A Nov. 6 Sunday Business article misstated that the Red Cross has received $2.2 billion for hurricanes Katrina and Rita. That number is the total for private donations collected toward hurricane relief; the Red Cross has received $1.3 billion for the Gulf Coast hurricanes. (Published 11/7/2005)

Disaster strikes -- a hurricane, a flood, a tsunami, a terrorist attack. People die. Buildings are destroyed. Communities are devastated. What do you do? If you're like millions of Americans, you reach for your wallet. And you give. And give. And give.

More than $2 billion in private donations for victims of the Sept. 11, 2001, attacks. More than $1.5 billion for those affected by the devastating tsunami that swept through Southeast Asia at the end of last year. And last week, donations to the victims of hurricanes Katrina and Rita hit $2.2 billion.

But what is it that triggers that "must-give" button in our heads? And why do some disasters push that button when others don't? The string of hurricanes that hit Florida last year brought in only $79 million. Last month's earthquake in Pakistan, which killed more than 70,000 people -- many times the number who perished in Hurricane Katrina -- has brought in a mere $45 million in donations from Americans.

International and domestic relief groups that deal with disasters say they sense that certain circumstances can trigger an outpouring of donations. Although they emphasize that they are constantly surprised by what galvanizes Americans (the massive giving to the tsunami in far-off Southeast Asia, for example), here's what they generally find:

"Natural" disasters beat manmade disasters. In other words, victims of hurricanes and tsunamis generally attract more donations than victims of war and other politically caused crises.

Oxfam America, for example, a relief group that works in 26 countries, received $250 million in donations for tsunami victims -- more than enough for that effort, said Nathaniel A. Raymond, spokesman for the relief organization.

But it is struggling to raise funds to help the millions of victims of the civil strife in the Darfur region of Sudan, where some 2 million people have been forced from their homes and into camps, and for the civil war in northern Uganda that has killed tens of thousands and driven more than 1.6 million people off their farms.

Sudden disasters beat slow-moving crises. Who among us didn't feel spurred to take action while watching people beg for help outside the Superdome and on the overpasses in New Orleans after Katrina?

A sense of urgency mobilizes donors.

"People's lives are clearly at stake, and it creates a strong impulse to give," said Bill Strathmann, chief executive of Network for Good, a charitable Web site that allows people to donate and volunteer to more than 1 million charities. "Americans want to help, not stand by feeling helpless."

But plodding disasters, such as the decades-long devastation of the AIDS crisis or the methodical lethality of a famine, often don't trigger such an outpouring.

"With famines, that's so slow and gradual," said Patrick M. Rooney, director of research for the Center on Philanthropy at Indiana University, which has studied patterns of charitable giving over the past 50 years.

"It's horrific, but it happens very slowly. If you were there one day and back the next day, there wouldn't be much difference."

The United Nations says hunger has killed 6 million people worldwide this year -- almost 25 times as many as died in the tsunami.

But relief groups labor to raise money for efforts to resolve problems like that -- such as for the victims of the widespread famine and drought in the West African nation of Niger, where 2.5 million people face food shortages after their crops were ravaged by drought and locusts.

There are exceptions to this rule -- particularly when celebrities get involved. The famine in Ethiopia in 1984-85 triggered a record outpouring of donations, but only after rock musician Bob Geldof organized Live Aid, the worldwide rock concert held in July 1985.

TV counts. Spectacular videos that allow viewers to imagine themselves at the scene make a huge difference. Those scenes of the World Trade Center as the planes struck, footage of the giant waves washing away people and buildings in Southeast Asia, and the fury of Katrina and its aftermath in New Orleans all galvanized donors.

With these three events, "you had videographic evidence of what the locality looked like before the incident, during the incident and after the incident," said Rooney.

Even the same types of disaster can bring differing reactions if one has dramatic footage and the other doesn't. Floods that have "debris everywhere, a house floating down a river, those kinds of visuals that go along with it," bring in more dollars than less visible -- but possibly more expensive -- floods that don't yield those pictures, said Lauri Rhinehart, director of disaster fundraising for the American Red Cross.

