See Scrimshaw 1998_26ya; Bleichrodt & Born 1994_30ya; and Qian et al 2005:

Thirty-seven reported studies, total 12,291 children, were analysed. The effect size was an increase of 0.83, 0.82, and 0.32 SIQP respectively, for the children living in IS communities compared with those living in ID areas with no iodine supplementation, with inadequate iodine supplementation, or children who had received iodine during their mothers’ pregnancy and after birth. These equal to 12.45, 12.3, 4.8 IQ points. Compared with that of children whose mothers were persistently exposed to ID, the total effect size of the 21 entries was an increase of 0.58 SIQP (8.7 IQ points) in the group receiving iodine supplementation during pregnancy. Furthermore, there was an increase on 1.15 SIQP of Binet or 0.8 SIQP on Raven Scale (17.25 or 12 IQ points) for children born more than 3.5 years after iodine supplementation program was introduced. The level of iodine nutrition plays a crucial role in the intellectual development of children. The intelligence damage of children exposed to severe ID was profound, demonstrated by 12.45 IQ points loss and they recovered 8.7 IQ points with iodine supplementation or IS before and during pregnancy

“IQ in the Ramsey Model: A Naïve Calibration”, Jones 2006_18ya:

In a recently published paper, Erica Field, Omar Robles, and Maximo Torero study the effects of iodine deficiency on educational attainment in Tanzania (Field, Robles and Torero2009). Theirs is the first published paper to link economically [important] outcomes to iodine deficiency, and it supports the idea of improved schooling outcomes after iodine supplementation. Interestingly, Field et al. also find bigger effects for females, which could be due either to biological differences or differences associated with intra-household resource allocation and social responses to increased cognition of males and females.

The original:

Our findings suggest that reducing fetal IDD has [substantial] benefits for child cognition: Children who receive iodine in utero attain an average of 0.33 years of education above siblings and older and younger children in their district. Furthermore, the effects appear to be substantially larger for girls, consistent with new evidence from laboratory studies in animals indicating greater cognitive sensitivity of the female fetus to in utero iodine deprivation, including sex-specific responses to maternal thyroid hormone restriction on expression of neural thyroid hormone receptors. The results are consistent across household and district fixed effects models and patterns of variation in estimated effects are consistent with predictions regarding the vulnerability of specific subpopulations to fetal IDD. Cross-country regression estimates indicate a strong negative influence of total goiter rate and strong positive influence of salt iodization on female school participation.

An example of the commercial opposition or conspiracy theorists is the Kazakhstan campaign; from “In Raising the World’s I.Q., the Secret’s in the Salt”, NYT:

In fact, Kazakhstan has become an example of how even a vast and still-developing nation like this Central Asian country can achieve a remarkable public health success. In 1999_25ya, only 29% of its households were using iodized salt. Now, 94% are. Next year, the United Nations is expected to certify it officially free of iodine deficiency disorders…Largely out of the public eye, they made terrific progress: 25% of the world’s households consumed iodized salt in 1990_34ya. Now, about 66% do.

…In some nations, iodization becomes tarred as a government plot to poison an essential of life—salt experts compare it to the furious opposition by 1950s conservatives to fluoridation of American water. In others, civil libertarians demand a right to choose plain salt, with the result that the iodized kind rarely reaches the poor. Small salt makers who fear extra expense often lobby against it. So do makers of iodine pills who fear losing their market. Rumors inevitably swirl: iodine has been blamed for AIDS, diabetes, seizures, impotence and peevishness. Iodized salt, according to different national rumor mills, will make pickled vegetables explode, ruin caviar or soften hard cheese. Breaking down that resistance takes both money and leadership…The country’s biggest salt company was initially reluctant to cooperate, fearing higher costs, a Unicef report said. Cardiologists argued against iodization, fearing it would encourage people to use more salt, which can raise blood pressure. More insidious, Dr. Sharmanov said, were private companies that sold iodine pills. “They promoted their products in the mass media, saying iodized salt was dangerous,” he said, shaking his head…Asked about the Unicef report saying that Aral Tuz initially resisted iodization on the grounds that it would eat up 7% of profits, the company’s president, Ontalap Akhmetov, seemed puzzled. “I’ve only been president three years,” he said. “But that makes no sense.” The expense, he said, was minimal. “Only a few cents a ton.”

