Can we talk about IQ and Lead Poisoning?

IQ tests are designed to measure raw intelligence, irrespective of educational level or age.

Hypothetically, a score of 100 is meant to indicate an intelligence that’s precisely the average.

You’d think that the United States, given our economic strength, would have an above-average IQ; turns out, that’s not the case, as studies indicate an average of 97 or 98, significantly lower than Japan’s 106.48 or 105 or South Korea’s 102 or 106.

And, yes, these international comparisons are controversial, but I’m looking at several generally-credible sources:

World Population Review gives a searchable table based on multiple sources, including Richard Lynn, separately available as a report, and The Intelligence of Nations, which is also available online in a full text.  PsychCentral and HealthLine provide similar information, which provides some support for the credibility of the data.

But what about the low-IQ countries?

According to the data, the lowest IQ is to be found in Nepal, at 43, with Liberia and Sierra Leone tied for second at 45 and Guatemala at third-lowest with 48.  Clearly this is unreasonable, though the precise reasons why individuals would score so poorly aren’t well explained in these reports, which focus on the statistical math, the test names, and so on.  In any case, according to the website HealthyPlace, the IQ range of 50 to 70 is classified as “mild intellectual disability” and corresponds to the ability to learn reading and math at the level ranging from grade 3 to grade 6.  From IQ 35 to 49, individuals can communicate but only simplistically.  Finally, according to Vox (and presumably findable in all manner of sources), in a population with an average IQ of 100, only 2.2% of the population has an IQ below 70 (and similarly, 2.2% have IQs above 130).

Even though these extremely low IQ results don’t make any sense, it isn’t impossible for a country to have a lower-than-100 average IQ, solely in terms of the types of tasks people are called on to do in simple agricultural villages, where traditions are passed on from one generation to the next and most work is simple and concrete.  If you were to imagine re-centering an IQ at some moderately lower level, you’d still have some people with the higher IQs, just not as many.  (This was suggested on a random twitter thread a while back.)  One of the key differences is really that lower-IQ people struggle with various abstract concepts, hypotheticals, and “theory of mind” (ability to put yourself in someone else’s shoes).  (No academic sources here, just a reddit post as an image on twitter, and a blog post by someone who at least claims as biography expertise in statistics and medicine and has publishing and teaching expertise.)  And a peasant farmer’s life isn’t really all that complicated, so it is easy to imagine natural selection being less likely over time to result in increased IQs in areas where a higher IQ doesn’t matter for daily life.

That being said, a lot of the explanation for low IQ test scores is attributed to lack of experience with test taking.  But it is known that childhood lead exposure reduces IQ.  One study available at the NIH website reported that, looking at individual children (rather than population averages), a blood level concentration of 10 micrograms per deciliter results in an IQ that’s 8 points less than someone with 1 microgram per deciliter.  Alternately, the study estimated that, for lead levels of up to 10 micrograms per dL, each microgram was associated with a drop in IQ of 1.37 points.  (For levels above 10 micrograms per dL, the additional net impact of more lead is much less than the impact of the smaller increases.)

That being said, the average lead level for children ages 1 – 5 in the US is 0.6 micrograms per dL, and 95% of children have levels less than 2.  That’s down from 15 and 29, respectively, in the period 1976 – 1980, with the dramatic drop mostly due to the elimination of leaded gasoline.

In contrast, lead levels in children in certain poor countries is much higher — though still less than was the case in the US 50 years ago.  According to The Lancet in 2021, among those countries with available data, the highest lead levels for children were

• Bangladesh, 7.87 (supposed IQ 74.33)
• Cameroon, 8.7 (supposed IQ 67.76)
• Dem Rep Congo, 7.46 (IQ 64.92)
• Egypt, 8.24 (76.32)
• Haiti, 6.00 (82.1)
• Nepal, 6.69 (42.99)
• Nigeria, 7.67 (67.76)
• Pakistan, 9.27 (80)
• Palestine, 9.30 (77.69)
• Senegal, 8.22 (77.37)
• Serbia, 7.80 (89.6)
• Uganda, 6.68 (76.42)

Again, the lack of many African countries doesn’t mean most of these were fine, but that most of these lacked data.

And separate from this data, there is a study of Afghan children screened for lead shortly after arriving to the United States.  The data isn’t presented in a format consistent with the other study, but the Lancet study includes children ages 0 – 14 years, and the Afghan study includes children ages 0 – 16 (but may have different age distributions, which is relevant because lead levels decline as children grow; I don’t know why).  Among those children, about 32% had lead levels above 5 micrograms/dL, and 7% had levels above 10 micrograms/dL.  It’s not quite clear how to interpret this or compare this to the other rates, but the entire reason I started looking at this in the first place was a report, originally at The Telegraph and picked up by MSN, headlined, “Why Afghans are being slowly poisoned by their evening meal,” which states that

Afghanistan has one of the world’s highest rates of lead exposure, with an average blood lead level nearly three times that of nearby India and almost five times that of China, according to the best available data

and explains that the standard cooking pot used in Afghanistan causes significant lead exposure.

So on the one hand, are these very high levels of lead exposure a tangible explanation of why IQ levels are so much lower in some countries than others?  (Of course, other factors such as childhood nutrition levels surely play a role, and that’s not even getting into the question of the role of natural selection.)  But if so, it would seem that they would be now experiencing the level of IQ-reduction due to lead that was the norm in the US 50 years ago.

I’ll stop here but there are no easy answers to this murky question.