Stock-level information produced by many active managers can aggregate into a public signal about the overall market. We show that daily return spreads between stocks with high and low active-manager ownership forecast next-day S&P 500 futures and ETF returns; market-timing strategies based on these spreads earn Sharpe ratios near one. Predictability concentrates among hedge funds and highly active mutual funds, strengthens in recessions and high-uncertainty states, exhibits no reversals, persists longer in tight-funding periods, and extends to global markets. A rational-expectations theory of specialized managers and slow-moving capital reconciles these findings and delivers new mechanisms of information aggregation. Structural estimation implies that market-wide information accounts for about 20% of model-implied stock-level signal variation among stocks heavily owned by active managers.