More recent research has highlighted just how good an investment gifts can be—no matter what you’re selling. In one experiment, the economist Armin Falk had a charity send about 10,000 letters to potential donors, asking them to give money. About a third of the would-be donors received only a letter. Another third received the letter accompanied by a postcard with a colorful drawing on it—a gift, the recipients were told, “from the children of Dhaka” that could be “kept or given to others.” The final third received the letter and four postcards.

The postcards were not much of a gift—they cost pennies apiece. But they led to dramatically higher response rates. One postcard increased the response rate by 17 percent; four postcards raised the rate by 75 percent. According to Falk’s back-of-the-envelope calculation, the four-postcard solicitation improved the profitability of the direct-mail campaign by about 55 percent relative to the no-postcard solicitation (after accounting for the cost of the postcards themselves).

Other fund-raising experiments leveraging reciprocity have seen similarly impressive results.