There is, however, a pretty good explanation for this kind of pattern. Researchers have found that people treat money as earmarked for particular kinds of spending, a tendency behavioral economists call “mental accounting.” If someone is buying rounds at the neighborhood bar, people tend to treat the money they didn’t spend as “beer money,” and sooner or later they tend to spend it disproportionately on beer. As a result, they end up drinking more beer than they had originally intended.