How Adding Iodine to Salt Boosted Americans’ IQ

By Lisa Raffensperger | July 23, 2013 3:00 pm

morton salt iodine

Iodized salt is so commonplace in the U.S. today that you may never have given the additive a second thought. But new research finds that humble iodine has played a substantial role in cognitive improvements seen across the American population in the 20th century.

Iodine is a critical micronutrient in the human diet—that is, something our bodies can’t synthesize that we have to rely on food to obtain—and it’s been added to salt (in the form of potassium iodide) since 1924. Originally, iodization was adopted to reduce the incidence of goiter, an enlargement of the thyroid gland. But research since then has found that iodine also plays a crucial role in brain development, especially during gestation.

Iodine deficiency today is the leading cause of preventable mental retardation in the world. It’s estimated that nearly one-third of the world’s population has a diet with too little iodine in it, and the problem isn’t limited to developing countries—perhaps one-fifth of those cases are in Europe (pdf), where iodized salt is still not the norm.

Iodine’s Natural Experiment

With this background, then, a group of economists saw a natural experiment: comparing the intelligence of children born just before 1924—the year iodization began—and those born just after. James Freyer, David Weil and Dimitra Politi used military data from the early 1900s 1920s, when World War II drove millions of men and women to enlist.

Recruits all took a standardized intelligence test as part of their enlistment. Researchers didn’t have access to the test scores themselves, but they had a clever substitute: smarter recruits were assigned to the Air Forces while the less bright ones went to the Ground Forces. This allowed the researchers to infer test scores depending on which branch a recruit was selected for.

Intelligence data were paired with birthdate and hometown, since iodine levels in the soil and water vary significantly from place to place. To estimate which regions were naturally high-iodine and which were low, the researchers referred to nationwide statistics collected after World War I on the prevalence of goiter.

In all, researchers had sufficient data on about 2 million male recruits born between 1921 and 1927.

Stark Improvements

The economists found that in the lowest-iodine areas—the bottom quarter of the study population—the introduction of iodized salt had stark effects. Men from these regions born in 1924 or later were significantly more likely to get into the Air Force and had an average IQ that was 15 points higher than their predecessors.

Nationwide, that averages out to a 3.5-point rise in IQ because of iodization, the researchers report in a paper for the National Bureau of Economic Research.

The initiative wasn’t without its drawbacks—sudden iodine supplementation among people who are deficient can cause thyroid-related deaths. The researchers estimate that 10,000 deaths in the decades after 1924 were caused by salt iodization.

But on the positive side, iodine deficiency and its symptoms were vanquished almost overnight. And iodine’s mental benefits may even help explain the Flynn Effect, which observes that IQ rose about 3 points per decade in developed countries throughout the 20th century. It’s been thought that improved health and nutrition were the driving forces of the Flynn Effect. Now, it appears that iodine alone was responsible for roughly one decade of that remarkable climb. All the more reason, then, for the rest of the world to follow suit and relegate iodine deficiency to the history books.

 Image by trekkyandy via Flickr

CATEGORIZED UNDER: Health & Medicine, Top Posts
MORE ABOUT: iodine, nutrition
  • Rejean Gagnon

    World War 2 began in 1939 for some, earlier for others, and 1941 for the US… There’s a mistake at “…from the early 1920s, when World War II drove millions of men and women to enlist.”

    • Elias Chan-Sui

      The sentence is a bit wonky. However, they are referring to the individuals enlisting in WWII who were born in the ’20s both before and after iodized salt was introduced.

    • Rob Neff

      They tried fixing that sentence but still couldn’t figure out how to get it right. Rejean is correct, the military data for WWII is early 40’s (since the study used U.S. data). A person born in 1924 would be 18 in 1942 and ready for enlistment.

    • Linda Galli

      They were figuring out when the men were born. If they entered the military at age 20 in 1942, they would have been born in 1922. My Dad was born in 1915 and was drafted in March of 1941. The US knew the war was coming and started a 6 mo. draft that started in the spring of 1941. Dad was one of the first that was drafted. He was released in Oct. of ’41. Then after Pearl Harbor he was called back to the military for the duration.

