Stocks in 2015 chalked up the worst year for the S&P 500 since the start of the bull market in 2009. The Nasdaq, meanwhile, posted its weakest gain in four years. On Thursday, the Nasdaq and S&P 500 closed the last trading session of 2015 with losses of 1.2% and 0.9%, respectively. For the year, the Nasdaq rose 5.7%, while the S&P 500 fell 0.7%. As 2016 begins, wariness is natural.