Two weeks ago, the S&P 500 was up 12.39% in 2012, while the Dow Jones Industrial Average was up just 8.04%. This represented a spread of 4.35 percentage points. As shown below, the YTD performance spread between the two indices has tightened considerably over the past two weeks, and it now sits at 3.22%.
The reason? Apple (AAPL). With a 4%+ weighting in the S&P 500 and a 0% weighting in the Dow, Apple's 10% decline over the past two weeks has caused the S&P 500 to underperform. The second chart below of Apple's share price looks nearly the exact same as the S&P 500/Dow spread chart.