Insights from psychology suggest competitive interference: when a firm advertises, consumers are less likely to think of its competitor, benefiting the advertiser by keeping it top of mind and suppressing competitor recall. We empirically investigate this phenomenon in the context of auto insurance, a sector featuring some of the most advertised brands in the US. Using a large-scale randomized field experiment, we examine the dynamic effect of advertising on an individual’s consideration of the advertised insurer. Despite our advertiser being well-known and one of the largest spenders on advertising, the magnitude of our observed effect surpasses those documented in prior literature. The immediate and carry-over effects of advertising depend heavily on competitive actions, consistent with psychological predictions; the advertising impact increases when competitors advertise. Together with other findings, our data support interference as a driver of the marginal effects of ads. We build and estimate a cognitive psychology-based consumer decision-making model, demonstrating that it predicts both in- and out-of-sample data better than the flexible standard advertising stock model.