The effect of Universal Salt Iodization on cognitive test scores in rural India

This paper studies the impact of a large-scale public health intervention in early life on cognitive skills in childhood. Iodine deficiency is the most common predictor of brain damage globally which has prompted over 140 countries to implement Universal Salt Iodization. While small-scale interventions report positive effects of iodine supplementation on cognition, the causal impact of salt iodization at scale is unknown across low-income countries. This study evaluates the effect of Universal Salt Iodization on cognitive test scores of school-aged children in rural India. I exploit exogenous variation in the timing of the exposure to the policy in early life, comparing children residing in naturally iodine sufficient and deficient districts over time, using a difference-in-differences strategy. Exposure to the program increased basic numeracy and literacy skills by at least 2.4 percentage points and improved school progression. It further raised literacy scores by 6.1% of a standard deviation for girls. The effects on test scores are higher for poor children and for those residing in, or nearby, the major salt producing state where iodized salt consumption was lower at baseline. This is the first study to show that a blanket fortification policy can deliver considerable, yet heterogenous, improvements in cognition in the medium run in a developing country context.