Smacking your child makes them behave more badly, a new study has warned.
Children who are smacked before they are five have more behavioural problems between the ages of six and eight than those spared the rod.
Those who were physically punished argued, fought, got angry, acted impulsively and disturbed ongoing activities more than those who were not.
And the more smacks the children got, the worse their behaviour.
The findings suggested physically disciplining young children may have the opposite effect to what was intended.
Since 2004 it has been illegal in the UK for parents or carers to smack their child, except where this amounts to "reasonable punishment."
Associate Professor Dr Elizabeth Gershoff of the University of Texas at Austin said: "Our findings suggest that spanking is not an effective technique and actually makes children's behaviour worse not better."
Historically, trying to determine whether smacking actually causes children to develop behaviour problems has been difficult because researchers cannot ethically conduct experiments that randomly assign parents to hit their children or not.
Prof Gershoff said: "Parents spank for many reasons, such as their educational or cultural background or how difficult their children's behaviour is.
"These same reasons, which we call selection factors, can also predict children's behaviour problems, making it difficult to determine whether spanking is in fact the cause of behaviour problems.
"We realised that the statistical method of propensity score matching could help us get as close to an experiment as possible."
The study examined data from 12,112 children who participated in the nationally representative Early Childhood Longitudinal Study.
When the children were five their parents reported how many times they had spanked their child in the past week
The researchers classified any child whose parent provided a number other than zero as having been physically disciplined.
The researchers then matched children who had been spanked with those who hadn't according to 38 child and family-related characteristics.
These included the child's age, gender, overall health, and behaviour problems at five; the parent's education, age, and marital status; the family socioeconomic status and household size; and factors related to parenting quality and conflict in the home.
Pairing the children in this way yielded two groups of children whose main difference was whether their parents had spanked them, effectively accounting for other factors that could plausibly influence the behaviour of both parent and child.
This allowed researchers to approximate the random assignment of participants to groups, a hallmark of experimental design.
The child's behaviour was then gauged by school reports when they were five, six and eight.
The teacher's report noted the frequency with which the children argued, fought, got angry, acted impulsively, and disturbed ongoing activities.
Children who had been spanked at the age of five showed greater increases in behaviour problems by age six and also by age eight when compared with children who had never been spanked.
A similar analysis found children who were frequent smacked behaved a lot worse than those who were smacked less frequently.
Prof Gershoff concluded: "The fact that knowing whether a child had ever been spanked was enough to predict their levels of behaviour problems years later was a bit surprising
"It suggests that spanking at any frequency is potentially harmful to children.
"Although dozens of studies have linked early spanking with later child behaviour problems, this is the first to do so with a statistical method that approximates an experiment."
The study was published in Psychological Science, a journal of the Association for Psychological Science.