It's difficult to pinpoint why millions of Americans wear wristbands that promote or support causes.

Perhaps, in the wake of Hurricane Katrina and the 9/11 terrorist attacks, people want to feel a sense of community. Or perhaps, after years of robust spending on luxury goods, people are feeling a tad guilty and want to give back by buying a band that benefits charity.

Both developments likely played a role in the wristband craze, said Cheryl Greene, chief strategy officer for the New York ad agency Deutsch Inc. And it's clear that today's zeitgeist is etched in silicone rubber.

"This thing is really huge," Greene said. "It lets the little guys show they care and that they're charitable without having their name on a wing of a hospital. It used to be nobody knew that but maybe the IRS and you."

While the current fad really took off with a yellow wristband promoted by cyclist Lance Armstrong's foundation, it can be traced to Vietnam POW/MIA bracelets worn in the 1970s. Now most every cause, including the fight against cancer and efforts to support U.S. troops, has a colored band. The trend has even spawned fakes. New York Attorney General Eliot Spitzer recently busted a ring that sold counterfeit LiveStrong bands, forcing it to turn over almost $112,000 to Armstrong's organization.

The wristbands have gone commercial, too. Companies receive brand recognition by selling them for charity. Some, such as Spencer Gifts, sell them just for fun, and profit. 7-Eleven introduced a scented "Slurpee" wristband in June with proceeds going to its foundation that grants college scholarships.

The total market for wristbands, for profit or not, has easily topped $500 million, said Daniel Howard, chairman of the marketing department at Southern Methodist University's Cox School of Business.