We examine the informal exchange of favors in societies such that any two individuals interact too infrequently to sustain exchange, but such that the social pressure of the possible loss of multiple relationships can sustain exchange. Patterns of exchange that are locally enforceable and...

We examine how countries' incentives to go to war depend on the "political bias" of their pivotal decision makers. This bias is measured by a decision maker’s risk/ reward ratio from a war compared to that of the country at large. If there is no political bias, then there are mutually...

We study collective decisions by time-discounting individuals choosing a common consumption stream. We show that with any heterogeneity in time preferences, utilitarian aggregation necessitates a present bias. In lab experiments three quarters of "social planners" exhibited present biases, and...

We study cascades of failures in a network of interdependent financial organizations: how discontinuous changes in asset values (e.g., defaults and shutdowns) trigger further failures, and how this depends on network structure. Integration (greater dependence on counterparties) and...

We present a dynamic model of network formation where nodes find other nodes with whom to form links in two ways: some are found uniformly at random, while others are found by searching locally through the current structure of the network (e.g., meeting friends of friends). This combination of...

We develop a model where agents obtain information about job opportunities through an explicitly modeled network of social contacts. We show that employment is positively correlated across time and agents. Moreover, unemployment exhibits duration dependence: the probability of obtaining a job...