Those who have managed credit poorly in the past

Those who have managed credit poorly in the past. "Their behavior was so poor that no one will lend them credit," says Watts.

It's difficult to make assumptions about income, too. "It tends to be lower -- and some middle -- income," says Nick Jacobs, spokesman for the National Foundation for Credit Counseling, an umbrella group of local nonprofit credit counseling services. "There are some middle-income people who just haven't built up enough of a file yet."

But some very wealthy individuals are also living without credit. "The very wealthy often don't have credit," says Linda Sherry, spokesperson for Consumer Action, a D.C.-based advocacy group. "They have no need for it -- they pay for things with their checkbooks."