We donate to charities for lots of reasons: because we're generally magnanimous people, because we care deeply about certain issues or because it's the only way to get Meg to stop talking about the plight of the endangered proboscis monkey.
And for men, there may be another force at play: a subconscious desire to impress the ladies.
Researchers in the United Kingdom reviewed thousands of online donation pages from the 2014 Virgin London Marathon. Runners participating in the marathon usually put up a fundraising page where they can raise money for a charity of their choice. And donations are made publicly, so the researchers could keep track of donors' names and how much they contributed.
It turned out that everyone was a little competitive on online donation platforms: people on average gave about £10 more after seeing others' large donations.
But men donating to attractive female fundraisers were extra competitive. They contributed £28 more than the last guy, on average.
"The results were quite surprising, actually, in that they were completely in line with the theory that men are hardwired to act competitively in this way," says Sarah Smith, an economist at the University of Bristol and a one of the researchers behind the study.
The researchers came to this conclusion by first having independent reviewers rate the attractiveness of each fundraiser's profile photo. They then looked at how much people donated to each fundraiser. When someone donated £50 or more, the researchers studied how subsequent donors reacted.
Attractive fundraisers raised more money, as did those whose profiles featured nice smiles. That worked for both male and female fundraisers. "Maybe not everybody can be the most attractive, but everybody can give a smile," Smith says. "That's something to keep in mind the next time you're trying to raise money."
This doesn't mean that people who donate to charities don't have noble intentions, Smith notes. People usually decide to donate because they care about a cause. But this subconscious competitiveness may be subtly influencing how much they choose to donate.
It's also worth noting that the researchers weren't able to account for the donors' sexual preferences or relationship status. "So we just don't know how being married or being attracted to other men would affect the competitiveness," Smith says. "But I think it's safe to assume if we excluded men have no incentive to compete for the fundraiser, we probably would have seen even more competiveness."
"This sort of thing happens in the animal kingdom all the time," says Nichola Raihani, an evolutionary biologist at University College London who worked with Smith on the paper. "The classic example would be the peacock. The male is so showy — trying to impress with his huge plume. And the female really holds all the cards. It's the same thing here, in the context of male donors."
By donating to charities, men signal that they're caring and generous, as well as wealthy, Raihani says. "We found that women aren't competitive with each other in the same way in this context," she notes. "And that may be because men and women prioritize different things when evaluating potential partners. Studies show that women are more likely to prioritize cues that the man can be a good provider."
Culture and hormones can help explain the behavior as well, notes David Geary, an evolutionary psychologist at the University of Missouri who wasn't involved with the study, which was published Thursday in Current Biology.
"Basic competitiveness is evolved and related in part to testosterone," Geary said in an email. But how the competitiveness is expressed depends on context. "You'd only find competitive donations in wealthy societies," he adds. Other cultures may define success in other ways.