  • Dickybow

    What!? Communist agents contaminating our pure God-given salt? This must stop immediately! (Don’t get me started on Fluoride…)

    • John Pinion

      PLEASE PLEASE PLEASE actually do the research before you make such a rash and reckless comment. From Wikipedia, “Iodine is required by higher animals, which use it to synthesize thyroid hormones, which contain the element.” Also from Wikipeida, “Consumption of fluoride at levels beyond those used in fluoridated water for a long period of time causes skeletal fluorosis.
      In some areas, particularly the Asian subcontinent, skeletal fluorosis
      is endemic. It is known to cause irritable-bowel symptoms and joint
      pain. Early stages are not clinically obvious, and may be misdiagnosed
      as (seronegative) rheumatoid arthritis or ankylosing spondylitis.” The body has a NUTRITIONAL NEED for Idodine, and NO NUTRITIONAL NEED for Fluoride.

      • Dickybow

        Oh John John, I know most Americans have a rather literal sense of humour but really! You’re 51 and sound reasonably well educated, could you not detect a hint of satire? No memory of Kubrick’s film Dr. Strangelove? Well done for researching how essential Iodine is in trace amounts; Here in Britain the condition known as ‘Derbyshire neck’ (Goitre) was associated with Iodine deficiency in the 17th Century. You are getting confused about Fluoride, at its simplest, no fluoride and teeth decay is accelerated, TOO MUCH fluoride and skeletal fluorosis occurs. ( I assume your drinking water suppliers know what concentrations are suitable, healthy and safe.) Your statement on GM foods in your other comments is highly suspect too.(GM foods toxic to animals? I don’t think so!)

        • David Zabrocki

          I thought your comment was funny, on point and good satire of our “chicken-little” sensibilities………but I’m Canadian

          • John Pinion

            I may misunderstand you, but it also may be easier for the two of you to view my perpective as an “American chicken-little sensibility” since the chances are lower, as Canadians and Brits, that you are being forced to drink fluoridated water (or, rather, have to go out of your way _not_ to drink it.)

        • Archies_Boy

          I’ll take a serving of fish and chips please — with iodized salt, of course! ;-D

        • John Pinion

          Yes, exactly, and the problem is in the numbers. The difference between the “effective” dose and “toxic” dose for fluoride is just very close (currently considered in the range of x3) For Iodine, that number is closer to x10. Not all genetically modified foods are toxic, but they certainly can be – after all, they’re designed in many cases to unnaturally produce toxins they never would in nature. What happens when that mechanism becomes deranged? Oh no – THAT would NEVER happen! (See, I DO know SATIRE…)

        • Will Morris

 again here it is don’t want you to miss out

        • jon

          Calcium fluoride is the best fluoridation but it is too expensive so governments use the cheaper form from industrial waste called hydrofluorosilicic acid, which is very toxic.

      • TYactive

        The body has a need for fluoride to strengthen teeth and saliva to prevent cavities. If you read the report – you will see that too much Iodine causes thyroid problems – but not enough and you get health problems. The same is true for fluoride. Fluoride is a micronutrient.

        • John Pinion

          Again, when the “theraputic” dose is as close to the toxic dose as it is, I question the use of the term “nutrient”. But irrespective of terminology, we are, in the modern world, being exposed to much more fluoride than when water fluoridation was started in the 1940’s, hence the need for diminished supplimentation.

        • 2nd amendment

          The so called flouride they use is left over from aluminum production. Not the naturally occurring substance in the soil

        • 2nd amendment

          Harvard flouride study

      • Will Wiegman

        Yeah, it was Nazis, not Communists!

      • Alex Tsban

        When I was 38 I had great teeth,the same year the citizens of my city voted “no” to fluoride in our water-I’m now 63 without any teeth left.

    • Follow The Money

      Dozens of studies have shown that children from areas with naturally higher fluoride exposures have lower IQ. Fluoride is known to affect thyroid function at certain exposure levels. It is likely that fluoride’s lowering of IQ works by disrupting thyroid function, probably in mothers, just the opposite of iodine acting to protect thyroid function thereby preventing low IQ. The only real question is whether the fluoride exposures of mothers and children in places with artificial fluoridation are sufficient to also lower IQ. Based on the existing science, it seems likely at least some will have lowered IQ due to fluoridation. The effect may be subtle and hard to detect without a very careful scientific study, which has not yet been done.

