  We study the impacts of business cycles on machine learning (ML) predictions.
Using the S&P 500 index, we find that ML models perform worse during most
recessions, and the inclusion of recession history or the risk-free rate does
not necessarily improve their performance. Investigating recessions where
models perform well, we find that they exhibit lower market volatility than
other recessions. This implies that the improved performance is not due to the
merit of ML methods but rather factors such as effective monetary policies that
stabilized the market. We recommend that ML practitioners evaluate their models
during both recessions and expansions.
