Americans are pretty generous. Three-quarters of American families give to charity -- and those who do, give an average of $1,800. Of course that means one-quarter of us don't give at all. What distinguishes those who give from those who don't? It turns out there are many myths about that.

To test them, ABC's "20/20" went to Sioux Falls, S.D., and San Francisco. We asked the Salvation Army to set up buckets at their busiest locations in both cities. Which bucket would get more money? I'll get to that in a minute.

San Francisco and Sioux Falls are different in some important ways. Sioux Falls is small and rural, and more than half the people go to church every week.

San Francisco is a much bigger and richer city, and relatively few people attend church. It is also known as a very liberal place, and since liberals are said to "care more" about the poor, you might assume people in San Francisco would give a lot.

But the idea that liberals give more is a myth. Of the top 25 states where people give an above-average percentage of their income, all but one (Maryland) were red -- conservative -- states in the last presidential election.

"When you look at the data," says Syracuse University professor Arthur Brooks, "it turns out the conservatives give about 30 percent more. And incidentally, conservative-headed families make slightly less money."

Researching his book, "Who Really Cares", Brooks found that the conservative/liberal difference goes beyond money:

"The people who give one thing tend to be the people who give everything in America. You find that people who believe it's the government's job to make incomes more equal, are far less likely to give their money away."

Conservatives are even 18 percent more likely to donate blood.

The second myth is that people with the most money are the most generous. But while the rich give more in total dollars, low-income people give almost 30 percent more as a share of their income.

Says Brooks: "The most charitable people in America today are the working poor."

We saw that in Sioux Falls, S.D. The workers at the meat packing plant make about $35,000, yet the Sioux Falls United Way says it gets more contributions of over $500 from employees there than anywhere else. Continued...