Projects, Papers, and Publications

New Working Papers:

Measuring Police Presence in US Cities using Smartphone Data
Joint with Katherine L. Christensen, Elicia John, Emily Owens, and Yilin Zhuo

While extensive, research on policing in America has focused on documented actions such as stops and arrests—less is known about patrolling and presence. We map the movements of over ten thousand police officers across twenty-three of America's largest cities by combining anonymized smartphone data with station and precinct boundaries. Police spend considerably more time in Black neighborhoods, a disparity which persists after controlling for density, socioeconomics, and crime-driven demand for policing. Our results suggest that roughly half of observed racial disparities in arrests derive from disparities in exposure, and that supervisor (but not officer) diversity reduces this.

Causal Estimation of Stay-at-Home Orders on SARS-CoV-2 Transmission
Joint with Yilin Zhuo, Malena de la Fuente, Ryne Rohla, and Elisa F. Long

Accurately estimating the effectiveness of stay-at-home orders (SHOs) on reducing social contact and disease spread is crucial for mitigating pandemics. Leveraging individual-level location data for 10 million smartphones, we observe that by April 30th—when nine in ten Americans were under a SHO—daily movement had fallen 70% from pre-COVID levels. One-quarter of this decline is causally attributable to SHOs, with wide demographic differences in compliance, most notably by political affiliation. Likely Trump voters reduce movement by 9% following a local SHO, compared to a 21% reduction among their Clinton-voting neighbors, who face similar exposure risks and identical government orders. Linking social distancing behavior with an epidemic model, we estimate that reductions in movement have causally reduced SARS-CoV-2 transmission rates by 49%.

Geographic Mobility in America: Evidence from Cell Phone Data
Joint with Devin G. Pope, NBER working paper 27072

Traveling beyond the immediate surroundings of one's residence can lead to greater exposure to new ideas and information, jobs, and greater transmission of disease. In this paper, we document the geographic mobility of individuals in the U.S., and how this mobility varies across U.S. cities, regions, and income classes. Using geolocation data for ~1.7 million smartphone users over a 10-month period, we compute different measures of mobility, including the total distance traveled, the median daily distance traveled, the maximum distance traveled from one's home, and the number of unique haunts visited. We find large differences across cities and income groups. For example, people in New York travel 38% fewer total kilometers and visit 14% fewer block-sized areas than people in Atlanta. And, individuals in the bottom income quartile travel 12% less overall and visit 13% fewer total locations than the top income quartile.

Suppliers and Demanders of Flexibility: The Demographics of Gig Work
Joint with Judy Chevalier, Peter Rossi, and Lindsey Currier

Platform gig work such as rideshare driving involves workers supplying flexibility to the platform, for example, providing service when demand is high. It also can be attractive to workers who demand flexibility, for example, workers with irregular commitments in other jobs. Who benefits the most (and least) from flexible work arrangements? Workers who supply labor price elastically provide flexibilty to the platform and receive above the platform-average compensation. In contrast, workers with the most time-variation in their reservation wage are demanders of flexibility and benefit from the availability of flexible work options. Using an empirical Bayesian model, we estimate driver-by-driver both the level and time variation in the driver reservation wage. We characterize the demographics of Uber drivers and explore the characteristics of drivers who supply flexibility and the characteristics of drivers who would drop out if the arrangement were less flexible. Our results run counter to several common intuitions about the costs and benefits of gig work.

Do Pre-Announcement Face-to-Face Interactions Increase the Returns to Acquisitions? Evidence from Smartphone Geolocational Data
Joint with Marco Testoni and Mariko Sakakibara

Information asymmetries make acquisitions risky, reducing firms' ability to gain from these transactions. We test whether face-to-face interactions with the target's employees before an acquisition provide informational advantages to the acquirer. For a sample of U.S. domestic acquisitions, we use smartphone geolocational data to measure the movement of people between merging companies' headquarters in the months before the acquisition announcement and use the number of visits between the two companies as a proxy for face-to-face interactions. Using bad weather conditions in the two companies' locations as a source of exogenous variation in intercompany mobility, we find evidence that more intense face-to-face interactions increase the abnormal returns on the acquirer's stock at the acquisition announcement.

