Ambient Light and Homicides

Abstract

This paper investigates the influence of ambient light on number of homicides by firearms in one of the most violent countries in the world. As exogenous variation in daylight hours we exploit the transition from Standard to Daylight Saving Time (DST) in some but not all Brazilian states to provide both within and between-states comparison around the transition. Using non-parametric regression discontinuity techniques, we find a sharp decrease in the number of homicides by firearms in Brazilian treated states (14.4%), while no statistical relationship is found among states not affected by the policy. The uncovered effect is mostly concentrated on hours directly affected by the shift in daylight. Moreover, comparing treated to untreated states in a Differences-in-Differences approach, we find that homicides by firearms decreased during DST months by about 3%. These estimates imply Daylight Saving Time is responsible for saving about 3,850 potential victims from 2006-2011.