Research participants were willing to donate 38% less, on average, to public broadcasting if the U.S. nonprofit offered a thank-you gift, in this case a pen, say George E. Newman and Y. Jeremy Shen of Yale University. A promised gift of a tote bag brought intended donations down 17%. A thank-you gift creates ambiguity in the donor's mind about whether the donation is supporting the charity or is a quid-pro-quo, the researchers say.

Source: The counterintuitive effects of thank-you gifts on charitable giving