…In the 1990s, when the campaign for iodization began, the world’s greatest concentration of iodine-deficient countries was in the landlocked former Soviet republics of Central Asia. All of them—Kazakhstan, Turkmenistan, Tajikistan, Uzbekistan, Kyrgyzstan—saw their economies break down with the collapse of the Soviet Union. Across the region, only 28% of all households used iodized salt. “With the collapse of the system, certain babies went out with the bathwater, and iodization was one of them,” said Alexandre Zouev, chief Unicef representative in Kazakhstan…That combination is missing in many nations where iodine deficiency remains a health crisis. In nearby Pakistan, for instance, where 70% of households have no iodized salt, there are more than 600 small salt producers. [see “Pakistan takes legal action”, Unicef; for further reading, the WHO’s Assessment of iodine deficiency disorders and monitoring their elimination]

average daily Japanese consumption of iodine ranges from 5.28-13.8mg of iodine; Patrick L. Iodine: deficiency and therapeutic considerations, although Gaby argues this is a misinterpretation: “the idea that Japanese people consume 13.8 mg of iodine per day appears to have arisen from a misinterpretation of a paper. In that paper, the average intake of seaweed in Japan was listed as 4.6 g (4,600 mg) per day, and seaweed was said to contain 0.3% iodine. The figure of 13.8 mg comes from multiplying 4,600 mg by 0.003. However, the 4.6 g of seaweed consumed per day was expressed as wet weight, whereas the 0.3%-iodine figure based on dry weight. Since many vegetables contain at least 90% water, 13.8 mg per day is a [large] overestimate of iodine intake. In studies that have specifically looked at iodine intake among Japanese people, the mean dietary intake (estimated from urinary iodine excretion) was in the range of 330 to 500 mcg per day, which is at least 2.5-fold lower than 13.8 mg per day.” Abraham replies “The urinary excretions of iodide reported by Ishizuki et al in references 7 and 8 of Gaby’s editorial are obviously from patients who were told to abstain from seaweed. We will quote other studies by Japanese investigators who reported urinary iodide levels 10 to 100 time higher than the values reported by Ishizuki et al .”↩︎

“The Impact of Iodine Deficiency Eradication on Schooling: Evidence from the Introduction of Iodized Salt in Switzerland”, Politi2014 (see also Field et al 2009_15ya):

I identify the impact of iodization in three ways: first, in a differences-in-differences framework, I exploit geographic variation in iodine deficiency, as well as the fact that the nationwide campaign to decrease iodine deficiency began in 1922_102ya. Second, I use spatial and temporal variation in the introduction of iodized salt across Swiss cantons, and examine whether the level of iodized salt sales at the time of one’s birth affected one’s educational attainment. Third, I employ a fuzzy regression discontinuity design and use jumps in sales of iodized salt across Swiss cantons to identify the effect of iodization, by comparing outcomes for those born right before and right after these sudden changes in the treatment environment. These approaches indicate that the eradication of iodine deficiency in previously deficient areas increased the schooling of the population [statistically-]significantly. The effects are larger for females than for males, which is consistent with medical evidence showing that women are more likely to be affected by iodine deficiency disorders than men.

“Salt Iodization and the Enfranchisement of the American Worker”, Adhvaryu et al 2013_11ya:

…We find substantial impacts of salt iodization. High school completion rose by 6 percentage points, and labor force participation went up by 1 point. Analysis of income transitions by quantile shows that the new labor force joiners entered at the bottom of the wage distribution and took up blue collar labor, pulling down average wage income conditional on employment. Our results inform the ongoing debate on salt iodization in many low-income countries. We show that large-scale iodized salt distribution had a targeted impact, benefiting the worker on the margin of employment, and generating sizeable economic returns at low cost.

