Aggregate stock market returns display negative skewness. Firm stock returns display positive skewness. The large literature that tries to explain the first stylized fact ignores the second. This article provides a unified theory that reconciles the two facts by explicitly modeling firm-level heterogeneity. I build a stationary asset pricing model of firm announcement events where firm returns display positive skewness. I then show that cross-sectional heterogeneity in firm announcement events can lead to conditional asymmetric stock return correlations and negative skewness in aggregate returns. I provide evidence consistent with the model predictions.