Dynamic Pricing in a Labor Market: Surge Pricing and Flexible Work on the Uber Platform
Joint with Michael Sheldon

This paper studies how the dynamic pricing of tasks in the "gig" economy influences the supply of labor. A large economic literature has explored labor supply when workers can flexibly choose how long to work each day. In a study of taxi drivers, Camerer et al. (1997) claim that drivers quit when they hit a daily income target, consequently driving less when hourly earnings are high. If general, this behavior would undermine the benefits of emerging "sharing economy" markets where tasks are dynamically priced. In this paper, we study how driver-partners on the Uber platform respond to the dynamic pricing of trips, known as "surge" pricing. In contrast to income-target findings, we find that Uber partners drive more at times when earnings are high, and flexibly adjust to drive more at high surge times. A discontinuity design confirms that these effects are causal, and that surge pricing significantly increases the supply of rides on the Uber system. We discuss the implications of these findings for earnings, flexible work, and the efficiency of dynamically-priced labor markets.


Published and Forthcoming Papers:

Racial Disparities in Voting Wait Times: Evidence from Smartphone Data
Joint with Kareem Haggag, Devin G. Pope, and Ryne Rohla
NBER working paper 26487, conditionally accepted, RESTAT

Equal access to voting is a core feature of democratic government. Using data from millions of smartphone users, we quantify a racial disparity in voting wait times across a nationwide sample of polling places during the 2016 U.S. presidential election. Relative to entirely-white neighborhoods, residents of entirely-black neighborhoods waited 29% longer to vote and were 74% more likely to spend more than 30 minutes at their polling place. This disparity holds when comparing predominantly white and black polling places within the same states and counties, and survives numerous robustness and placebo tests. We shed light on the mechanism for these results and discuss how geospatial data can be an effective tool to both measure and monitor these disparities going forward.

Computing the Conditional Entry-State Distribution in Erlang Loss Systems
Joint with Bobby Nyotta and Fernanda Bravo
Operations Research Letters
, May 2021
We consider an Erlang loss system with state-dependent arrival rates. Given the system is in steady-state and there are j customers being served, the system operator may wish to know about the distribution of arrival states for the j customers in service. Specifically, they may want the steady-state probability that any given customer entered when the system had i servers busy, given j customers are currently being served. We term this metric the conditional entry-state distribution, and develop an algorithm to compute it.

Nursing Home Staff Networks and COVID-19
Joint with Judy Chevalier and Elisa F. Long
Proceedings of the National Academy of Sciences, Jan 2021

Nursing homes and other long-term care facilities account for a disproportionate share of COVID-19 cases and fatalities worldwide. Outbreaks in U.S. nursing homes have persisted despite nationwide visitor restrictions beginning in mid-March. An early report issued by the Centers for Disease Control and Prevention identified staff members working in multiple nursing homes as a likely source of spread from the Life Care Center in Kirkland, Washington to other skilled nursing facilities. The full extent of staff connections between nursing homes—and the role these connections serve in spreading a highly contagious respiratory infection—is currently unknown given the lack of centralized data on cross-facility employment. We perform the first large-scale analysis of nursing home connections via shared staff and contractors using device-level geolocation data from 50 million smartphones, and find that 5.1 percent of smartphone users who visit a nursing home for at least one hour also visit another facility during our 11-week study period—even after visitor restrictions were imposed. We construct network measures of connectedness and estimate that nursing homes, on average, share connections with 7 other facilities. Controlling for demographic and other factors, a home's staff-network connections and its centrality within the greater network strongly predict COVID-19 cases. Traditional federal regulatory metrics of nursing home quality are unimportant in predicting outbreaks, consistent with recent research. Multivariate regressions comparing demographically and geographically similar nursing homes suggest that 49 percent of COVID cases among nursing home residents are attributable to staff movement between facilities.

