The last time I made an impulse buy was Saturday night. I was swept into a bidding war with a burly man at a tattoo art auction, and in the end spent $275 on a terribly lovely piece of original flash that features, among other things, a hula-dancing wolf and a cockroach sporting a banner with the word “YUMMY.” Frivolous? Perhaps. But I was in a good mood and it was for a good cause—to support the chronically ill 9-month-old son of a NYC tattooist.

However, a new psychological study suggests that if I were sad and self-absorbed on Saturday, I may have paid even more.

Every single American bank note is printed at Bureau of Engraving and Printing facilities, with ink and on paper each made, separately, in only one factory in the world. And yet at any given time, there is at least $70 million worth of fake U.S. currency floating around, 75 percent of which is in $100 bills. In large part because of the advent of inexpensive scanners and printers, more and more fake bills are entering circulation: From 1997 to 2007, the amount of passed counterfeit bills nearly doubled, to $64.9 million.