What's the principle of 'reciprocity' and how does it work in fundraising?

What it boils down to is that it's a basic human impulse to reciprocate a favor or a gift. You know, you get invited over for dinner, you feel like you need to have that person over to your place for dinner, right? We can't help it -- we keep score. That's reciprocity and it's good for the survival of our species.

You get, you give.

Check this out: groups that send out fundraising letters with a gift for the recipient -- often something like those preprinted return address labels -- report that the rate of positive response in the form of a donation almost DOUBLES, going from 18% to 35%.