Apple's correlation with the S&P 500 index increases more often when the stock declines than when it increases. Over rolling 90-day periods, the average correlation of Apple to the S&P 500 is about 0.7 when the stock declines, and 0.5 when it rises. Since Sept. 30, shares of Apple are down 35.2% as of Friday morning after recovering some following Thursday's $14 collapse. The fall was driven primarily on news of slowing iPhone sales.
The iPhone creator, and mother to Siri, has been among the largest holding in the index since its late 2000's resurgence. Its importance to investor portfolios has grown significantly in the years following as index investing has exploded in popularity, with the S&P 500 standing as the most common passive fund proxy. Currently Apple is about 3.2% of the index, down from a peak of 4.5% in late October.