We infer motives for trade initiation from market sidedness. We define trading as more two-sided (one-sided) if the correlation between the number of buyer- and seller-initiated trades increases (decreases), and assess changes in sidedness (relative to a control sample) around events that identify trade initiators. Consistent with asymmetric information, trading is more one-sided before merger news. Consistent with belief heterogeneity, trading is more two-sided before earnings and macro announcements with greater dispersion in analyst forecasts, and after news with larger announcement surprises. We examine the codeterminacy of sidedness, bid-ask spread, volatility, number of trades, and order imbalance.