The predictability of asset returns is one of the more controversial topics in financial economics. According to the Efficient Markets Hypothesis (EMH), all information in investors’ information set should be incorporated into asset prices, thus leaving asset returns unpredictable. However, researchers and practitioners have identified a number of variables that seem to predict returns, at least in-sample. Some examples of variables that have been shown to predict aggregate market returns are inflation (Fama and Schwert, 1977), the default spread and the term premium (Fama and French, 1989) and the dividend-price ratio (e.g. Campbell and Shiller, 1988; Fama and French, 1988). Other financial ratios, such as the book-to-market ratio (e.g. Kothari and Shanken, 1997) and the earnings ratio (e.g. Lamont, 1998) have also been shown to predict returns. Recently, other variables that are not price-based have also been found to predict the aggregate stock market, such as the output gap (Cooper and Priestley, 2008). Traditionally, these variables have been thought to capture time-varying risk, a concept which is not in conflict with a modern interpretation of informationally efficient markets. In addition there is a growing literature that abandons the idea of informational efficiency; this literature argues that behavioral biases induce stock return predictability. In one recent example Baker and Wurgler (2007) argue that investor sentiment predicts returns. Another recent example is Hong, Torous and Valkanov (2007) who find that certain industries lead the aggregate market; they argue that this is due to underreaction to information.
In this paper I examine how changes in the oil price affect stock markets. Oil is by far the most important commodity and as of September 2008 energy accounted for around 75% of the Goldman Sachs Commodity Index (S&P GSCI). Crude oil accounted for 40% of the entire index and Brent crude oil for another 15%, thus leaving oil products with a weight of around 55% of the entire commodity index. The individual commodities in the index are weighted by their respective world production quantities. Given its large weight, it is not surprising that changes in oil prices may be an important factor for fluctuations in the economy. It is also plausible that oil price changes are important for understanding changes in stock prices. However, no consensus seems to have been reached regarding the impact of oil price changes on stock returns. Two recent papers, Driesprong, Jacobsen and Maat (2008) and Pollet (2004), document that oil price changes predict stock returns. Driesprong et al. (2008) show that the predictability is strongest for developed markets and less pronounced for emerging markets.