Consumer Search in the U.S. Auto Industry: The Role of Dealership Visits
Joint with Dan Yavorsky and Elisabeth Honka
Quantitative Marketing and Economics, October 2020

In many markets, consumers visit stores and physically inspect products before making purchase decisions. We view the inspection of a product at a retail location as a search for product fit. We quantify the cost and benefit from searching for product fit using a discrete choice model of demand with optimal sequential search. In these models, the benefit of searching is measured by the standard deviation of the product fit and has, heretofore, been fixed to one for identification purposes. With an exogenous search cost shifter, both the cost and benefit of searching can be separately identified. Our empirical setting is the U.S. automotive market. We assemble a unique data set containing individual-level smartphone geolocation data that inform us about dealership visits. We also obtain information on new vehicle purchases from proprietary DMV registration data. Our exogenous cost shifter is the distance a consumer must travel to visit a dealership. Our results show that the benefit provided by dealerships to consumers is substantial. Failure to estimate the standard deviation of the product fit leads to biased search cost and consumer surplus estimates and to inaccurate predictions regarding the number of searches consumers conduct.

Political Storms: Emergent Partisan Skepticism of Hurricane Risks
Joint with Elisa Long and Ryne Rhola
Science Advances, September 2020
Mistrust of scientific evidence and government-issued guidelines is increasingly correlated with political affiliation. Survey evidence has documented skepticism in a diverse set of issues including climate change, vaccine hesitancy, and, most recently, COVID-19 risks. Less well understood is whether these beliefs alter high-stakes behavior. Combining GPS data for 2.7 million smartphone users in Florida and Texas with 2016 U.S. presidential election precinct-level results, we examine how conservative-media dismissals of hurricane advisories in 2017 influenced evacuation decisions. Likely Trump-voting Florida residents were 10 to 11 percentage points less likely to evacuate Hurricane Irma than Clinton voters (34% versus 45%), a gap not present in prior hurricanes. Results are robust to fine-grain geographic controls, which compare likely Clinton and Trump voters living within 150 m of each other. The rapid surge in media-led suspicion of hurricane forecasts—and the resulting divide in self-protective measures—illustrates a large behavioral consequence of science denialism.

News and Geolocated Social Media Accurately Measure Protest Size
Joint with Zachary C. Steinert-Threlkeld, Anton Sobolev, and Jungseock Joo
American Political Science Review, November 2020
Larger protests are more likely to lead to policy changes than small ones are, but whether or not attendance estimates provided in news or generated from social media are biased is an open question. This letter closes the question: news and geolocated social media data generate accurate estimates of protest size variation. This claim is substantiated using cellphone location data from more than 10 million individuals during the 2017 United States Women's March protests. These cellphone estimates correlate strongly with those provided in news media as well as three size estimates generated using geolocated tweets, one text-based and two based on images. Inferences about protest attendance from these estimates match others' findings about the Women's March.

The Value of Flexible Work: Evidence from Uber Drivers
Joint with Judy Chevalier, Peter Rossi, and Emily Oehlsen
Journal of Political Economy, December 2019
Participation in flexible contract work has increased dramatically over the last decade, often in settings where new technologies lower the transaction costs of providing labor flexibly. One prominent example of this is the ride-sharing company Uber, which allows drivers to provide (or not provide) rides anytime they are willing to accept prevailing prices for this service. An Uber-style arrangement offers workers flexibility in both setting a customized work schedule and also adjusting it throughout the day. Using high-frequency data of hourly earnings for Uber drivers, we document the ways in which drivers utilize this real-time flexibility and we estimate the driver surplus generated by this flexibility. We estimate how drivers' reservation wages vary in high frequency from hour to hour, which allows us to study the surplus and supply implications of both flexible and traditional work arrangements. Our results indicate that, while the Uber relationship may have other drawbacks, Uber drivers benefit significantly from real-time flexibility, earning more than twice the surplus they would in less flexible arrangements. If required to supply labor inflexibly at prevailing wages, they would also reduce the hours they supply by more than two-thirds. The implications of our findings for the future of flexible work are discussed.