Politi2015, “The effects of the generalized use of iodized salt on occupational patterns in Switzerland”↩︎

“The Economic Effects of Micronutrient Deficiency: Evidence from Salt Iodization in the United States”, Feyrer et al 2008_16ya (quotes are from 2008_16ya draft, not 2013 final version):

For instance, Sachs (2003_21ya) shows that efforts to eliminate malaria have substantial effects on income, through their effect on health, reduced absenteeism, etc. Acemoglu & Johnson2007 look at the effect of health interventions and find little effect. Weil (2007_17ya) charts a middle course…For example, Bleakley (2007_17ya) examines the effects of hookworm eradication in the American South in early 20^th century and finds [substantial] effects on education and future incomes of those cohorts that benefited from the intervention.

• Bleakley, Hoyt, “Disease and Development: Evidence from Hookworm Eradication in the American South”, Quarterly Journal of Economics, February 2007_17ya, 122 (1), 73–117 [but see Roodman’s contrary analysis, “Questioning the evidence on hookworm eradication in the American South”, based on analyzing the de-aggregated data.]

…Figure 1 illustrates the geographic distribution of goiter in the US as measured among World War I recruits (we discuss the data further below). In 1924_100ya iodized salt was introduced in the United States explicitly to reduce the goiter rate. This intervention rapidly reduced the incidence of iodine deficiency. Iodization of salt in the US provides a particularly good natural experiment due to the geographic distribution of the disease combined with a rapid, complete treatment. Since there are large in utero effects of iodine deficiency, we should see a [substantial] difference between those born before and after the introduction of iodized salt in locations with low levels of environmental iodine. Those living in high iodine regions provide a control group…The probability of assignment to the Air Corps rises [substantially] in low iodine (ie. high goiter) counties in the years after the introduction of iodized salt. In the lowest iodine regions, our estimates suggest a 10–20% increase in the probability of a man being assigned to the Air Corps after iodization. Using information about average scores of Air and Ground Force recruits we can infer a one-quarter to one half standard deviation increase in average test scores in these regions. The average level of iodine deficiency in the US was [substantially] lower than in the highest regions, so the overall effect in the US was much more modest, though iodization was undoubtedly extremely cost effective. The increase in cognitive ability due to salt iodization may have contributed a small amount to trend rise in measured IQ that took place over the course of the twentieth century, the so-called Flynn Effect…Behrman & Rosenzweig2004 find that differences in birth weight among identical twins are reflected in differences in school attainment and adult earnings…More cites to be added, including Case & Paxson.

• “Economic Status and Health in Childhood: The Origins of the Gradient”, Case et al 2002

Typically, non-deficient populations differ from iodine-deficient populations by approximately 10 IQ points, whereas the whole normal IQ distribution of a population shifts to the left as a result of iodine deficiency⁷. In endemic areas, cretinism can affect up to 15% of the population (de Benoist et al eds 2004_20ya). Bleichrodt & Born 1994_30ya estimate that the average IQ of iodine-deficient groups is 13.5 points lower than the non-deficient groups…Scrimshaw (1998_26ya) provides a list of studies and experiments that have been conducted, and which have shown the hindering effects on mental development of iodine deficiency in utero.

• 7: Scrimshaw (1998_26ya) provides a list of studies and experiments that have been conducted, and which have shown the hindering effects on mental development of iodine deficiency in utero.

• de Benoist, Bruno, Maria Andersson, Ines Egli, Bahi Takkouche, and Henrietta Allen, eds, Iodine status worldwide: WHO Global Database on Iodine Deficiency, Geneva, Switzerland: World Health Organization, 2004

• Bleichrodt, N and M P Born, “A meta-analysis of research on iodine and its relationship to cognitive development,” in John B. Stanbury, ed., The damaged brain of iodine deficiency, New York: Cognizant Communication, 1994_30ya, pp. 195–200

