Using stock-level data from the securities lending market, we develop a new empirical strategy to examine how financial markets process information. A key challenge in this literature is identifying when new—especially private—information arrives. We address this challenge by proxying information arrivals with increases in shorting demand, which captures informed traders’ reaction to new signals. We motivate our method within a theoretical model and validate it empirically using leading measures of informed trading. When shorting demand increases, we observe higher return volatility, higher volume–volatility correlation, and lower price informativeness. These findings align with a differences-of-opinion model in which overconfident investors, acting on their perceived informational advantage, trade so aggressively that market prices become less reflective of fundamentals. Our results hold even after excluding public signals, such as earnings announcements, highlighting the securities lending market’s role in revealing private information.