The Secret Money Machine

AN OLD POKER JOKE GOES LIKE THIS: If you look around the table and you can't spot the sucker, the sucker is probably you.

Looking around for places to invest their money, Americans in recent years have taken their seats at Wall Street's big casino as never before. Fewer than seven years after the crash, 61% of small investors' money is riding on publicly traded securities, up from a 46% share in 1987. More than $1.3 trillion has flowed into mutual funds since the 1990s began, bringing the total to $2 trillion.

Americans have had lots of good, sensible reasons for...