An empirical analysis of mean reversion of the S&P 500’s P/E ratios

Numerous authors have suggested that the price-earnings (P/E) ratio can be used to predict the future movement of stock prices. Such arguments are based on the belief that P/E ratios are mean-reverting. However, are the S&P P/E ratios really mean reverting? A review of the literature finds arguments on both sides, but the issue of mean reversion has not been tested adequately. Using unit roots and multiple structural breaks, we explicitly show that the P/E ratio is stationary around multiple breaks, which means that it will eventually revert to some long-run means. This result supports evidence that high P/E ratios relative to the current long-run mean will be followed by slow growth in stock prices and/or high earnings growth.