The academic system incentivizes p-hacking, where researchers select estimates and statistics with statistically significant p-values for publication. We analyze the complete process of Granger causality testing including p-hacking using Monte Carlo simulations. If the degrees of freedom of the underlying vector autoregressive model are small to moderate, information criteria tend to overfit the lag length and overfitted vector autoregressive models tend to result in false-positive findings of Granger causality. Researchers may p-hack Granger causality tests by estimating multiple vector autoregressive models with different lag lengths and then selecting only those models that reject the null of Granger non-causality for presentation in the final publication. We show that overfitted lag lengths and the corresponding false-positive findings of Granger causality can frequently occur in research designs that are prevalent in empirical macroeconomics. We demonstrate that meta-regression models can control for spuriously significant Granger causality tests due to overfitted lag lengths. Finally, we find evidence that false-positive findings of Granger causality may be prevalent in the large literature that tests for Granger causality between energy use and economic output, while we do not find evidence for a genuine relation between these variables as tested in the literature.