This one comes from the mind of none other than former Federal Reserve Chairman Alan Greenspan, known as "the oracle" for his uncanny ability to keep a pulse on the U.S. economy. According to media reports, Greenspan had a number of his own favorite economic indicators, including trends in dry cleaning (a luxury reserved for flush times, he believed) and strangely, sales of men's underwear [source: NPR].
If you look at a graph of men's underwear sales over the past several decades, it's almost comically flat. Men don't run out and buy underwear because it's fashionable; they buy it when they need it, and only when they need it. Since men's underwear represents the ultimate non-luxury item, Greenspan theorized that any slight dip in sales signaled a significant drop in discretionary income [source: Kurtzleben].