Even when they are being generous, men are competitive.
British researchers examined evidence from various online fundraising sites and found that men often donate more money to a campaign run by an attractive woman—if another man has already made a large donation to the same project. (Tweet This)
The behavior provides evidence of something called "competitive altruism." Some psychologists have previously observed that people tend to act more generously if they believe they are being watched by others, for example.
In the new study, the researchers pulled data from the online fundraising platform for the London Marathon—giving them a "real world context" in which to examine the phenomenon.
Runners raising money for a charity create a profile that has personal information along with a picture. Most of the donors tend to know the fundraisers personally, and the profiles allow users to see the identities of the givers, the amounts they gave, and the order in which the donations came in.
The team selected profiles of nearly 4,500 fundraisers and had participants rate them for attractiveness. They then looked at each fundraiser's donation history, and found a few patterns.
First, both men and women donate more money to fundraisers managed by people deemed more attractive. But men also did something else women did not do: If a man made an exceptionally large donation to an attractive female fundraiser (i.e., an amount twice that of the average donation), other men would soon follow with bigger donations of their own.
Read MoreHigher education wealth gap widens
The researchers. whose study was published Thursday in the journal Current Biology, said men may be trying to signal to women that they are capable of cooperative behavior. Altruism and generosity can be somewhat perplexing from an evolutionary perspective, and some scientists think that some generous acts are in part fueled by competition.
"How we behave in relation to altruism and giving is shaped by a lot of different factors," said study co-author Sarah Smith of the University of Bristol. "So yes, people want to do good, but there are a lot of other motivations that are determining how they behave."
By trying to appear most generous, men might be signaling that they would make good partners to attractive women. They may be trying to show they are cooperative and "pro-social," attributes women tend to value highly in men.
Read MoreAt what age can you really retire?
When considering charitable giving, economists are expanding their range of possible factors beyond traditional issues like income, price or tax relief, Smith said.
"Individual preferences are still the main drivers for 'doing good,' and price matters, too, in terms of tax relief, but we have to think about social interactions in order to really understand behavior," Smith said.
The researchers believe that the behavior is likely subconscious, and that men are not aware they are competing this way.
"We don't know," Smith said, "but we were giving people the benefit of the doubt."