…It had been documented that large amounts of iodine could cause hyperthyroidism to develop in some adults and thyrotoxicosis in others…thyrotoxicosis might occur as a result of iodization in those individuals that have suffered from long-term iodine deficiency and whose goiters have become nodular. In such cases, iodine supplementation causes the output of hormone to jump to toxic levels…The number of enlisted men started increasing in 1942_82ya, and it peaked in the second semester of that year, as well as the first semester of 1943_81ya. People born after iodization enlisted in large numbers starting the first semester of 1943_81ya. The proportion of recruits going to the Air Force was particularly low in 1943_81ya…How large are these effects? The highest goiter places have goiter rates of 30. The coefficients (from Table 4) suggest that about 15% more recruits from the highest goiter regions go into the AAF after salt iodization. From Appendix A we know that the Air Force recruits have, on average 9 point higher AGCT scores (almost a half a standard deviation). The average increase for the recruits from that section is therefore 0.15 time 9 points, or 1.35. So the average cognitive ability in the section goes up by greater than one twentieth of a standard deviation. Examining the high-low regressions yields similar figures. In the high goiter group, we have a 5–10% higher assignment rate to the AAF after salt iodization. 10% times 8 AGCT points results in 0.8 points higher on average, or about a twentieth of a standard deviation. This implies a twentieth of a standard deviation increase in cognitive ability for 25% of the US population.

The final version finds a more substantial effect:

The estimated coefficient on the high-goiter dummy in table 5 is in the range 2.5–8.7 percentage points. Similarly, the logit specification in Table 8 implies a marginal effect of iodization on the probability of entering the AAF in the range of 3–7.8 percentage points. Comparing these estimates to Table 9, we see that the lower range of these estimates are consistent with iodization raising IQ by 15 points (that is, X = 15), which is a reasonable expectation given the work of (Bleichrodt and Born 1994_30ya). However, the higher range of estimates is larger than any reasonable estimate of the increase in IQ that would have resulted from iodization. Given the uncertainty surrounding the selection process and prior literature on iodization we do not consider the larger results plausible. In any case, our results are consistent with a substantial effect in line with the existing literature.

But unfortunately, the detailed mechanisms & psychometrics are insufficient to support a concrete estimate of IQ gains, as taken literally, their results indicate an IQ gain somewhere around 30 points; this is sufficiently large as to prove that the analysis is not fully capturing the dynamics and the estimates cannot be taken literally aside from proving that there was probably a substantial gain. (Based on the many problems with the iodization literature, suggesting 15 points is probably an overestimate.)

Another interesting paper exploits the similarly abrupt introduction of iron fortification into the USA ~WWII: “Ironing Out Deficiencies: Evidence from the United States on the Economic Effects of Iron Deficiency”, Niemesh2015.↩︎

“Effects of drinking water with high iodine concentration on the intelligence of children in Tianjin, China” (slides), Liu et al 2009_15ya:

The results indicated that for children who resided in regions with a very high water iodine concentration, there was on average a reduction of IQ by nearly nine points (b = −8.7, 95% CI: −15.9 to −1.4) in comparison with those who resided in a non-high water iodine area. When comparing children living in areas with a high water iodine concentration with those in the non-high areas, a slight reduction of IQ by about two points was observed, however, it was not [statistically-]significant. There was also a [statistically-]significant association between year of birth and IQ, which seemed to exhibit a dose-response relationship. There was a progressive increase in IQ by about four (p = 0.072) and eight (p = 0.015) points for children born in 1996_28ya and 1997_27ya when compared with those who were born in 1995_29ya, the year China first implemented the national dietary iodine supplement programme.

Analyzed 88 samples; 47 fell below USFDA recommended level and their labeled level: Dasgupta PK, Liu Y, Dyke JV. “Iodine nutrition: iodine content of iodized salt in the United States”↩︎

Salt in cooking meat loses ~60% of iodine; Wang et al 1999_25ya, Effects of storage and cooking on the iodine content in iodized salt and study on monitoring iodine content in iodized salt↩︎

“Iodine status of UK schoolgirls: a cross-sectional survey”, Vanderpump et al 2011_13ya; see also “The prevalence of iodine deficiency in women of reproductive age in the United States of America”, Hollowell et al 2007↩︎

