Tis the season for giving… blood? U.S. hospitals struggle to maintain blood supplies, but there’s one simple way to increase donations from the public, according to a study co-authored by Johns Hopkins business professor Mario Macis.

Why is a business professor studying blood supply? Mostly because it turns out to be a clear case of supply and demand.  Advertising a $5 gift card reward for donors increased the likelihood of giving by 26 percent; a $10 gift card doubled that, and a $15 gift card caused a rise of 72 percent.  (These were all donors who had donated before.)

“Because of the big gap between supply and demand, as an economist I wanted to know whether economic incentives could be introduced into this market that would stimulate people to donate blood,” Macis said. “We learned that this was indeed the case.”

But getting rewards for donating blood (or other body parts) is considered questionable by some.  Getting cash for blood, organs, or body parts is illegal.  But as of December 1, bone marrow donors are allowed to receive compensation — whereas before, such arrangements could’ve gotten you five years in prison.

Macis also found that surprise rewards seemed to backfire.  When donors who didn’t know about the gift card program were given one after they gave blood, their rate of subsequent donations actually went down — perhaps because “these donors might have seen the unexpected gifts as an affront to their intrinsic motivation to commit a social-minded act.”