, new international research has shown.
Infrequent use of spanking defined as one or two times a month doubled the risk of aggression in five-year-olds.
It is the first time research has shown increased aggression could not be explained away by other factors, such as high stress in the mother or maltreatment or neglect of the child.
Child psychologist Bob Montgomery has called for a public health campaign to cut out smacking.
Dr Montgomery believes this would lead to a reduction in schoolyard and street violence.
Prevention starts in the home and skills learnt from parenting programs such as the Triple P parenting course could alter lives, Dr Montgomery said.