Other research has asked people to imagine what they would do for those closest to them - but this is the first study to put it to a physical test.
Psychologists from six institutions across the UK, carried out three separate studies over two-decades, each of which asked participants to impose pain upon themselves from a ski-training exercise, in exchange for a reward given to a particular person.
Participants were told they were "working" for a particular person, such as their sister, cousin or best friend - but they were not told that the researchers wanted to see how much they would do for an individual until their part in the study was complete.
England city-dwellers and Zulus living in rural South Africa took part.
The participants squatted against a wall as if sitting on a chair.
The longer they maintained the posture, the greater reward they earned.
But the longer they stayed there, the more pain they had to bear - as the body's weight was born by the thigh muscles.
After about 100 seconds, the pain increases significantly as the thighs bear the whole weight of the body.
In all the groups studied, people held the position longer and so endured more pain the closer they were to the beneficiary.http://news.bbc.co.uk/2/hi/health/6591829.stm