The key to raising a lot of money for charity may be beauty.
Research has shown men give more money after seeing that others have donated large amounts and to beautiful female fundraisers.
Scientists from University College London (UCL) and the University of Bristol, who conducted the study, believe this response is unlikely to be conscious and could have an evolutionary function.
Theories predict that generous actions can signal hidden qualities - such as wealth or desirable personality attributes - to potential partners.
The study, published on Friday in Current Biology, found people on average donate about £10 more after seeing other large donations.
But when large amounts are made by men to attractive female fundraisers, subsequent donations from other men increase by a further £28 on average.
Co-author Dr Nichola Raihani from UCL (Life Sciences) said: "We looked at why people behave generously in real-world situations, even when there is no obvious benefit to them in doing so.
"We found a remarkably strong response with men competing to advertise generosity to attractive women, but didn't see women reacting in a similar way, showing competitive helping is more a male than female trait."
The researchers reviewed 2,561 fundraising pages from the 2014 London Marathon and found 668 which met the study criteria.
Each needed to include an image of the fundraiser whose gender was identified and attractiveness verified independently.
The pages also had to feature large donations from people who could be assigned a gender to measure the responses of subsequent donors.
A large donation was defined as double the mean donation on the page and at least £50 - it was typically around £100.
The team calculated the average donation using up to 10 donations before a large sum was given.
The responses of up to 15 donors following that donation were then studied in 12 categories defined by the gender and attractiveness of the fundraiser and the gender of the donor.
Each fundraising page was shown to four independent reviewers who rated how attractive they found the fundraiser on a scale of 0-10, with 10 being "extremely attractive".
Both men and women fundraisers who were smiling were perceived as more attractive than those who were not and received more donations.
Dr Raihani added: "It's fascinating that evolutionary biology can offer insights into human behaviour even in the modern world.
"People are really generous and their reasons for giving to charity are generally not self-serving but it doesn't preclude their motives from having evolved to benefit them in some way.
"Take eating for example, our primary drive is to dispel the feeling of hunger, which is pleasurable, but the evolutionary purpose is to make sure we don't starve and die.
"Generous behaviours can be seen in a similar way - the motivation for performing them doesn't have to be the same as the evolutionary function."
Professor Sarah Smith, co-author from the University of Bristol, said the study could have practical implications for future fundraising.
"Fundraising pages provide a fascinating real-life laboratory for looking at charity donations," she said.
"Previously, we saw how donors responded to how much other people had given. Now we see that the response depends - albeit sub-consciously - on the fundraiser's attractiveness.
"On a practical level, there are implications for how fundraisers can raise more money for charities.
"To London marathon fundraisers, I would say get your generous friends to donate early and make sure you put a good picture up, preferably one in which you are smiling."
Competitive Helping In Online Giving is published in Current Biology.