I present a dynamic general equilibrium model in which commonality in bank assets endogenously changes over the business cycle and shapes systemic risk. To reduce individual risks, banks diversify, increasing portfolio overlap and hence the similarity of their exposures to fundamental shocks. Systemic financial crises burst at the end of credit booms when productive investment opportunities are exhausted, banks' diversification incentives are strong, and their portfolios are highly correlated. A calibrated model is able to match key moments related to frequency, severity, and the economy's behavior around systemic crises.

We propose a tractable model in which risk, at both the micro and macro levels, is endogenous and driven by incentives. In the model, each firm chooses the mean and the variance of its productivity process, as well as how it covaries with the productivity of other firms. Aggregate risk arises when firms select productivity processes that are correlated with one another. The theory predicts that larger firms and those with lower markups are less volatile and less correlated with aggregate productivity. We find support for these predictions in the data. Through their impact on risk-taking decisions, distortions such as taxes and markups can make GDP more volatile in equilibrium. In a calibrated version of the model, removing distortions significantly reduces GDP volatility.

We show that socially responsible investors can have a negative impact by slowing the pace of firm reform. Investors with broad prosocial preferences value acquiring dirty firms with high negative production externalities because they can reform these firms. The anticipation of trading gains for dirty firms decreases the incentive of current firm owners to reduce externalities proactively, potentially causing delay in reform. The presence of financial investors—alongside socially responsible investors—can exacerbate delay. Investment mandates through which socially responsible investors commit to paying a premium for green firms can incentivize reform in a timely manner.

We propose a model of the financial system in which banks are individually prone to runs and connected through fire sales. Strategic complementarities within and across banks amplify each other, making heterogeneity in bank risks a key factor shaping the fragility of each bank and the entire system. As long as different banks are interconnected, an increase in heterogeneity stabilizes all banks. Reductions in asset commonality, bank-specific disclosures, and even broad-based policies such as asset purchases and liquidity requirements can enhance stability by increasing bank heterogeneity.

Supply chain disturbances can lead to substantial increases in production costs. To mitigate these risks, firms may take steps to reduce their reliance on volatile suppliers. We construct a model of endogenous network formation to investigate how these decisions affect the structure of the production network and the level and volatility of macroeconomic aggregates. When uncertainty increases in the model, producers prefer to purchase from more stable suppliers, even though they might sell at higher prices. The resulting reorganization of the network tends to reduce macroeconomic volatility, but at the cost of a decline in aggregate output. The model also predicts that more productive and stable firms have higher Domar weights—a measure of their importance as suppliers—in the equilibrium network. We provide a basic calibration of the model using U.S. data to evaluate the importance of these mechanisms.