The American wealthy are hands down the most philanthropic in the world. Americans dominate global giving lists and surveys consistently show that the U.S. rich are far more likely to make philanthropy a priority.

But some say they could give more. One recent study found that a large share of people making $200,000 or more give only 2.8 percent of their income to charity. Other studies show that multi-millionaires donate only about one percent of their wealth to charity (though billionaires tend to give a higher percentage).

Read more: Stars of 'Nerd Philanthropy' Dominate Top Giver List

Warren Buffett and Bill Gates launched their Giving Pledge in large part to persuade the super-rich and the non-super-rich alike to give more to charity. As Buffett told me in 2011, "The hope is that our larger population ends up giving a larger proportion of their income to fund philanthropy."

So why don't the rich give more?

A new study of multi-millionaires offers some answers. The study, by SEI Private Wealth Management, found that 82 percent of wealthy families believe that having more money means you have a greater obligation to be philanthropic.

But the respondents (worth $10 million or more) listed three main reasons for not giving more. First, nearly half said they needed more confidence "that the level of their wealth would continue to support their lifestyle and their family." Second, they said they would give more if the markets improved.

Finally, a third of those polled said they needed to "find something they could be more passionate about."

Read more: Did 2013 Tax Hikes Slow 2012 Charitable Giving?

The first two reasons aren't all that surprising. If they had more money, the wealthy would give more of it away.

But the third reason is worth noting. While more money helps, it's also important to be motivated by a cause. And if you care enough about a particular problem, you'll be more inclined to sacrifice some of life's comforts to solve it.

Money, in other words, isn't the only driver of philanthropy. It's about the heart as much as the wallet.

Jeff Roberson / AP

If Panera Bread pay-what-you-can experiment serves as a test for a people's inner goodness, it appears honesty usually wins out. The company says 60 percent of people leave the suggested amount, and 20 percent leave more.

Hungry but short on cash? You might be in luck if you live near one of the three Panera Bread locations that allow customers to pay what they can for their meals.

These cafes, which serve as part of the nonprofit arm for the St. Louis-based chain, rely on customers' sense of goodness and honesty to raise money for charitable programs. Instead of a tradtional menu and cash register, you can leave a lone penny or a fat wad of Benjamins in a donation box. The three locations (Clayton, Mo.; Dearborn, Mich.; and Portland Ore.) each bring in between $3,000 to $4,000 above costs.

This pay-what-you-can form of progressive charity is far from revolutionary: Community kitchens that operate like the Panera locations can be found across the country; a nonprofit gym in Euless, Texas, offers physical therapy without a price tag; and New York's Metropolitan Museum of Art and other musuems have for decades eliminated ticket counters in favor of donation boxes.

There's evidence that the name-your-price model is catching on with for-profit businesses (or at least being used as a marketing ploy):

• A decade ago, Priceline.com shook up the travel industry by allowing people to name their own price for airline tickets and hotel rooms. The company has de-emphasized this service in recent years, focusing on a standard comparison search format, but it is a still a core part of Priceline's business (and advertising campaign).
• Early this spring, Gap launched a "say your price" online program where shoppers can bid for clothing and accessories. If the website accepts your bid, you can print out a coupon to use in stores.
• Progressive has trumpeted its "Name Your Price" program for auto insurance. "Get a quote, then adjust the price to find a package that's right for you," its website says. Sounds great, but what you're really doing is raising or lowering your insurance premium.
• Delta airlines lets people name their price to be bumped off flights.
• In 2007, alternative rock band Radiohead released a downloadable album, "In Rainbows," without a price tag. Fans could pay what they wanted or nothing at all.

What do you think? Is a new trend in the making? Can a true pay-what-you-can business model work for a for-profit company?