CHICAGO (Reuters Life!) - U.S. Christians have a reputation for generosity -- but when it comes to supporting their own churches, it turns out most are stingy.

A parishioner holds her rosary beads as she prays at Immaculate Conception Catholic Church in Chicago, Illinois April 10, 2008. REUTERS/John Gress

The extent of their penury is outlined in a new book, “Passing the Plate: Why American Christians Don’t Give Away More Money” by Christian Smith and Michael Emerson.

It reveals that 20 percent of U.S. Christians give no money to their church, and that many who do donate fall far short of the 10 percent of income their churches traditionally expect.

Christianity is the dominant U.S. religion, with an estimated 226 million followers in 2005, and 140 million belonging to churches. The U.S. population exceeds 305 million.

“If American Christians could somehow find a way to move to practices of reasonably generous giving, they could generate, over above what they currently give, a total of another $133.4 billion a year to devote to whatever purposes and needs they would choose,” according to the book.

“What good in the world U.S. Christians could do with an additional $133.4 billion, year after year, is almost unimaginable, simply astonishing, nearly beyond comprehension,” said Smith, of the University of Notre Dame, and Emerson, of Rice University.

A mere $500 million, for instance, would close the funding gap needed to eradicate polio globally by 2010, they estimate, while $10 billion would sponsor 20 million needy children worldwide for food, education and healthcare.

The book cites a number of sources for its figures on giving. One found that 22 percent of all U.S. Christians gave nothing, 71 percent gave less than two percent of their income and nine percent gave 10 percent or more.

Smith said in an interview that there is no simple explanation for the lack of contributions. Factors range from the pressure on consumers to spend money on themselves to how money is collected and how much information is given to people. Parental example can also be an influence.

Another factor may be that people’s formal link with organizations has declined over time along with “a sort of responsibility to uphold institutions, and also an overall decline in trust of social institutions -- media, banking and so on,” Smith said.

“The less people trust the less they will give,” he added.

Megachurches could be playing a part because they appeal to the unaffiliated offering casual encounters but asking very little up front except attendance and short, entertaining services, he suggested.

But Mormons, who appear to give seven times more as a percentage of their income than Catholics, are the exception. Smith said the book does not include them in its overall calculations of giving because “they are so sociologically distinctive in terms of giving” that they deserve a separate mention.

“Mormons have a much higher expectation. They teach tithing much more conscientiously. Every year you meet with a local bishop who asks you if you tithed, and if you haven’t there are consequences,” Smith said.

Although the book only examined giving in the United States and did not compare levels of giving elsewhere in the world, it said Americans are more generous with charities than people in other advanced industrialized nations. American religious believers are more generous than non-believers.

What the impact of the current economic upheaval on giving will be remains to be seen.

“In general I expect people to give less,” said Smith, the director of Notre Dame’s Center for the Study of Religion and Society, adding that giving fell precipitously during the Great Depression of the 1930s.