We investigate the role of “arbitrageurs,” who exploit price discrepancies between redundant securities. Arbitrage opportunities arise endogenously in an economy populated by rational, heterogeneous investors facing investment restrictions. We show that an arbitrageur alleviates these restrictions and improves the transfer of risk amongst investors. When the arbitrageur behaves noncompetitively, taking into account the price impact of his trades, he optimally limits the size of his positions due to his decreasing marginal profits. When the arbitrageur is subject to margin requirements and is endowed with capital from outside investors, the size of his trades and capital are endogenously determined in equilibrium.

We study the stock market's reaction to aggregate earnings news. Prior research shows that, for individual firms, stock prices react positively to earnings news but require several quarters to fully reflect the information in earnings. We find a substantially different pattern in aggregate data. First, returns are unrelated to past earnings, suggesting that prices neither underreact nor overreact to aggregate earnings news. Second, aggregate returns correlate negatively with concurrent earnings; over the last 30 years, for example, stock prices increased 5.7% in quarters with negative earnings growth and only 2.1% otherwise. This finding suggests that earnings and discount rates move together over time and provides new evidence that discount-rate shocks explain a significant fraction of aggregate stock returns.

We explore the role of interfirm alliances as a mechanism for sharing technological knowledge. We argue that knowledge flows between alliance partners will be greater than flows between pairs of nonallied firms, and less than flows between units within single firms. Using patent citations as a proxy for knowledge flows, we find results that are consistent with these expectations. We then explore how firm characteristics affect knowledge flows within alliances and find positive effects due to technological, geographic, and business similarities between partners. We use alliance data from MERIT, patent data from the USPTO, and firm data from Compustat.