It is widely assumed that women are more charitable than men.

A new poll puts a number on it: Wealthy women give away nearly twice as much as of their wealth as their male counterparts.

In a survey due out this week, Ledbury Research for Barclay's Wealth found that women in the U.S. give an average 3.5% of their wealth to charity. The survey of 500 people with investible assets of $1 million or more found that men give an average of 1.8%

And it isn't just a U.S. phenomenon. In the U.K., women give an average of 0.8%, compared with 0.5% for men, the survey shows.

Yes, the finding confirms stereotypes. But it also has big implications for the future of philanthropy.