Every so often, I see a newspaper article which states that the wealthiest people in my country make less substantial contributions to philanthropy (relative to their wealth) than do their American counterparts. Invariably, our higher taxes are cited as a reason why this is so, supposedly leading some to form the opinion that they are already giving their fair share to the less fortunate. If this is a significant consideration, it would be reasonable to also expect a change in the rate of philanthropic contributions when a country alters the amount of tax it collects from its wealthy.

I'm interested in whether this is so in the US, given that it's long had a culture of philanthropy by the wealthy. Has such a relationship with taxes been observed there?

I have no particular definition of "wealthy" in mind and am open to any that allows for comparison of data from different periods.