What is especially hard in investing is timing. If you followed Matt's advice and put all your investments to index fund (say Vanguard Total Stock Market ETF, https://www.google.com/finance?q=NYSEARCA) in the late 2007, you would have lost almost 50% of your savings in a year. And this example is not far fetched. My startup got acquired in 2007 and I invested some of that money to the stock market in late 2007. Not all, fortunately.