There are two things you can always count on: public radio pledge drives and the local blood bank asking for a donation of a very different sort.

Both kinds of giving can fill you with a sense of goodwill. But, let's be honest, the tote bags help, too.

When it comes to blood donations, though, ethical concerns and risk have led to limits on incentives for donors in many places. The World Health Organization has set a goal for governments around the world to reach completely voluntary and nonremunerated donations of blood by 2020.

The WHO declaration, made in Melbourne, Australia, in 2009, came, in part, because the assembled public health officials asserted their belief that "paid donation can compromise the establishment of sustainable blood collection" from volunteers. Also, paying people to give blood may hurt the quality of donations.

Three economists argue in the latest issue of Science that it's time to rethink restrictions. In the last few years there have been some real-world experiments with incentives that suggest they can help increase donations without causing trouble.

In Switzerland, the reward of a lottery ticket valued at a little over five bucks lifted donations by about 5 percentage points from a base of about 42 percent of the population that typically gave blood. In the U.S., a $10 gift card increased donations by 7 percentage points over the usual 13 percent donation level.

The economists, including Nicola Lacetera of the University of Toronto Mississauga, wrote that the incentives led to quick and localized upticks in donations, which could help with blood shortages.

There are a few things that can be done to minimize potential problems. Offering incentives framed as rewards, rather than outright cash payments, would make sense, for instance.

And giving rewards to people who show up to donate — not just to those people who complete donation — could reduce the incentive for people to lie in order to qualify as a donor. "This practice may be critical for blood safety when incentives are offered," the economists write.

The details can be fiddled with, and researched some more. "But there should be little debate that the most relevant empirical evidence shows positive effects of offering economic rewards on donations," the authors conclude.

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