Scientists have come up with evidence to suggest parents may smack their children much harder than they intend to.
Researchers say parents may use more force than they intend to
They say this is because everyone - from infants to professional boxers - may underestimate the power of their physical actions.
The scientists believe the phenomenon occurs because of the way the brain is programmed.
Dr Sukhwinder Singh Shergill and other Wellcome Trust researchers at University College, London, assessed six pairs of people in "tit-for-tat" situations.
They applied a fixed force to the finger of one member of each pair. These volunteers were in turn then asked to apply the same force to the second member in each pair.
The second group of volunteers were then asked to apply the same force to the first group again. The cycle was repeated eight times.
By the end, the researchers found that the force being applied was 14 times greater than that originally applied.
They found that the volunteers increased the amount of force they were using at each turn by at least a third. In some cases, the force increased by 50%.
But when participants were asked to apply the same force remotely, by operating a joystick, they were surprisingly accurate.
The researchers said the findings may explain why children who have been fighting say "they hit me harder".
"The results showed that to get the same feeling of force, you need to exert more force," said Dr Shergill.
"It is well known that a system in the brain de-emphasises the effects of our own actions, but this is the first time it has been measured."
Dr Shergill suggested the findings could have implications for a wide-range of people, including parents.
"It may not be possible for parents to accurately judge the force they apply when they smack their children and this experiment would suggest that they will smack harder than they think or intend," he said.
The researchers believe that the phenomenon is a result of mixed messages in the brain.
When a person makes a movement they send a signal to a specific area of the brain, telling it what to expect.
This forewarning causes a person to apply more force than they intend to.
They believe the same process is involved when a person tries to tickle themselves.
It is almost impossible to tickle yourself, largely because the brain knows what to expect.
However, it is possible so long as the brain is caught unaware. This usually only occurs when someone else tries to tickle you.
The researchers are planning to carry out further tests to try to identify the area of the brain responsible for this phenomenon.
However, they already believe their findings could have implications for people with schizophrenia.
Many people with schizophrenia lose their ability to tell whether they or someone else is responsible for some of their physical actions. This can include anything from moving an arm to speaking.
In many cases, they believe someone or something else has caused them to act in a certain way.
The researchers believe that this may occur because their brains are no longer able to predict the impact of their actions.
"We now hope to use imaging techniques to look at the brains of healthy subjects and those with schizophrenia, to identify which brain areas are responsible for this activity and which areas don't work if things go wrong," said Dr Shergill.
"Eventually, it may be possible to use drugs or electrical stimulation to reactivate that region."
The study is published in the journal Science.