The Effect of Partisanship and Political Advertising on Close Family Ties (or the final published version here)
Joint with Ryne Rhola
Science, June 2018
Awarded a Nature: Editor's Choice. Additional analyses and tables are in the supplementary materials, data and code are here, and full-size graphics and slides are here.
Research on growing American political polarization and antipathy primarily studies public institutions and political processes, ignoring private effects including strained family ties. Using anonymized smartphone-location data and precinct-level voting, we show that Thanksgiving dinners attended by opposing-party precinct residents were 30-50 minutes shorter than same-party dinners. This decline from a mean of 257 minutes survives extensive spatial and demographic controls. Dinner reductions in 2016 tripled for travelers from media markets with heavy political advertising—an effect not observed in 2015—implying a relationship to election-related behavior. Effects appear asymmetric: while fewer Democratic-precinct residents traveled in 2016 than 2015, political differences shortened Thanksgiving dinners more among Republican-precinct residents. Nationwide, 34 million person-hours of cross-partisan Thanksgiving discourse were lost in 2016 to partisan effects.

Future Tense and Economic Decisions: Controlling for Cultural Evolution
Joint with Sean Roberts and James Winters
July 2015
A previous study by Chen demonstrates a correlation between languages that grammatically mark future events and their speakers' propensity to save, even after controlling for numerous economic and demographic factors. The implication is that languages which grammatically distinguish the present and the future may bias their speakers to distinguish them psychologically, leading to less future-oriented decision making. However, Chen's original analysis assumed languages are independent. This neglects the fact that languages are related, causing correlations to appear stronger than is warranted (Galton's problem). In this paper, we test the robustness of Chen's correlations to corrections for the geographic and historical relatedness of languages. While the question seems simple, the answer is complex. In general, the statistical correlation between the two variables is weaker when controlling for relatedness. When applying the strictest tests for relatedness, and when data is not aggregated across individuals, the correlation is not significant. However, the correlation did remain reasonably robust under a number of tests. We argue that any claims of synchronic patterns between cultural variables should be tested for spurious correlations, with the kinds of approaches used in this paper. However, experiments or case-studies would be more fruitful avenues for future research on this specific topic, rather than further large-scale cross-cultural correlational studies.

The Effect of Language on Economic Behavior: Evidence from Savings Rates, Health Behaviors, and Retirement Assets
American Economic Review, April 2013
Editor’s choice: Science Magazine, Vol 339(4)
Additional analyses and tables in an online appendix; data and code are in a zip file here.
Languages differ widely in the ways they partition time. In this paper I test the hypothesis that languages that do not grammatically distinguish between present and future events (what linguists call weak-FTR languages) lead their speakers to take more future-oriented actions. First, I show how this prediction arises naturally when well-documented effects of language on cognition are merged with models of decision making over time. Then, I show that consistent with this hypothesis, speakers of weak-FTR languages save more, hold more retirement wealth, smoke less, are less likely to be obese, and enjoy better long-run health. This is true in every major region of the world and holds even when comparing only demographically similar individuals born and living in the same country. The evidence does not support the most obvious forms of common causation. I discuss implications of these findings for theories of intertemporal choice.

