Source: http://www.businessweek.com/articles/201...

Most of this cash use is impossible to track. That’s one of cash’s virtues for those hiding from the police, the IRS, marketers, and data miners, and in the U.S., many assume these $100 bills are mostly used by criminals or tax evaders. Some, such as Harvard University economist Kenneth Rogoff, have suggested that the $100 bill be taken out of circulation.

But in Greene and Schuh’s survey, 5.2 percent of consumers readily admitted to carrying one or more $100 bills. The researchers found no evidence these people were anything but law abiding, Schuh says. There's a less nefarious reason the share of $100 bills in circulation has doubled in the past 20 years. It takes more $100s to make a big purchase than it used to: Inflation has cut the value of the $100 bill, the largest bill in circulation, by 45 percent over roughly the same 20-year period.

In fact, people who carry around lots of cash aren't that different from anyone else, with one important distinction: They make more money. Consumers who make more than $75,000 are about 50 percent more likely to be carrying around $100 than those who make less than $35,000. That might not seem surprising—more money in the paycheck means more in the wallet—but it undercuts the idea that wads of cash are mainly for poor people who don’t have access to banks or credit cards. In fact, holders of more than $100 were more likely to have a credit card, though they tended to pay it off every month. They were also more likely to be older than 55 and have more than a high school education. Beyond those, the study concluded, “other demographic factors are not statistically significant.”