Gunton JE, Hams G, Fiegert M, McElduff A. Iodine deficiency in ambulatory participants at a Sydney teaching hospital: is Australia truly iodine replete?; see also Clifton et al 2013.↩︎

Hoption Cann SA. “Hypothesis: dietary iodine intake in the etiology of cardiovascular disease”:

In the general United States (US) population, a declining trend in urinary iodine levels has been observed as estimated through the large National Health and Nutrition Examination Surveys (NHANES I, 1971–74; and NHANES III, 1988–94) [3]. The proportion of the US population with moderate to severe iodine deficiency (<50 μg iodine/L in urine) has more than quadrupled in the last 20 years, 2.6% in NHANES I vs 11.7% in NHANES III [3]. This decline may be due to a reduced intake of iodized salt. For example, Enstrom et al. [4] compared data from nationally representative samples of the American population from two time periods, 1980^–₂1982_42ya vs 1990^–₂1992_32ya. Based on dietary intake studies from over 10,000 individuals, no change was observed in sodium intake from food, while there was a 65% decline in sodium intake from discretionary (iodized) salt. Adventitious sources of iodine have also declined with reductions in the use of iodine in the dairy industry and in commercial bread production [5]. Comparable trends have been observed in other countries that iodize salt. For example, public health recommendations to reduce salt intake have been implicated in the decreasing iodine status in Australia and New Zealand [6]. Similarly, a recent decline in median urinary iodine levels in Austria may be due to lower salt intake as well as the availability of noniodized salt in Austria after joining the European Union in 1995_29ya [7]. In Greenland, as imported foods have replaced consumption of traditional foods such as fish and sea mammals, there has been a gradual decline in iodine status. Andersen et al. [8] found that median urinary iodine excretion declined with the degree of decrease in the traditional lifestyle. Evidence of iodine deficiency was seen in non-Inuit subjects, who had the lowest levels. Thus, in countries where there are changing dietary patterns, including those long established iodization programs, [substantial] declines in iodine intake may occur.

See also Caldwell et al 2005; Pearce et al 2004; Trumpff et al 2013; and Gahche et al 2013↩︎

Bautista did not report standard deviations, but did report ns, means, and p-values, which permits the standard deviation to be derived, following the Cochrane Handbook (“7.7.3.3 Obtaining standard deviations from standard errors, confidence intervals, t values and p values for differences in means”): the degrees of freedom = 198, so with a reported p = 0.57, t = 0.569; then the standard error is the means divided by the t value ((70.31 − 69.43) / 0.569); and finally the standard deviation: 1.55 / √(1⁄100 + 1⁄100) = 10.96.↩︎

Southon reported control & experimental scores split by gender, which need to be re-pooled:

• control

  1. n = 9 + 20 = 29

  2. mean = ((60.6 × 9) + (65.6 × 20)) / (9 + 20) = 64.1

  3. standard deviation = (((9 − 1) × 2.3) + ((20 − 1) × 1.7)) / (9 + 20 − 2) = 1.87

• active

  1. n = 10 + 12 = 22

  2. mean = ((10 × 63.4) + (12 × 64.4)) / (10 + 12) = 63.94

  3. standard deviation = (((10 − 1) × 2.6) + ((12 − 1) × 2.6)) / (10 + 12 − 2) = 2.6

See Table 7; 12 month followup scores are reported grouped in IQ deciles, and if one assumes scores in each decile average to around the median (since they should be following a bell curve, this will overstate standard deviations), one can calculate out the figures in R as follows:

# experimental
sd(c(rep(65,12), rep(75,10), rep(85,17), rep(100,19), rep(115,1), rep(125,1), rep(135,0)))
[1] 14.59699
mean(c(rep(65,12), rep(75,10), rep(85,17), rep(100,19), rep(115,1), rep(125,1), rep(135,0)))
[1] 85.25

# control
sd(c(rep(65,30), rep(75,13), rep(85,25), rep(100,26), rep(115,5), rep(125,0), rep(135,1)))
[1] 16.19141
mean(c(rep(65,30), rep(75,13), rep(85,25), rep(100,26), rep(115,5), rep(125,0), rep(135,1)))
[1] 83.6