(Dow Jones) Despite the hype around Bill Gates and Warren Buffett's Giving Pledge last year, the 50 biggest private donors in the U.S. gave less in 2010 than in any year since 2000.

It is unclear if the shaky economy is entirely to blame, or whether some other factor-such as accounting issues-contributed.

Donors on the Chronicle of Philanthropy's 2010 "Philanthropy 50" list committed a total of $3.3 billion to charity last year, the worst result since the list's inception a decade ago. That compares with "Philanthropy 50" totals of $4.1 billion in 2009 and $15.5 billion in 2008. In 2006, the top 50 philanthropists gave out a whopping $50.7 billion.

Broader gauges also reflect the big-donor drought. Worldwide, the 400 charities that raise the most from private sources saw an 11% drop in donations in 2009, the Chronicle of Philanthropy reported late last year.

A drop in overall giving makes perfect sense in light of the 2008 financial crisis, which hit many wealthy people's investments, and tax uncertainties in recent years. But these factors don't fully explain the decline in "Philanthropy 50" generosity, according to Richard Mittenthal of TCC Group, a consultancy to non-profits.

"These people are billionaires," says Mittenthal. "A blip in the economy, even a big blip, or uncertainty about the estate tax isn't going to have an effect (on their giving), not by themselves. But their psychological impact may be causing people to wait."

Until Congress late last year granted a two-year extension of tax cuts enacted under former President George W. Bush, many people had anticipated increases in taxes on income, capital gains and estates. That could have clouded some wealthy people's giving plans.

Another factor may have been a kind of lag in how the financial crisis hits the books of charitable foundations.

Many foundations are mandated to give away 5% of their funds every year, explains Kris Putnam-Walkerly of Putnam Community Investment Consulting. "But that's based on an average of the previous 12 to 16 quarters. We saw this delayed reaction after the dot-com bubble and after 9-11."

Also, the Giving Pledge itself may have put a brake-however slight and temporary-on some high-wealth donations.