Experts warn: don't get greedy on S&P 500 gains — diversify your portfolio

Low-cost S&P 500 index funds have delivered strong returns, but experts caution against over-concentration in the biggest US stocks. They recommend adding other asset classes to portfolios to reduce volatility and improve diversification. The growing dominance of a handful of mega-cap companies in the index raises risk for investors who hold only S&P 500 funds.