Productivity shocks play a central role in real business cycles as an exogenous impulse to macroeconomic activity. However, measured Solow-Prescott residuals do not behave as an exogenous impulse. Rather, econometric evidence provided in this paper indicates that (1) money, interest rates, and government spending Granger-cause these impulses, and (2) a substantial component of the variance of these impulses (between one quarter and one half) is attributable to variations in aggregate demand. These results are robust to a number of econometric issues, including measurement errors, specification of the production function, and certain forms of omitted real variables.