This article relies on a large-scale field experiment in Mexico to measure the effects of two ability-grouping models (tracking and heterogeneous/bimodal groups) on student learning outcomes during middle school. Both strategies yielded an average learning gain of 0.08 of a standard deviation....

We explore how social network exposure to COVID-19 cases shapes individuals' social distancing behavior during the early months of the ongoing pandemic. We work with de-identified data from Facebook to show that U.S. users whose friends live in areas with worse coronavirus outbreaks reduce their...