      Some of the studies of fluoride and IQ have simultaneously examined iodine and IQ, and have found that low iodine greatly exacerbates fluoride’s ability to lower IQ. Thus, people with “low” iodine and “high” fluoride are most at risk of decreased IQ. In the US and many other developed countries today, an amazingly large percentage of child-bearing-age women are deficient in iodine. A recent study in the UK found that those women that were iodine deficient (more than 50% were deficient!) had children with lower IQ than those women who were iodine-sufficient. This was a careful study that controlled for other factors like socio-economic status.

      This is a very serious public health issue that can’t be dismissed with glib comments about “communist conspiracies” or references to the movie Dr. Strangelove.

      • Dickybow

        As a retired Environmental Scientist (specialising in Pollution Chemistry & Hydrology) I agree that domestic water quality is a serious subject. Give me the Epidemiological references to your ‘dozens of studies’ or accept my retort of UTTER BOLLOCKS. (You have a nice day now…)

        • Will Morris
          • Nathan Merrill

            Crazy people ranting about communists contaminating their precious bodily fluids.

            So, pretty much exactly what he was making fun of?

            As far as actual scientific studies have found, the only “ill effect” that fluoridation of water has caused is tooth discoloration, which is otherwise harmless. Unless you are exposed to high levels of fluoride, it is harmless – as they say, the dose makes the poison, and in small quantities (the concentration found in fluoridated water supplies) it is therapeutic. There are other ways of supplementing people with fluoride.

            The idea that it lowers IQ is unsubstantiated and indeed appears to be outright false. Sorry.

          • LettaMego

            no truth what-so-ever in the above post..

      • LettaMego

        This IS a very serious public health issue…and to add to that, our decades long wars have contaminated much of the water with perchlorate…perchlorate has a higher affinity for the thyroid than iodine.. fluoride also causes thyroid problems… This lack of iodine also contributes to breast tumors.. lowering IQ, and obesity.. and varying degrees of cretinism.

    • 2nd amendment

      Agree should be a way to sue for poisoning the water with toxins. Especially flouride. My family drinks nothing but filtered water and flouride is extremely difficult to remove. Most filters won’t touch it. Making it extremely expensive. Iodine does help your body not obsorb flouride which is great since its in everything we eat for the most part.

  • m12345

    Americans are among the stupiiest people in the world, are you sure you’re adding enough iodine… triple it please.

    • Morrie Chamberlain

      You can thank stupid American scientist Vinton Cerf for creating the internet which enables you to post that remark. You can thank stupid American Dr. H. Edward Roberts for inventing the micro computer on which you probably typed those words. Stupid American Ted Hoff invented the microprocessor which made the PC possible and stupid American Jack Kilby invented (with many others) the integrated circuit which makes microprocessors possible. Stupid American Thomas Edison had a part to play in electricity as well. And so on.

      • JonFrum

        Life is full of little intelligence tests. When you feed a troll, you get one answer wrong, and your score is recorded accordingly.

      • m12345

        Roberts would be sixth generation UK. So youll have to discount the four colonising countries genes, sorry.

        You’re proposing 5 generations is enough time for people to evolve their intelligence “radically” beyond anything genes can, just by culture. An interesting proposition. Completely flawed but interesting.

        One might ask how many generations does it take before you can say a populations iq is directly related to their country. Ie effects have influenced enough generations.

        But I suppose you brought it down to competition of inventions.

        As I said your best inventions then are massive debt, massive unemployment, cities being removed, jobs lost over-seas, yes you have invented a lot of these things.

        • John

          6th generation? That’s dilution some generations ago. You’re a rather desperate troll.

      • LettaMego

        Yes, in the past, Americans were intelligent.. but now we have fallen prey to a combination of powerful, well-focused special industrialized
        interests, together with public inattention and the indifference of the
        scientific community… so that now our land, air and water contain perchlorate and heavy metal .. driving our IQs down.. We have whole cities of cretins. And whole blogs… they mostly just curse and call people names. Instead of cretins we call them trolls. Actually it’s not all our fault… We’re sadly misinformed and uninformed by the media.. because the media is owned by the special interest groups that are poisoning us.

    • David Zabrocki

      At least Europe places and emphasis on quality of life over the value of constant work.

    • eirikr1

      And where does that leave *your* loser nationality that hasn’t even landed on the moon, much less been there multiple times? plan on catching up anytime soon troll?

