People may think they suffer a lot for their relatives - now scientists say a 20-year study shows that we do.
People will suffer for their relatives
Researchers from around the UK have been looking at how willing 150 people were to bear physical pain for others.
People do more to help a relative, even at a cost, because it helps the continuation of the genes, the British Journal of Psychology study says.
A psychology expert said being a blood relation had a significant, but not comprehensive, effect on actions.
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.
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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.
George Fieldman, who started work on the project 20 years ago at Oxford and is now at Buckingham Chilterns University College, said: "We found people will do more for their loved ones - irrespective of whether they like them or not.
"And the closer you're related to someone, the more pain you will go through."
He added: "It shows that genes count. You might be doing something for, say, your beloved children, but you're also doing something which is led by an interest in the immortality of your genes."
Dr Fieldman said people did things for the friends because they hoped for "reciprocal" benefits, which could help them and their families.
And Dr James Thompson, a senior lecturer in psychology at University College London, said: "Like vampire bats, we favour our own. Bats regurgitate blood for their relatives; humans bear pain, do favours and give money.
"However, even vampire bats give some blood to their greater community; and so do humans, through blood donation, general helpfulness and donations to charity.
"This excellent study shows that blood relatedness has a very significant impact on human altruism, though it does not explain it all.
"The greater question is whether we should accept that blood relatedness will always dominate our behaviour, in which case family, extended family, tribe and clan will have to be the basis for social policy."