Yet every year, less than 10 percent of those who are eligible to give blood do so, according to the American Red Cross, though blood is needed somewhere in the United States every two seconds.

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So what gives?

Some experts say it is the nature of the need to act, more than it is the act of donating.

“Every time there’s a disaster or an attack, in those moments people feel like they don’t have control over that situation,” said Arthur Markman, a psychology professor at the University of Texas at Austin. “One response is to reassert your control, because that feels better than events creating that. You look for something tangible to do that allows you to take control.”

To fill the daily need of patients at the nation's 2,600 hospitals, the American Red Cross must collect nearly 14,000 blood donations every day, according to its website. In 2017, blood drives across the United States have seen far fewer donors than in the past, say Red Cross officials. It was so bad this summer, on July 5 the organization said it was 61,000 donors short of what was immediately needed. Because fresh blood can be stored for only four to six weeks (and can't be frozen for later use), there is a continual need for the precious fluid.

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In a study published in the Journal of Consumer Research in September, researchers found that independent-minded folks view public appeals from blood centers as a social pressure to be like everyone else. But during a national emergency, when everyone's focus is on the public tragedy, there is a communal willingness to do something that is clearly not felt at other times of the year.

Markman cites another reason for the soaring number of blood donations in the aftermath of events like the Las Vegas massacre: the psychological theory of terror management.

“The theory of terror management taps into how existential concerns motivate people,” Colin Zestcott told The Washington Post last year. Zestcott, formerly at the University of Arizona, is now a psychologist at the State University of New York at Geneseo. “When people are worried about their mortality they want to maintain self-esteem and grab on to anything that makes them feel like they’ll live on.”

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Markman says the overwhelming charitable response to disasters, like the one in Las Vegas, fits well with the psychological theory of terror management, which has been gaining acceptance for some two decades.

“One of the ways in which we [people make themselves feel immortal] is to connect back to the community through a blood bank, or gathering for a vigil, or going to a religious institution. . . . Because it could have been them [who died]. . . . What lives on after your physical body? What can you do differently tomorrow to enhance your memory in other people?”