The old adage that loving couples feel each other's pain has been proved to be right.
Men and women reported similar levels of pain
A team from University College London found that merely knowing your loved one is suffering is enough to activate pain centres in the brain.
The study, publised in Science, is based on 16 couples.
The researchers measured activity in the brain while painful stimulation was applied to the woman's right hand - or to the right hand of her partner.
They found that watching the pain suffered by a partner was enough to trigger activity in some - but not all - of the brain's pain centres.
Simply watching pain being inflicted was enough to trigger an empathetic response, making the witness aroused and emotional.
In effect, the brain flinches when it sees someone in pain in the same way that we might wince.
This is the same kind of response triggered by anticipating that you are about to be hurt.
Women who reported the strongest feelings of empathy experienced the greatest brain activity.
However, simply witnessing pain was not enough to stimulate the pain centres that register the actual physical sensation of hurt.
Lead researcher Dr Tani Singer said: "The results suggest that we use emotional representations reflecting our own subjective feeling states to understand the feelings of others.
"Probably, our ability to empathize has evolved from a system for representing our own internal bodily states.
Dr Singer said the research was the first time that brain imaging technology had been used to show how we empathise with the plight of others.
She said: "Our human capacity to 'tune in' to others when exposed to their feelings may explain why we do not always behave selfishly in human interactions but instead engage in altruistic, helping behaviour."