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Can’t say parasites causes low IQ either

Ever since Richard Lynn started compiling national intelligence averages (national IQs), there has been egalitarian minded researchers using these to find evidence of non-genetic causes of group differences. One frequently mentioned and somewhat plausible cause is infectious diseases and parasites. These problems used to be much more common in the past (so could maybe explain some of the Flynn effect of rising IQ scores), and also show a predictable worldwide pattern. WHO produces this map in 2004:

Burden of all infectious diseases, worldwide in 2004, measured in disability-adjusted life years

There’s some evidence from Western populations that infections in childhood might cause lower intelligence levels later on, presumably because the body must divert resources to fighting off invaders rather than optimizing development. For instance, from the 1970s in the United Kingdom:

The possibility that some of the common childhood infections lead to unrecognized impairments of neurological function was examined in 43 820 Birmingham children whose intelligence was assessed in the 11-plus examination. Mean verbal reasoning scores were lower for children who had had measles or pertussis than for those who had had neither of these diseases. However, since attack rates and measured intelligence are related inversely to social class, the lower scores of children with measles and pertussis may be due to class differences which are not eliminated completely by standardization for maternal age and birth order. Mean scores were a little higher for children who had had rubella than for those who had not, and it is suggested that this difference may be due to more frequent reporting of the disease by the more intelligent mothers.

Or Swedish data from 1973 to 1992:

Objectives  To test the association of childhood infection with IQ and adult NAP, to find whether shared familial confounding explains the infection-NAP and IQ-NAP associations, and to examine whether IQ mediates and/or moderates the childhood infection-NAP association.

Design, Setting, and Participants  Population-based longitudinal cohort study using linkage of Swedish national registers. The risk set included all Swedish men born between 1973 and 1992 and conscripted into the military until the end of 2010 (n = 771 698). We included 647 515 participants in the analysis.

Measurement of Exposures  Hospitalization with any infection from birth to age 13 years.

Main Outcomes and Measures  Hospitalization with an International Classification of Diseases diagnosis of NAP until the end of 2011. At conscription around age 18 years, IQ was assessed for all participants.

Results  At the end of follow-up, the mean (SD) age of participants was 30.73 (5.3) years. Exposure to infections, particularly in early childhood, was associated with lower IQ (adjusted mean difference for infection at birth to age 1 year: –1.61; 95% CI, −1.74 to −1.47) and with increased risk of adult NAP (adjusted hazard ratio for infection at birth to age 1 year: 1.19; 95% CI, 1.06 to 1.33). There was a linear association between lower premorbid IQ and adult NAP, which persisted after excluding prodromal cases (adjusted hazard ratio per 1-point increase in IQ: 0.976; 95% CI, 0.974 to 0.978). The infection-NAP and IQ-NAP associations were similar in the general population and in full-sibling pairs discordant for exposure. The association between infection and NAP was both moderated (multiplicative, β = .006; SE = 0.002; P = .02 and additive, β = .008; SE = 0.002; P = .001) and mediated (β = .028; SE = 0.002; P < .001) by IQ. Childhood infection had a greater association with NAP risk in the lower, compared with higher, IQ range.

Conclusions and Relevance  Early childhood is a sensitive period for the effects of infection on IQ and NAP. The associations of adult NAP with early-childhood infection and adolescent IQ are not fully explained by shared familial factors and may be causal. Lower premorbid IQ in individuals with psychosis arises from unique environmental factors, such as early-childhood infection. Early-childhood infections may increase the risk of NAP by affecting neurodevelopment and by exaggerating the association of cognitive vulnerability with psychosis.

There‘s a lot of more similar studies. I couldn’t find the holy grail, namely, a sibling control where the mother was sick during pregnancy of one child but not another, and later age IQ measures of both siblings, nor could I find sibling controls for children themselves getting infected and their later IQ scores. If these associations still exist in these models, it would help rule out ‘self-selection’ into disease exposure, as smarter parents presumably take better care to avoid themselves and their children getting sick (or mutational load causes both lower intelligence and weaker immune systems).

But if we go along with the assumption there is some causality, especially in bad cases of early childhood infections, one could use this along with the map above to argue that the global distribution of intelligence has at least one identifiable and remediable non-genetic cause, or at least, a mediator. Care here must be taken in interpretation since a country’s ability to use proper health measures itself reflects intelligence, so the distribution of the disease burden would also be partly heritable based on a hereditarian model (we showed this was the case using the spread of HIV since this post-dates the measurements of national IQs). A few published studies took this route of arguing for environmental causes of national IQs:

The 2010 study got the most attention, and was maybe the first to get published in a popular journal, and as a result, it has gathered 392 citations.

But not anymore, says the journal:

Following the publication of this article, Proceedings B was recently made aware of potential problems with the underpinning datasets used in the analyses, which were drawn from published sources [1,2]. The editors’ attention was drawn to the fact that the datasets on between-country variation in IQ had been the subject of several critiques claiming that they contain substantial inaccuracies and biases that throw substantial doubt on inferences made from them, and that these problems had not been resolved in revised versions of the dataset used by Eppig and colleagues. Upon detailed scrutiny, the editors found these claims to be convincing and asked Eppig and colleagues for their response. While the authors acknowledged at least some of the claimed flaws, they maintained that the inferences from the data were nevertheless reliable.

Proceedings B publishes research of outstanding scientific excellence and importance, conforming to recognized standards of scientific procedure in terms of methodology and ethical standards. Journal policy stipulates retraction where editors have clear evidence that the findings are unreliable (and may invalidate the conclusions of the paper). After carefully considering the dataset, the critiques, the authors’ response and the potential harms created by using a dataset that appears to portray human populations in some geographical regions as of below normal intelligence on average, the editors concluded that the manifest problems in the data warranted retraction in order to uphold these standards.

Notice the unnamed, uncited source of criticism. This is not how science is supposed to work.

Attempting to get national intelligence articles retracted is a long-term favorite hobby of Rebecca Sear, an activist anthropologist, who wrote a preprint with her criticism, shotgun argumentation style. This preprint has been cited as grounds for other retractions already, so maybe it was also behind this one. She has a friend called Cathryn Townsend, who does the same kind of thing (both are White women). Were they behind this one too? Retraction Watch covered this retraction, but no mention is made of the responsible censorship activists. However, it does note the timing:

In July 2023, the editorial team was made aware of criticism about the dataset used by Eppig et al. in a 2010 paper in Proceedings B. After considering a wide array of evidence, including the original data set, subsequent critiques and the authors’ response, the editors concluded problems with the study were sufficient to call its conclusions into question and warrant retraction.

The decision to retract was made in January 2024, and the authors were informed at that time, the spokesperson added.

I searched online to see if I could find the culprit. I found this 2020 Twitter thread (archive) by some random activist professor called Levi Gray that didn’t get much attention:

Given the lack of attention, this thread probably didn’t cause the retraction 4 years later.

But the censorship duo published a commentary criticizing a fertility and intelligence study in August of 2023 in a different journal by the same publisher (The Royal Society), so maybe they sent their complaints a bit earlier the same year, which eventually lead the publisher to censor this study.

If preventable diseases and infections are a major cause of lower intelligence in the developing world and if intelligence is the chief cause of socioeconomic development, this would be a good reason to target developmental aid at these problems. Unfortunately, the censors will not allow even this to be discussed. The effect of this is, as Linda Gottfredson put it, “hurting those we intend to help”, in a 2005 piece about suppression of intelligence research. It has, sadly, not become less relevant, but more so.