Are Women Overinvesting in Education? Evidence from the Medical Profession
Joint with Judith Chevalier
Journal of Human Capital, Summer 2012
Recent literature has documented that women earn significantly lower returns than men to investing in professional degrees. However, these papers have not addressed the question of whether this gap is large enough to render professional degrees poor financial investments for women. To study this, we examine whether becoming a physician is a positive net-present-value investment for women. We sidestep some selection issues associated with measuring the returns to education by comparing physicians to physician assistants, a similar profession with lower wages but much lower up-front training costs. We find that the median female (but not male) primary-care physician would have been financially better off becoming a physician assistant. This result is partially due to a gender-wage gap in medicine. However, it is mostly driven by the fact that the median female physician simply doesn't work enough hours to amortize her upfront investment in medical school. In contrast, the median male physician work many more hours, easily enough to amortize his up-front investment. We discuss the robustness of our results to other medical specialties and their relevance to gender-wage gaps more broadly. We discuss other sources of returns to education that rationalize these investments by women.

Intertemporal Choice and Legal Constraints
Joint with Alan Schwartz
American Law and Economic Review, Spring 2012
We study the effect of legal constraints in an environment in which agents face demand shocks they would like to smooth, but also have weakness of will: agents' long and short run preferences are misaligned. Some agents are sophisticated—they know they will make inconsistent intertemporal choices—while other agents are naive. The consequent public policy problem is complex. The state should facilitate consumer borrowing to help agents smooth consumption and cushion the effect of shocks, but should also facilitate pre-commitment, to help agents control excessive present-biased preferences. We show that in many simple settings, naive and sophisticated agents make similar consumption/savings choices, which simplifies the policy problem. We also show that all agents borrow when they experience consumption shocks, and that agents with relatively strong present-biased preferences who face relatively mild consumption shocks will borrow to finance excessive current consumption. Other agents save appropriately. Legal constraints that severely restrict agents' access to credit thus would be over-inclusive. Offering agents access to both a liquid and an illiquid savings vehicle appears to be welfare improving relative to either allowing agents complete freedom to borrow or strongly restricting their access to the credit market. Creating and regulating such vehicles are public goods that the market will not supply.

The Evolution of Decision-Making Under Risk: Framing Effects in Monkey Risk Preferences
Joint with Venkat Lakshminarayanan & Laurie Santos
Journal of Experimental Social Psychology, May 2011
When making choices between risky options, human decision-makers exhibit a number of framing effects. One of the most prominent framing effects is the tendency for decisionmakers to evaluate gambles relative to a reference point, and to act risk-seeking when prospects are framed as losses but risk-averse when identical prospects are framed as gains. This tendency for risk-preferences to reverse between loss and gain frames has been termed the reflection effect, and is one of the primary predictions of Prospect Theory. Here, we explore whether non-human primates exhibit a similar reflection effect. Using a token-trading task, we show that capuchin monkeys (Cebus apella) exhibit an analogous reversal of risk preferences depending on whether outcomes are presented as gains or losses, suggesting that similar framing effects also influence choice in non-human primates. This finding suggests that the mechanisms that drive framing effects in humans may be evolutionarily ancient, extending broadly across the primate order.

How to Study Choice-Induced Attitude Change: Strategies for Fixing the Free-Choice Paradigm
Joint with Jane Risen
Social and Personality Psychology Compass, December 2010
The theory of cognitive dissonance has been among the most influential theories in social psychology for the last 50 years. Support for the theory has come primarily from three experimental paradigms: free-choice, induced compliance, and effort justification. Recently, Chen and Risen (2010, Journal of Personality and Social Psychology, 99, 573-594) have argued that although the free-choice paradigm reliably finds a "spreading of alternatives" (i.e., after making a choice, participants' evaluations of the chosen item improve and evaluations of the rejected item decline), these results cannot be interpreted as evidence for dissonance reduction or attitude change. Unlike the other dissonance paradigms, participants self-select how they are treated in the free-choice paradigm, making it impossible to know whether spreading is because of the choice process or the information that is revealed about participants' existing preferences. The current paper has two goals. First, we will describe the criticism developed by Chen and Risen (2010) and situate the criticism within the broader study of dissonance. Second, we will offer four suggestions for how researchers can isolate the effect of the choice process and properly test for choice-induced attitude change in the free-choice paradigm.