    • Nathan Merrill

      Average IQ in the US is the same as in the UK, despite the US having 13% of its population coming from a racial demographic whose average IQ is a full SD below the UK average, and having a vast number of immigrants from countries which have not fully undergone the Flynn Effect.

      The developed world is the smartest place in the world; the only undeveloped country with high IQ is China.

      • LettaMego

        I have some doubts about those facts.

    • Tec

      I would agree that America appears to be stupid policy wise, only due to what I call life’s losers who choose to go into politics and liberals who want to run everyone else’s lives have taken over power in this country.
      Liberals in my opinion do lack the ability of a mature thought process taking charge of our government makes us look like children are in charge and running things around here.
      The best and the brightest don’t go into government they go into business and no matter what you say about our political policies it doesn’t apply to the best and the brightest that run the worlds best business and leading edge technologies.
      Say what you want about the dimwits running our government, a very small group of power hungry losers, the typical American makes up the greatest country that ever existed and your envy and self loathing oozes from your every word about our country.

      • LettaMego

        It’s gotten to the point that everyone talks like a Fox News commentator. It all sounds smart but makes no sense….
        What’s really frightening is most Americans actually don’t know the difference between commentary and facts.. Yes, our IQs are falling, our life spans are shortening…we’ve lost just about everything that has to do with freedom, privacy… and a healthy environment.. Our intelligence is being wiped out by perchlorate and other toxins .. like lead… making us increasingly stupid. We might have a chance if we weren’t so misinformed and uninformed.. As long as Made-in-China General Electric is feeding us propaganda in the media, and we believe it… we’re pretty much going down.

    • LettaMego

      Rocket fuel ( perchlorate ) displaces iodine in tissues… our tissues have a higher affinity for perchlorate than for iodine. The US Government is killing more people here in the USA than they are killing enemies. Our decade after decade wars have cause the pollution of almost all of North America with rocket fuel… That’s probably the chief cause of our obesity and ignorant name calling and stupidity.

  • Wayne Lusvardi

    The author of this article must not know much about research design. All that I can see that this study measured was the difference in social class between two groups. Higher income people might have had better diets and vice versa. In science, you have to try to falsify or disprove your thesis which this study did not do. Moreover, it used proxy measures not real measurements of iodized salt intake. Maybe higher income people ate more fish which has iodine in it? Maybe one community was predominantly Catholic and during that era ate fish on Fridays regularly? Or maybe one community had high levels of perchlorate in the water which can block iodine absorption? The Flynn Effect? Was that Errol Flynn?

    • John Pinion

      James Robert Flynn PhD FRSNZ (born 1934), aka Jim Flynn, Emeritus Professor of Political Studies at the University of Otago in Dunedin, New Zealand researches intelligence and is famous for his publications about the continued year-after-year increase of IQ scores throughout the world, which are now referred to as the Flynn effect.

      • Wayne Lusvardi

        Errol Flynn was a movie actor and my comment about the Flynn Effect was sarcastic.

        • John Pinion

          (I just thought I would add that for those who, like me, didn’t know and would be prompted by your comment to look it up.)

    • Rob Neff

      I disagree. They used iodine levels by geographic region, and compare pre-1924 numbers to post-1924 numbers. This would correct for social classes and fish intake, etc, as the geographic regions didn’t change.

      They might need to consider that older people enlisting (born before 1924) might be enlisting because they’re not smart enough to find other work, so you’re picking the bottom of the barrel, but then, in the early 40’s pretty much every able-bodied young male was expected to become involved in the war effort, so even that wouldn’t have much effect (unlike the Viet Nam era, where those with political connections could get into the National Guard or have deferments, etc.)

  • colindenronden

    They used to take the smarter fellows into the air force in Australia too. Army guys were referred to as drongos (a flightless bird), which became a bit of an insult as it implied stupidity.

  • Angie Eddy

    As much as I’d like to believe that Americans are superiorly intelligent, this article and research is a joke! I’ve seen this research all over the internet regurgitated in various forms. I’m so glad to see some intelligent conversation on the topic here. There are several initial questions and issues I had with the study without having read the actually research article. Several variables were conveniently not
    considered in the equation of this study.

    #1 – correlation is not equal to causation.

    #2- Military testing and IQ testing is not the same thing

    and both have changed over the years.

    #3- the recruitment qualifications for Air Force and the

    Army may have changed based on technology or need. Did anyone look into that? Why weren’t the actual scores included in the study?

