There are few areas where skepticism is more important than how one invests one's life savings. Yet intelligent and educated people, some of them naïve about finance and others quite knowledgeable, have been ruined by schemes that turned out to be highly dubious and quite often fraudulent. The most dramatic example of this in American history is the recent announcement that Bernard Madoff, a highly regarded money manager and a former chairman of Nasdaq, has for years been running a very sophisticated Ponzi scheme, which by his own admission has defrauded wealthy investors, charities and other funds of at least $50...