Being altruistic - helping others without thought of reward - is particularly attractive to women, research finds.
But both men and women find those who are altruistic more attractive.
The results come from three studies including over 1,000 people.
People were asked about the behaviours they looked for in a mate.
Some of the suggested behaviours included donating blood and volunteering in a local hospital.
Women were more keen on these altruistic traits in a potential partner.
Dr Tim Phillips, the study's first author, explained that humans were not the only species to display altruism:
"Evolutionary theory predicts competition between individuals and yet we see many examples in nature of individuals disadvantaging themselves to help others.In a subsequent study, researchers asked people in couples how much they appreciated altruistic behaviour.
In humans, particularly, we see individuals prepared to put themselves at considerable risk to help individuals they do not know for no obvious reward."
Those that preferred altruistic traits had partners who displayed these more.
Dr Phillips said:
"For many years the standard explanation for altruistic behaviour towards non-relatives has been based on reciprocity and reputation - a version of 'you scratch my back and I'll scratch yours'.Dr Tom Reader, who co-authored the study, said:
I believe we need to look elsewhere to understand the roots of human altruism.
The expansion of the human brain would have greatly increased the cost of raising children so it would have been important for our ancestors to choose mates both willing and able to be good, long-term parents.
Displays of altruism could well have provided accurate clues to this and genes linked to altruism would have been favoured as a result."
"Sexual preferences have enormous potential to shape the evolution of animal behaviour.The study was published in the British Journal of Psychology (Phillips et al., 2008).
Humans are clearly not an exception: sex may have a crucial role in explaining what are our most biologically interesting and unusual habits."
Comment: If people are attracted to altruism, natural selection would dictate that generations to come would be more generally altruistic than the present ones. That would be good news, for a change. But it is probably not as simple as that. Free will is real, and that means that the future is always open.