For people who don't have a lot of time to spend on their investments, Warren Buffett's prescription of 90 percent in the S&P 500 and 10 percent in short term U.S. government bonds is 'great,' says MIT professor Andrew Lo. But Lo, who is also the founder of an investment company called AlphaSimplex, argues investors should ideally be more diversified, adding exposure to currencies, commodities and foreign stocks. The S&P, he says, is 'one big bet on the U.S. economy.' He acknowledges, however, that retail investors cannot be expected to follow the ups and downs of currencies and commodities. But retail investors may not be so eager to turn their money over to a professional money manager with a complex pitch.