Working Papers

Market Segmentation in Informal Public Transportation: Evidence from Johannesburg's Minibus Taxi Networks  [JMP]
(with F. Christopher Eaglin)

How does spatial market segmentation affect firms' ability to meet demand across space? We study the market for public transportation in Johannesburg, South Africa, where private associations of minibus owners segment the city into distinct territories. In contrast, the demand for urban mobility is inherently interconnected, with a quarter of commuter trips originating in one association’s territory and ending in another’s. We study the frictions that associations face on these "between-territory" routes. Using GPS traces for over 40 million minibus trips and 9 million commuter trips, we present two complementary empirical results that quantify these frictions. First, we use an expected, cyclic mobility demand shock – the sharp increase in recreational mobility following monthly pay dates – to trace out the supply curve of minibus services by route type. The supply elasticity is close to 1 on routes contained within an association’s territory but is significantly lower on between-territory routes (0.4). We estimate that if between-territory supply were as elastic as "within-territory" supply, aggregate wait time for commuters would decrease by one million minutes per day, or approximately 4 minutes per trip. In our second exercise, we use exogenous fleet reductions due to bus breakdowns and repossessions to show that associations prioritize maintaining service on between-territory routes over within-territory routes, indicating that between-territory routes are more profitable at the margin. In a model of minibus allocation, our observed empirical patterns correspond to more convex costs on between-territory routes, reflecting the need for associations to coordinate with each other on these routes.

The Role of the 'Fare' in Welfare: Public Transportation Subsidies and their Effects on Low-Income Households
(with Seth Chizeck)
Honorable Mention, UEA Prize for Best Student Paper, 2024

Can reducing public transit fares improve mobility and socioeconomic outcomes for low-income individuals? We conduct a randomized experiment that offers fare discounts to 9,544 low-income households in one large U.S. county. Households are randomly assigned to receive either no discount, a 50% discount, or a 100% discount on all public transit trips for 16 to 19 months. We measure participants' mode-specific travel behavior using a combination of smartphone GPS data, high-frequency surveys, and farecard transactions. GPS data indicates that free fares increase transit ridership by 43% relative to status quo prices, accompanied by a decrease in private vehicle trips. Half-price fares yield no change in transit ridership. There is suggestive evidence that fare reductions decrease the overall frequency and spatial breadth of travel, implying the need for other measures when quantifying improvements in mobility. Our confidence intervals rule out increases of more than 3.2 percentage points in the likelihood of being employed during the first year, and we rule out increases in first-year earnings of more than $864. We find minimal downstream effects on health care consumption, social services receipt, or self-reported health and well-being. Fare prices appear to play a limited role in the socioeconomic lives of poor families.

Demand for Urban Exploration: Evidence from Nairobi
(with Joshua Dean and Gabriel Kreindler)

Growing cities in low- and middle-income countries offer increased market access, yet taking advantage of urban opportunities requires that residents explore  unfamiliar surroundings. We study urban exploration in a sample of 800 casual workers in Nairobi. The median person commutes 7.8 km but has never been to half the neighborhoods within 75 minutes from where they live. To quantify preferences for familiar locations, we offer short-term employment and experimentally induce familiarity by training participants in either familiar or unfamiliar locations. Participants are willing to travel 3.5 km further or take a pay cut worth 22% of the median daily wage to work in a familiar location. This familiarity premium is fully offset after one visit to an unfamiliar neighborhood. These effects persist for a different paid opportunity 2-4 months after the intervention, and participants report visiting training neighborhoods outside our study. Our results are consistent with unfamiliarity making people view neighborhoods more negatively, and we find little evidence that navigation costs or exploration risk can explain these results. We use additional job choice elicitation methods to demonstrate two further barriers to exploration. We show that participants only partially anticipate that one visit will eliminate the familiarity premium, and that participants are less likely to consider unfamiliar neighborhoods  as locations to visit. Our results suggest that one-time exploration frictions are an important component of urban mobility costs in cities like Nairobi.

Nudging Parents out the Door: The Impacts of Parental Encouragement on School Choice and Test Scores
(with Guthrie Gray-Lobe, Michael Kremer, Joost de Laat, and Cole Scanlon)

This study evaluates a large-scale SMS outreach program to engage caregivers of students in private primary schools in Kenya. Using a two-stage randomization design, we tested two types of weekly SMS messages: growth mindset encouragement and personalized performance information. We find two main effects: First, outreach improved test scores by 0.07 standard deviations, with particularly strong gains among initially lower-performing students. This improvement generates 12 learning-adjusted years of schooling per US$100 spent—making it highly cost-effective relative to other education interventions. Second, outreach increased student exit rates by 4.6-5.0 percentage points, with effects concentrated among higher-achieving students (6.3-8.8 percentage points). We develop a theoretical model of vertically differentiated schools where parental engagement affects both learning production and school choice. The model shows that when parents update their understanding of education production through engagement programs, they become more sensitive to perceived school quality differences. This increased sensitivity can lead lower-quality schools to forgo implementing engagement programs—even when costless—as enhanced parental discernment accelerates student exits. Our findings suggest a role for third-party provision of parent engagement programs in competitive education markets.

Works In Progress

Motorcycle Taxis and Urban Congestion: Evidence from Nairobi's "Boda Boda" Ban

Publications

Mandatory Retirement and Age, Race, and Gender Diversity of University Faculties
(with Daniel Ho and Anne McDonough), American Law and Economic Review, 2021
ALER Best Empirical Paper Award, 2022
Replication data and code

While many have documented the changing demographics of universities, understanding the effects of prohibiting mandatory retirement (“uncapping”) has proved challenging. We digitize detailed directories of all American law school faculty from 1971–2017 and show that uncapping in 1994 had dramatic effects. From 1971 to 1993, the percent of faculty above 70—when mandatory retirement would typically have been triggered—remained stable at 1%, but starting in 1994, that proportion increased to 14%. We use a permutation test of moving cohorts to show that these increases are attributable to uncapping. Roughly 39% of faculty members would counterfactually have been subject to mandatory retirement. Effects were less pronounced at public schools, which were more likely to have defined benefits retirement plans. Second, we show that schools with the highest proportion of faculty over 70, and thus most impacted by uncapping, also exhibit the slowest integration of female and minority faculty members. Our study highlights crosscutting effects of civil rights laws: preventing age discrimination can have collateral effects on racial and gender integration. 

Menu labeling, Calories, and Nutrient Density: Evidence from Chain Restaurants
(with Daniel Ho, Rebecca Potash, and Anne McDonough), PLOS One, 2020

The Food and Drug Administration’s menu labeling rule requires chain restaurants to prominently display calories, while leaving other nutritional information (e.g., fat, sodium, sugar) to the request of consumers. We use rich micronutrient data from 257 large chain brands and 24,076 menu items to examine whether calories are correlated with widely used “nutrient profile” scores that measure healthfulness based on nutrient density. We show that calories are indeed statistically significant predictors of nutrient density. However, as a substantive matter, the correlation is highly attenuated (partial R2 < 0.01). Our findings (a) suggest that the promise of calorie labeling to improve nutrient intake quality at restaurants is limited and (b) clarify the basis for transparency of nutrient composition beyond calories to promote healthy menu choices.