Drama counts. Consider Katrina vs. the quartet of hurricanes -- Charley, Jeanne, Frances and Ivan -- that struck Florida last year. Americans donated $79 million to assist victims of those four hurricanes, compared with the billions of dollars for Katrina. Four consecutive hurricanes were "incredibly bad luck" for Floridians, said Rooney. "But they did not hit the exact same area, and they didn't hit with the same ferocity as Katrina did."

Timing counts. Tsunamis roared through Southeast Asia on Dec. 26, 2004, right when U.S. families were watching television, many of them surrounded by holiday presents. "When you see the juxtaposition of that affluence and abundance versus those children who had been orphaned by the tsunami and whole communities destroyed . . . that certainly pulled on our hearts and philanthropic impulses," said Rooney.

Similarly, while the Red Cross has received $2.2 billion for hurricanes Katrina and Rita, it has received a whopping $93,000 for Hurricane Wilma, which struck southern Florida late last month. Granted, Katrina caused vastly more damage and disrupted more lives than Wilma. But relief agencies say that more Americans might have ponied up for Wilma if they hadn't already been through the emotional roller coaster of witnessing Katrina.

Ease of giving makes a big difference. Pull out that credit card and click 'n give; increasingly, donors are turning to the Internet to make their contributions after disasters. More than one in four dollars that the Red Cross has received for hurricanes Katrina and Rita has come through its Web site, redcross.org.

"It's analogous to an impulse purchase in the retail industry," said Strathmann of Network for Good, through which 100,000 people have donated $12.5 million to 300 charities. "You're driven to give, the ability is right there, and you can take action. And I think that's a good thing," he added.

Personal experience helps. As the United States becomes increasingly multicultural, so is the response to large-scale tragedies. Indian and Sri Lankan communities in the United States raised tens of millions of dollars for tsunami relief, while Muslims are mustering the same kind of effort for victims of last month's earthquake in Pakistan.

As for Katrina, many Americans had visited the Gulf Coast, knew people who live there or knew of people who live there.

"Pretty much everybody in America had [only] one or two degrees of separation to somebody who had been impacted by Katrina," said Strathmann. Hence the reason earthquakes in places such as Pakistan or Iran tend to generate anemic response among those with no ties to the region.

"I think it's difficult for people to imagine themselves in a remote village in Pakistan," said Rhinehart. However, she adds: "It doesn't make the need any less."

The power of personal ties also explains why some charities use child sponsorships to raise money -- where contributors "sponsor" a child in a poor country (even though, as most child-sponsorship charities make clear, their money goes to the child's community, not to the individual child).

To personalize distant tragedies, charities try to bring the experience home to affluent Americans.

Oxfam America, for example, has had success with fundraising dinners where people are randomly assigned to one of three groups: Diners in the "poor" group sit in the floor and are served only rice. Those in the "middle-class" group get rice and beans, and the "wealthy" diners get a three-course meal.

Simple beats complex. Relief groups try to communicate to donors that the world is complicated and that positive change to crises such as poverty and religious and ethnic strife doesn't come quickly, easily or cheaply.

Indeed, while people like to feel they have made an immediate difference in the lives of the desperate, relief groups urge Americans not to forget that some solutions take years, not days.

Disaster giving doesn't supplant donations to other causes. Contrary to popular belief, Americans generally don't reduce giving to their regular charities when they send off some change to an unexpected disaster here or abroad, researchers said.

That's because individual giving to disasters tends to be small -- three-quarters of the people who donated to a Sept. 11 charity gave $100 or less -- so people can comfortably keep up their other giving, said Rooney. The strength of the economy has far more impact on charitable giving than do disasters. While many charities blamed Sept. 11 giving for reduced contributions in 2001, Rooney and other researchers say the 2001 recession -- not the Sept. 11 donations -- was actually the cause.

Nonetheless, charitable groups are urging donors to keep up the flow of dollars during this fall and winter's "giving season." And, to spur them on, Congress recently passed a bill increasing tax deductions for charitable donors. Until the end of the year, and to any charity, individuals can write off charitable deductions equal to their adjusted gross income, compared with the usual limit of 50 percent of their adjustable gross income. Tax experts say the provision will probably help only wealthy charitable donors. But backers hope everyone gets the message -- that it isn't just hurricane victims who need help.