The top 10 holdings together made up about 37% of the whole fund -- because the S&P 500 index is market-cap weighted. That means larger companies will move the needle more than smaller companies.

The S&P 500 is still a smart all-around option, but there's good reason to start thinking about other index funds.

Competition in the passive index fund space is intense. While active managers try to stand out with performance, strategy or brand reputation, index fund providers compete primarily on scale and fees.

This mutual fund encompasses 500 of America's biggest companies. Investing in it plugs you into most of the U.S. economy. Another way to invest in it is via an exchange-traded fund (ETF). Image source ...