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According to Mike Maloney, the dividend yield is the second best way to measure a stocks value (after the Price Earnings Ratio). The ratio indicates how much a company pays out in dividends each year relative to its share price. In other words, it measures how much "bang for your buck" you are getting from dividends. In the absence of any capital gains, the dividend yield is effectively the return on investment for a stock. The lower the dividend yield, the less you get for your investment and hence the more overvalued a stock. The historic S&P 500 Dividend Yields were deducted by Robert Shiller and published in his book Irrational Exuberance.
In recent years, rather than paying out a dividend, share repurchases have become a popular way for companies to return value to their shareholders. This new practice partly explains the lower dividend yields that we have been experiencing.