Men donate generously through fundraising web sites when the fundraiser is an attractive woman, according to a new study.
Men also tend to shell out more money after seeing that other men have donated large amounts.
"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," said study co-author Nichola Raihani from the University College London (UCL).
The study found that people on average give about 10 pounds more after seeing others' large donations.
When the large donations are made by men to attractive female fundraisers, subsequent donations from other men increase by a further 28 pounds on an average.
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The scientists say this response by men is unlikely to be conscious and could have an evolutionary function as theories predict that generous actions can signal hidden qualities, such as wealth, to potential partners.
The researchers reviewed 2,561 fundraising pages from the 2014 London marathon and found 668 that met the study criteria.
For both men and women, fundraisers who were smiling were perceived to be more attractive than those who weren't and received more donations.
"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," said co-author professor Sarah Smith from the University of Bristol.
"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," Raihani concluded.
The study was published in Current Biology.