Standard incentive theory models provide a rich framework for studying informational problems but assume that contracts can be perfectly enforced. This paper studies the design of self-enforced relational contracts. I show that optimal contracts often can take a simple stationary form, but that self-enforcement restricts promised compensation and affects incentive provision. With hidden information, it may be optimal for an agent to supply the same inefficient effort regardless of cost conditions. With moral hazard, optimal contracts involve just two levels of compensation. This is true even if performance measures are subjective, in which case optimal contracts terminate following poor performance.

The combinatorial clock auction has become popular for large-scale spectrum awards and other uses, replacing more traditional ascending or clock auctions. We describe some surprising properties of the auction, including a wide range of ex post equilibria with demand expansion, demand reduction, and predation. Our results obtain in a standard homogeneous good setting where bidders have well-behaved linear demand curves, and suggest some practical difficulties with dynamic implementations of the Vickrey auction.

We develop a model in which firms set impersonal salary levels before matching with workers. Wages fall relative to any competitive equilibrium while profits rise almost as much, implying little inefficiency. Furthermore, the best firms gain the most from the system while wages become compressed. In light of our results, we discuss the performance of alternative institutions and the recent antitrust case against the National Resident Matching Program.

Early admissions are widely used by selective colleges and universities. We identify some basic facts about early admissions policies, including the admissions advantage enjoyed by early applicants and patterns in application behavior, and propose a game-theoretic model that matches these facts. The key feature of the model is that colleges want to admit students who are enthusiastic about attending, and early admissions programs give students an opportunity to signal this enthusiasm.

The past decade has seen the explosive emergence of online advertising as a major source of revenue for Internet publishers. Analyses of this phenomenon are mostly conducted in the sway of Google’s hugely successful search advertising program. In the early days of the Internet, before Google, virtually all advertising revenues were related to simple display ads. Yet by 2008 search advertising accounted for over $10.5 billion of the $23.4 billion in total online advertising, and pundits were forecasting continued growth at rates of 12 % per year over the next five years.1 Internet advertising markets have broken sharply from the advertising markets for traditional media. In the older media, every consumer that received a particular magazine, listened to a particular radio program, or watched a particular TV show would read, hear or see the same advertisement. An advertiser that wanted to reach an audience with particular characteristics could do so only within narrow limits. For example, a beer company might advertise on televised football games and a maker of fashion clothing might advertise in women's magazines. Although publications do some tailoring of their offerings, as when a newspaper has different local editions, audience mix is nevertheless

We present new evidence on consumer liquidity constraints and the credit market conditions that might give rise to them. We analyze unique data from a large auto sales company serving the subprime market. Short-term liquidity appears to be a key driver of consumer behavior. Demand increases sharply during tax rebate season and purchases are highly sensitive to down-payment requirements. Lenders also face substantial informational problems. Default rates rise significantly with loan size, providing a rationale for loan caps, and higher-risk borrowers demand larger loans. This adverse selection is mitigated, however, by risk-based pricing.

Premiums in health insurance markets frequently do not reflect individual differences in costs, either because consumers have private information or because prices are not risk rated. This creates inefficiencies when consumers self-select into plans. We develop a simple econometric model to study this problem and estimate it using data on small employers. We find a welfare loss of 2-11 percent of coverage costs compared to what is feasible with risk rating. Only about one-quarter of this is due to inefficiently chosen uniform contribution levels. We also investigate the reclassification risk created by risk rating individual incremental premiums, finding only a modest welfare cost.

The platform design, the process that helps potential buyers on the internet navigate toward products they may purchase, plays a critical role in reducing search frictions and determining market outcomes. We study a key trade-off associated with two important roles of efficient platform design: guiding consumers to their most desired product while also strengthening seller incentives to lower prices. We use simple theory to illustrate this, and then combine detailed browsing data from eBay and an equilibrium model of consumer search and price competition to quantitatively assess this trade-off in the particular context of a change in eBay's marketplace design.

We document some early effects of how mobile devices might change Internet and retail commerce. We present three main findings based on an analysis of eBay's mobile shopping application and core Internet platform. First, early adopters of mobile e-commerce applications appear to be people who already were relatively heavy Internet commerce users. Second, adoption of the mobile shopping application is associated with both an immediate and sustained increase in total platform purchasing, with little evidence of substitution from the core platform. Third, differences in user behavior across the mobile applications and the regular Internet site are not yet so dramatic.

We estimate the sensitivity of Internet retail purchasing to sales taxes using eBay data. Our first approach exploits the fact that a seller's location—and therefore the applicable tax rate—is revealed only after a buyer has expressed interest in an item. We document how adverse tax “surprises” reduce the likelihood of purchase and shift subsequent purchases toward out-of-state sellers. We then use more aggregated data to estimate that every one percentage point increase in a state's sales tax increases online purchases by state residents by almost 2 percent, while decreasing their online purchases from state retailers by 3–4 percent.