    #4 – goiters might contribute to low intelligence, but low

    iodine is not the sole cause of goiters.

    #5 – goiters are actually more common in women and people

    over 50 years old.

    #6- could the low iodine (high goiter) areas actually be due

    to education reform or social class and nutrition?

    Who funded this research grant?!!!

    • Follow The Money

      The paper is 43 pages long! If you read it, your questions may all be answered. While this type of study can never provide a conclusive answer, it seems to be a sound study which offers strong evidence that low iodine can reduce IQ.

  • weensplosion

    Good read!

  • Queen Mennon

    They lie. they have not been putting iodine in the salt. I grew up with a box of salt that said: “This Salt does not contain iodine. A necessary ingrediant.” they sell two kinds of salt on the grocery shelf: that with iodine and that without iodine and I know for a fact, many americans use the one without the iodine in it.

    • LettaMego

      Take a look at perchlorate… it has a higher affinity for they thyroid than iodine.. We all turning into cretins. Our decades of wars are killing more of us than our enemies.

  • steveday72

    Of course, anyone who has seen statistics used by people to prove _their_ point of view of agendas, will know the phrase “Correlation does not equal causation.”

    While certainly interesting. Using statistics alone for medical research is utterly pointless. For starters, the study only looked at a specific sample of the population.What about men who were too mentally challenged to have even been called-up in the first place? (ie: already listed as disabled). What about the effect on women

    Lastly, it could equally be said that preventing pregnant women or nursing mothers from drinking alcohol was the deciding factor in raising their children’s IQ scores…. US Prohibition was between 1920-1933, so any men (used in the study’s sample) aged 21 or younger in 1941, were born during the period that alcohol was banned.

    There. Can I have my $250k study grant please!

  • LettaMego

    Perchlorate displaces Iodine in tissue.. including the thyroid gland….
    Perchlorate is not removed by filtering it… unless it’s reverse osmosis filtering.
    There’s also the little problem of the idiots in government forcing the most toxic element on earth, fluoride, on us by adding it to our drinking water. Fluoride also causes iodine deficiency.. by mimicking iodine… Fluoride is very difficult to filter out of water… it you use water softeners or regular water filters you are not removing fluoride.
    We the people have ALL the symptoms of being drained of iodine….
    Hair loss or thinning hair
    lowering IQs
    Weight gain
    Intolerance to cold temperatures
    Muscle aches and pains
    We’ve gotten so stupid, people believe ridicule and name calling makes sense… in arguments..
    Then there’s the ever growing problem that the iodine deficiency also causes breast tumors..

  • Renee Shreve

    Interestingly, fluoride and bromine (additives archaically not required to be listed as ingredients), compete with iodine and because of this they have been called the “Halogen Bullies”. It is now well known that they interfere with iodine absorption, cause thyroid problems, and the dysregulation of many body systems, understandably. Fluoride increase has been repeatedly associated with lowered IQ points. Fluoride, unlike bromine, is not an essential nutrient. Fluoride occurs naturally in surface waters at a world-wide average of only 0.05 ppm, and in mother’s milk at about 0.004 ppm, but based on an out-dated and politically-charged policy stance, fluoride is put into our water system 14 to 80 times the 0.05 ppm naturally occurring world-wide average, and well over a hundred times what is now known a formula-fed baby should be exposed to.

    Fluoride is naturally available in tea leaves — a very, very inexpensive and readily available source of fluoride for those who feel that preventing a half a cavity a mouth is worth having fluoride compete with iodine within their own — or their developing child’s — body. Free country — right?

    For current information on fluoride’s impact on IQ, see “Our Daily Dose”, a film by Jeremy Seifert on YouTube.

  • Albert Aruse

    You guys should put the newest comments first, otherwise they will never get read like they others had their spot in the sun. It wouldn’t matter if there were two comments but cmon for God’s sakes.

    Anyway you can argue that iodization of salt reduced goiter almost overnight in goiter belts like Cleveland and Akron, but my point stands that straight iodine (without ANY amount of potassium iodide) would do even better probably. I am a person sensitive to potassium iodide and got lucky to figure it out. “Nascent” iodine works but “Lugol’s” (with added pot iodide) destroys me in a big way. Do you believe all research papers? The conclusions have to be logical. You don’t think Bayer Corp pays for results first and asks for truth later? It happens believe me. I was a chemist.


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