We criticize the theories used to explain the size distribution of cities. They take an empirical fact and work backward to obtain assumptions on primitives. The induced theoretical assumptions on consumer behavior, particularly about their inability to insure against the city-level productivity...

Extreme losses are the major concern in risk management. The dependence between financial assets and the market portfolio changes under extremely adverse market conditions. We develop a measure of systematic tail risk, the tail regression beta , defined by an asset's sensitivity to large...

Both in practice and in the academic literature, models for setting margin requirements in futures markets use daily closing price changes. However, financial markets have recently shown high intraday volatility, which could bring more risk than expected. Such a phenomenon is well documented in...