When men donate to charity it's not so much the giving that counts but the desire to compete with other men for the attentions of attractive women, according to a study of charitable donations.

Men will give significantly more money to charity if they see that other men have already given large amounts and when the person asking for the money is an attractive woman, researchers have discovered.

Women, on the other hand, appear to be unaffected by the looks of the male fundraisers, which has led scientists to believe that there is a biological difference between the sexes when it comes to the motivation behind donations.

The findings of the study have put a new slant on online fundraising through charitable websites such as JustGiving, which has produced its own complex etiquette surrounding how much to donate, and whether to do it anonymously.

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The study analysed nearly 700 fundraising webpages from the London 2014 marathon, where the picture of the fundraiser was published, and their gender was identified so that their attractiveness could be verified independently by four reviewers of the opposite sex using a 10-point score.

In each case, the donations were posted in sequence, along with the name of the donor (unless they had opted for anonymity) which other donors were then able to see before deciding on their own donation.

"This creates a potential tournament in which donors may compete by responding to how much others have given," said Professor Sarah Smith of the University of Bristol, who co-authored the study published in the online journal Current Biology.

"Fundraising pages provide a fascinating real-life laboratory for looking at charity donations... now we see that the response depends, albeit subconsciously, on the fundraiser's attractiveness."

The analysis showed that people on average gave about £10 (NZ$20) more after seeing other peoples' large donations, but when large donations were made by men to attractive women, subsequent donations from men increased a further £28 (NZ$50) on average.

"We don't think that males are seeing large donations from other males to attractive women, and thinking 'Yeah, I'll give more than him because she will find me more attractive'," said Nichola Raihani of University College London, the lead author of the study.

"I think it's more likely that humans have an evolved psychology that motivates us to behave in ways that would have been, on average, adaptive in our evolutionary past, and may still be now," Dr Raihani said.

The scientists suggest that men are unwittingly displaying signals of their generosity and wealth, which in evolutionary terms are perceived by women as attractive qualities for a potential sexual partner. Men, on the other hand, are more influenced by physical attractiveness.

"Men put a higher emphasis on signals of fertility such as youth and hip-to-waist ratio, while women tend to put greater stress on wealth and social status," Dr Raihani said. "One way that men can signal these resources is to make an ostentatious display of wealth, such as making big donations."

Researchers also found that bigger donations were made to fundraisers whose pictures showed them smiling.