The S&P 500 index's historical returns are about 10 percent annually and closer to 7 percent when adjusted for inflation, explains Investopedia. Over 40 percent of total historical returns are from dividends. The S&P launched on March 4, 1957, states S&P Dow Jones Indices. The one-year return in September 2015 was negative at -2.65 percent. The index closed at $44.06 on its launch date and at $1,958.03 on Sept. 18, 2015, notes Yahoo! Finance.
The S&P 500 is an index of 500 stocks that includes the most widely held stocks traded on the Nasdaq and New York Stock Exchange, according to Kimberly Amadeo for About.com. Only U.S. companies with market capitalizations over $1 billion and a per share price over $1 are included in the index. At least 50 percent of a company's stock has to be available to the public for a company to be eligible for inclusion in the index.
Standard & Poor's analysts select the companies included in the index, explains Investopedia. The largest sectors in the index are information technology, finance and energy, states Amadeo. The 10 largest companies in the index, such as Apple, General Electric and Berkshire Hathaway, represent more than 17 percent of the index's total market capitalization.