One car accident victim can require as much as 100 pints, and the American Cancer Society tells us that of the nearly 1.7 million people who’ll receive a cancer diagnosis this year, many will need blood, even daily, during chemotherapy.

But while roughly 40 percent of the U.S. population is eligible to donate blood at a given time, less than 10 percent do every year. And since blood can’t be manufactured or created in a lab, it’d be good if we better understood what gets people to give.