Abstract
We study how heat over early childhood affects foundational numeracy in ten sub-Saharan African countries. We link the sixth round of the Multiple Indicator Cluster Surveys (MICS6), which tests children aged 7 to 14 at home, to daily ERA5 temperatures, and follow each child’s exposure to hot days from conception to the test date, comparing children born at different times in the same location. A one standard deviation increase in hot days lowers numeracy by 0.014 SD in the first 1,000 days and by 0.012 SD at ages 2 to 5, about one month of learning in each case; heat after age 6 has no detectable effect. The similar effects in both windows point to accumulation rather than a single sensitive period. The timing of heat determines who bears the cost: losses from heat in the first 1,000 days are similar across households, while losses from preschool-age heat fall on poorer, rural, and male children and children of less-educated mothers. Sibling comparisons suggest that parents respond to heat in the preschool years, shifting more-exposed children toward private or religious schools, but not to heat in the first 1,000 days.