How Choice Affects and Reflects Preferences: Revisiting the Free-Choice Paradigm
Joint with Jane Risen
Journal of Personality and Social Psychology, October 2010
After making a choice between two objects, people evaluate their chosen item higher and their rejected item lower (i.e., they "spread" the alternatives). Since Brehm's (1956) initial free-choice experiment, psychologists have interpreted the spreading of alternatives as evidence for choice-induced attitude change. It is widely assumed to occur because choosing creates cognitive dissonance, which is then reduced through rationalization. In this paper, we express concern with this interpretation, noting that the free-choice paradigm (FCP) will produce spreading, even if people's attitudes remain unchanged. Specifically, if people's ratings/rankings are an imperfect measure of their preferences, and their choices are at least partially guided by their preferences, then the FCP will measure spreading, even if people's preferences remain perfectly stable. We show this, first, by proving a mathematical theorem that identifies a set of conditions under which the FCP will measure spreading, even absent attitude change. We then experimentally demonstrate that these conditions appear to hold, and that the FCP measures a spread of alternatives, even when this spreading cannot have been caused by choice. We discuss how the problem we identify applies to the basic FCP paradigm as well as to all variants that examine moderators and mediators of spreading. The results suggest a reassessment of the free-choice paradigm, and perhaps, the conclusions that have been drawn from it.

This paper builds on an earlier working paper:
Rationalization and Cognitive Dissonance: Do Choices Affect or Reflect Preferences?

Is Choice a Reliable Predictor of Choice? A Comment on Sagarin and Skowronski
Joint with Jane Risen
Journal of Experimental Social Psychology, February 2009
This is a response to a paper published in the JESP, which is a critique of a working paper of mine, "Rationalization and Cognitive Dissonance: Do Choices Affect or Reflect Preferences?". That working paper is here, and the critique of it which I am responding to is here.
In a recent working paper, Chen argues that a methodology central to the cognitive dissonance literature (the free-choice paradigm) has suffered from an inability to separately measure how much choices affect people's preferences, and how much they simply reflect those preferences, by failing to fully control for the fact that subjects tend to choose goods they prefer (Chen 2008). Although Sagarin and Skowronski concede this, they discount Chen's argument, claiming that for revealed preferences to completely account for observed choice-effects the relationship between choice and preference would have to be unrealistically high. In this comment, we argue that their critique both misses the crux of Chen's analysis, and is incorrect. Specifically, to properly test whether choices affect preferences, it is essential that researchers experimentally control for revealed preferences rather than speculate how much of a role they may play. Moreover, Sagarin and Skowronski's critique rests on two fundamental errors—a misunderstanding of the function of the null-hypothesis and a misunderstanding of preference-measurement psychometrics. These errors leads S&S to suggest alternative experimental designs which while a good first step, do not address the problems identified by Chen (2008).

The Evolution of Rational and Irrational Economic Behavior: Evidence and Insight from a Non-human Primate Species
Joint with Laurie Santos
This is a book chapter from Neuroeconomics: Decision Making and the Brain,
edited by Paul Glimcher, Colin Camerer, Ernst Fehr, and Russell Poldrack.
Academic Press: Elsevier, 2009
The book can be browsed and ordered here, we are chapter 7.

The Endowment Effect in Capuchin Monkeys
Joint with Venkat Lakshminarayanan & Laurie Santos
Philosophical Transactions of the Royal Society: Biological Sciences, December 2008
In humans, the capacity for economically rational choice is constrained by a variety of preference biases: humans evaluate gambles relative to arbitrary reference points; weigh losses heavier than equally sized gains; and demand a higher price for owned goods than for equally preferred goods that are not yet owned. To date, however, fewer studies have examined the origins of these biases. Here, we review previous work demonstrating that human economic biases such as loss aversion and reference dependence are shared with an ancestrally related New World primate, the capuchin monkey (Cebus apella). We then examine whether capuchins display an endowment effect in a token trading task. We identified pairs of treats (fruit discs versus cereal chunks) that were equally preferred by each monkey. When given a chance to trade away their owned fruit discs to obtain the equally valued cereal chunks (or vice versa), however, monkeys required a far greater compensation than the equally preferred treat. We show that these effects are not due to transaction costs or timing issues. These data suggest that biased preferences rely on cognitive systems that are more evolutionarily ancient than previously thought—and that common evolutionary ancestry shared by humans and capuchins may account for the occurrence of the endowment effect in both species.

