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. 2017 Apr;15(2):355-387.
doi: 10.1093/jeea/jvw002. Epub 2017 Apr 8.

The Cognitive Effects of Micronutrient Deficiency: Evidence from Salt Iodization in the United States

James Feyrer  1 , Dimitra Politi  2 , David N Weil  3
Affiliations

The Cognitive Effects of Micronutrient Deficiency: Evidence from Salt Iodization in the United States

James Feyrer et al. J Eur Econ Assoc. 2017 Apr.

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 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 upward trend 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: Cognitive ability; Flynn Effect; human capital; productivity.

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Figures

Figure 1:
Figure 1:
Iodized salt advertisements
Figure 2:
Figure 2:. Distribution of goiter rates across World War I sections
Source: Love and Davenport (1920)
Figure 3:
Figure 3:. Goiter and iodine content of water
Source: McClendon (1924) and Love and Davenport (1920)
Figure 4:
Figure 4:. Percentage of Air Forces Recruits with above average AGCT scores
Source: US Air Force Historical Study #76: Classification and Assignment of Enlisted Men in the Air Arm, 1917–1945.
Figure 5:
Figure 5:. AGCT scores for AAF versus all other enlistees
Source: National Archives and Records Administration (2002)
Figure 6:
Figure 6:. Air Force demand and selectivity in WWII
Source: National Archives and Records Administration (2002)
Figure 7:
Figure 7:. US deaths from exophthalmic goiter, 1910–1960
Source: Grove and Hetzel (1968)
Figure 8:
Figure 8:. Deaths from exophthalmic goiter, by state group, 1920–1940
Source: U.S. Department of Commerce (various years)
Figure 9:
Figure 9:. Probability of joining the Air Forces
Source: National Archives and Records Administration (2002) and Love and Davenport (1920)
Figure 10:
Figure 10:. Residual probability of joining the Air Forces
Source: National Archives and Records Administration (2002) and Love and Davenport (1920)
See this image and copyright information in PMC

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