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.