Working Paper: NBER ID: w19233
Authors: James Feyrer; Dimitra Politi; David N. Weil
Abstract: Iodine deficiency is the leading cause of preventable mental retardation in the world today. The condition, which was common in the developed world until the introduction of iodized salt in the 1920s, is connected to low iodine levels in the soil and water. We examine the impact of salt iodization on cognitive outcomes in the US by taking advantage of this natural geographic variation. Salt was iodized over a very short period of time beginning in 1924. We use military data collected during WWI and WWII to compare outcomes of cohorts born before and after iodization, in localities that were naturally poor and rich in iodine. We find that for the one quarter of the population most deficient in iodine this intervention raised IQ by approximately one standard deviation. Our results can explain roughly one decade's worth of the upwardtrend in IQ in the US (the Flynn Effect). We also document a large increase in thyroid related deaths following the countrywide adoption of iodized salt, which affected mostly older individuals in localities with high prevalence of iodine deficiency.
Keywords: iodine deficiency; cognitive outcomes; salt iodization; Flynn effect; public health
JEL Codes: I18; I28; J24; N32
Edges that are evidenced by causal inference methods are in orange, and the rest are in light blue.
| Cause | Effect |
|---|---|
| cognitive improvement (D91) | Flynn effect (I25) |
| military draft data (H56) | causal link (C22) |
| salt iodization (L50) | IQ scores (C43) |
| iodized salt (L66) | thyroid-related deaths (I12) |
| iodine deficiency (I12) | cognitive scores (D91) |