Modeling a Presidential Prediction Market
Joint with Jonathan E. Ingersoll and Edward H. Kaplan
Management Science, August 2008
Prediction markets now cover many important political events. The 2004 presidential election featured an active prediction market at where securities addressing many different election-related outcomes were traded. Using the 2004 data from this market, we examined three alternative models for these security prices with special focus on the electoral college rules that govern US presidential elections to see which models are more (or less) consistent with the data. The data reveal dependencies in the evolution of the security prices across states over time. We show that a simple diffusion model provides a good description of the overall probability distribution of electoral college votes, while an even simpler ranking model provides excellent predictions of the probability of winning the presidency. Ignoring dependencies in the evolution of security prices across states leads to considerable underestimation of the variance of the number of electoral college votes received by a candidate, which in turn leads to overconfidence in predicting whether or not that candidate wins the election. Overall, the security prices in the Intrade presidential election prediction market appear jointly consistent with probability models that satisfy the rules of the electoral college.

The Taste for Leisure, Career Choice, and the Returns to Education
Joint with Judith Chevalier
Economics Letters, May 2008
We develop a simple methodology to estimate the returns to education despite heterogeneous labor/leisure preferences. The labor supply behavior of doctors and physician assistants is consistent with people choosing between the two careers based on differing tastes for leisure.

This paper previously appeared as the working paper:
The Taste for Leisure, Career Choice, and the Returns to Education: Evidence from the Medical Field
Joint with Judith Chevalier

Do Harsher Prison Conditions Reduce Recidivism? A Discontinuity-Based Approach
American Law and Economics Review Distinguished Article Prize of 2008
Joint with Jesse Shapiro
American Law and Economic Review, June 2007
We estimate the causal effect of prison conditions on recidivism rates by exploiting a discontinuity in the assignment of federal prisoners to security levels. Inmates housed in higher security levels are no less likely to recidivate than those housed in minimum security; if anything, our estimates suggest that harsher prison conditions lead to more post-release crime. Though small sample sizes limit the precision of our estimates, we argue that our findings may have important implications for prison policy, and that our methodology is likely to be applicable beyond the particular context we study.

How Basic are Behavioral Biases? Evidence from Capuchin-Monkey Trading Behavior
Joint with Venkat Lakshminarayanan & Laurie Santos
Journal of Political Economy, June 2006
Behavioral economics has demonstrated systematic decision-making biases in both lab and field data. Do these biases extend across contexts, cultures, or even species? We investigate this question by introducing fiat currency and trade to a colony of capuchin monkeys, and recovering their preferences over a range of goods and gambles. We show that capuchins react rationally to both price and wealth shocks, but display several hallmark biases when faced with gambles, including reference-dependence and loss-aversion. Given our capuchins' inexperience with trade and gambles, these results suggest that loss-aversion extends beyond humans, and may be innate rather than learned.

This paper previously appeared as the working paper:
The Evolution of Our Preferences: Evidence from Capuchin-Monkey Trading Behavior
Joint with Venkat Lakshminarayanan & Laurie Santos

Some Thoughts on the Adaptive Function of Inequity Aversion: An Alternative to Brosnanís Social Hypothesis
Joint with Laurie Santos
Social Justice Research, June 2006
In this commentary, we review and question Brosnan's hypothesis that inequity aversion (IA) evolved as a domain-specific social mechanism. We then outline an alternative, domain-general, account of IA. As opposed to Brosnan's social hypothesis, we propose that IA evolved from more general reward mechanisms. In particular, we argue reference-dependence and loss-aversion can account for the evolution of IA in primates. We discuss recent work on reference-dependence and explore how it may have given rise to inequality-averse behavior in social settings. We conclude with suggestions for future work examining the proximate mechanisms that give rise to IA.

