Historically, September has always been the cruelest month for stock market investors. (For a startling visual, check out this chart from the Big Picture.) But despite generations of brainpower devoted to understanding September's bizarre seasonal affective disorder, no one seems to have a good theory as to why market indices invariably seem to plunge once everyone returns from their summer vacation.
Maybe this year will be different. The month isn't yet half over, of course, and anything could happen, but the Dow Jones Industrial Average, NASDAQ and the S&P 500 all hit highs for the year on Thursday.