We often pledge money out of the goodness of our hearts. But there are other motives driving us to open up our wallets, ones that evolved over thousands of years and that we don’t even think about when we’re giving online.

Like when men, without even realizing it, try to impress good-looking women.

That’s the likely explanation behind the findings from a new study into the differences between how men and women donate to online fundraising pages. Men tend to pledge more money when they see other men donating large sums, especially when the fundraiser is an attractive woman, found the study, which was published in the journal Current Biology on Thursday.

Researchers say men may be signaling to women that they are kindhearted. Think of it like a peacock’s feathers, except in dollars and cents.

Although the men may be acting out of altruism, “one benefit might be, in doing this, you get to signal to members of the opposite sex that you are a worthy individual to be chosen for partnership,” said study co-author Nichola Raihani of University College London’s genetics department. “But it’s not enough to signal to members of the opposite sex; you also have to take into account what your competitors are doing and make sure your signal is at least as salient or competitive as [your] competitor.”

Raihani and co-author Sarah Smith, an economics professor at the University of Bristol, combed through thousands of personal fundraising pages posted by 2014 London marathon runners. They zeroed in on 668 that met the study’s criteria, such as including a photo and a list of donors who had genders listed. An independent panel of four reviewed every profile picture to rank its attractiveness on a one-to-10 scale.

The researchers found that the average donation across the pages ran about 50 British pounds, or about $75 U.S. If a recent donor gave double that amount, both men and women would then give about 10 British pounds more than the average of 50.

But men would give four times more when the fundraiser was an attractive woman and the most recent big donor was a man.

The researchers looked to see if women did the same when the fundraising page belonged to a good-looking man, and other donors were women. They didn’t. “Female donors do not compete in this way,” the authors found.

Smith said they looked at various combinations of donors and fundraisers. “We didn’t set out to go after the men but it turned it out it was only the men” showing this competitive helping behavior.

Her earlier research showed that people use previous donations as a guide in determining how much to donate online. In this new study, men and women gave more when a previous donation was large. But more money came in when men saw an attractive women asking.

The authors caution not to think of men in the study as calculating and manipulative givers.

More likely, evolution has pushed men in such a way that behaviors that would more likely result in mating also end up making them feel good. It’s like eating because you are hungry and not because you’re worried about dying, even though hunger evolved as a way to avoid starving. “It’s kind of like subconscious evolved psychology,” Raihani said.

Smith added: “A lot of work recognizes that males are kind of competitive, but I think it’s interesting to think this competition may not be around ‘macho’ behaviors. They’re also interested in demonstrating, subconsciously, their more generous characteristics.”

Plus, it’s better than trying to impress women via push-up competitions.

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