In a word, VOLATILITY. The Nasdaq is a collection of small-cap companies, many of which won't be around 5 years from now. A select few, like the Dells, Microsofts and Apples of the world, will rise to greatness. Yes, the returns are a bit higher, but so is the risk (as measured by standard deviation). The S&P 500 are the blue-chips. The guys that have been around a while, many paying dividends like clockwork for decades. They form the backbone of the stock market. But why choose between them? You can own a TSM (Total Stock Market) fund that captures both. Or you can invest in two separate funds that do the same, weighted in a way you find appealing (the S&P accounts for 80% of market cap, so a market weighting would have 80% S&P 500, 20% small cap fund).