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When consumers concentrate their purchases at a single firm, firms that offer more products than their rivals gain market share for all their products. These spillovers induce firms to offer a greater variety of products rather than lower prices, and a concentrated industry with few large firms can arise if spillovers are strong enough. This article presents a simple model that illustrates this mechanism explicitly. The empirical analysis documents strong demand spillovers in the retail segment of the U.S. mutual fund industry, in which fees are nontrivial, families offer many funds, and the market is quite concentrated. Instead, spillovers are weaker, fees are lower, families offer fewer funds, and the market structure is more fragmented in the institutional segment.

This paper investigates how trading frictions vary with the thickness of the asset market by examining patterns of asset allocations and prices in commercial aircraft markets. The empirical analysis indicates that assets with a thinner market are less liquid—i.e., more difficult to sell. Thus, firms hold on longer to them amid profitability shocks. Hence, when markets for assets are thin, firms' average productivity and capacity utilization are lower, and the dispersions of productivity and of capacity utilization are higher. In turn, prices of assets with a thin market are lower and have a higher dispersion. (JEL A12, L11, L93)

We estimate a model of illicit drugs markets using data on purchases of crack cocaine. Buyers are searching for high-quality drugs, but they determine drugs' quality (i.e., their purity) only after consuming them. Hence, sellers can rip off first-time buyers or can offer higher-quality drugs to induce buyers to purchase from them again. In equilibrium, a distribution of qualities persists. The estimated model implies that if drugs were legalized, in which case purity could be regulated and hence observable, the average purity of drugs would increase by approximately 20 percent and the dispersion would decrease by approximately 80 percent. Moreover, increasing penalties may raise the purity and affordability of the drugs traded by increasing sellers' relative profitability of targeting loyal buyers versus first-time buyers. (JEL D12, K42, L15, L65)

We investigate the effects of the institutional settings of the US health care system on individuals' life-cycle medical expenditures. Health is a form of general human capital; labor turnover and labor-market frictions prevent an employer-employee pair from capturing the entire surplus from investment in an employee's health. Thus, the pair underinvests in health during working years, thereby increasing medical expenditures during retirement. We provide empirical evidence consistent with the comparative statics predictions of our model using the Medical Expenditure Panel Survey (MEPS) and the Health and Retirement Study (HRS). Our estimates suggest significant inefficiencies in health investment in the United States. (JEL D14, D91, G22, I11, J32)

Abstract
We study the role of new-product quality for the dynamics of durable-good expenditures around the Great Recession. We assemble a rich dataset on US new-car markets during 2004–12, combining data on transaction prices with detailed information about vehicles' technical characteristics. During the recession, a reallocation of expenditures away from high-quality new models accounts for a significant decline in the dispersion of expenditures. In turn, car manufacturers introduced new models of lower quality. The drop in new-model quality persistently depressed the technology embodied in vehicles, and likely contributed to the slow recovery of expenditures.