Modeling Reciprocation and Cooperation in Primates: Evidence for a Punishing Strategy
Joint with Marc Hauser
Journal of Theoretical Biology, May 2005
Quicktime Movie Clip: Tamarins playing games (Tamarins play a repeated game, pulling to provide the other with food)
Experiments in which animals interact strategically with a conspecific or search over some experimentally constructed domain are becoming increasingly common. While these experiments hold the promise of illuminating surprisingly sophisticated behavior, the analysis of the resulting data is often quite coarse. For example, simply tallying the number of observations consistent with a particular behavioral theory fails to utilize systematic variation among observations inconsistent with the theory. Using a new data set generated by cotton-top tamarin monkeys playing a repeated food-exchange game, we apply a maximum-likelihood estimation technique (more commonly used to study human economic behavior) which utilizes much more of the information in these data, and which uncovers unexpectedly sophisticated cooperative behavior from our subjects. Tamarin cooperation remains stable as long as there are no more than two consecutive defections by one player, a strategy that resembles tit-for-two-tats. We present these supplementary results to our previous work on cooperation, detail the basic framework of our data analysis, enumerate the benefits of a maximum-likelihood approach in experimental settings such as ours, and suggest other areas in which these techniques may be fruitful.

Give Unto Others: Genetically Unrelated Cotton-Top Tamarin Monkeys Preferentially Give Food to Those Who Altruistically Give Food Back
Joint with Marc Hauser, Frances Chen & Emmeline Chuang
Proceedings of the Royal Society, Nov 2003
Quicktime Movie Clip: Tamarins playing games (Tamarins play a repeated game, pulling to provide the other with food)
Altruistic food giving among genetically unrelated individuals is rare in nature. The few examples that exist suggest that when animals give food to unrelated others, they may do so on the basis of mutualistic or reciprocally altruistic relationships. We present the results of four experiments designed to tease apart the factors mediating food giving among genetically unrelated cotton-top tamarins [Saguinus oedipus], a cooperatively breeding New World primate. In Experiment 1 we show that individuals give significantly more food to a trained conspecific who unilaterally gives food than to a conspecific who unilaterally never gives food. The apparent contingency of the tamarins’ food giving behavior motivated the design of Experiments 2-4. Results from all three experiments show that altruistic food giving is mediated by prior acts of altruistic food giving by a conspecific. Specifically, tamarins do not give food to unrelated others when the food received in the past represents the byproduct of another’s selfish actions (Experiments 2,3) or when a human experimenter gives them food (Experiment 4) as did the unilateral altruist in Experiment 1. In contrast, if one tamarin gives another food without obtaining any immediate benefit, then the recipient is more likely to give food in return. Overall, these results provide clear evidence of altruistic food giving among genetically unrelated individuals, indicate that tamarins discriminate between altruistic and selfish actions, and provide the necessary foundation for the evolution of reciprocal altruism.


Working Papers:

One-Way Essential Complements
Joint with Barry Nalebuff
While competition between firms producing substitutes is well understood, less is known about rivalry between firms producing complements. In this paper we study markets for one-way essential complements, where one good is essential to the use of the other but not vice versa. For example, an operating system is necessary for the software that runs on it, just as many speakers depend on a consumer owning an iphone. When separate firms produce the essential and the dependant good, how is surplus divided, and can a monopolist in the essential good leverage that monopoly into the market for dependant goods? Related to this is the incentive for firms to create complements to the essential good. We develop a general framework for analyzing markets with one-way complementarities, and compare our results with traditional results from the literatures on bundling, monopoly price discrimination, and versioning. Among other results, we show that a essential monopolist has no incentive to degrade rival complementary products, which suggests that a monopoly internet service provider will offer net neutrality.