As we mentioned earlier, the positive correlation of stocks and oil might arise because both are responding to underlying shifts in global demand. For a simple test of that hypothesis, we apply a decomposition suggested by James Hamilton, a macroeconomist and expert in oil markets at the University of California-San Diego. In a post from the end of 2014, Hamilton proposed estimating an equation relating changes in oil prices to changes in copper prices, changes in the ten-year Treasury interest rate, and changes in the dollar, then using the value of the oil price predicted by that equation to measure the effect of demand shifts on the oil market. [2] The premise is that commodity prices, long-term interest rates, and the dollar are likely to respond to investors’ perceptions of global and US demand, and not so much to changes in oil supply. For example, when a change in the price of oil is accompanied by a similar change in the price of copper, this method concludes that both are responding primarily to a common global demand factor. While this decomposition is not perfect, it seems reasonable to a first approximation.

We apply Hamilton’s method, using daily data. For reasons of data availability, we start the sample in mid-2011. Data are in percentage changes (more precisely, log changes), except for the change in the ten-year rate, which is a simple difference. We estimate the equation for data through mid-2014, then use that equation to predict the oil price that would have obtained if the only shocks to the oil market had been on the demand side (the light blue line in Figure 3). The estimated coefficients on all variables (see Appendix 1 at end of post) are highly significant, both economically and statistically.

Comparing the predicted and actual decline in oil prices, we find that something in the range of 40-45 percent of the decline in oil prices since June 2014 can be attributed to unexpectedly weak demand. (This range is very similar to that obtained by Hamilton in a different sample.) The results are not much affected if we drop changes in the value of the dollar from the equation, or if we replace the ten-year Treasury yield with the slope of the yield curve (the difference between the two-year Treasury yield and the ten-year yield).