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What Caused over a Century of Decline in General Intelligence? Testing Predictions from the Genetic Selection and Neurotoxin Hypotheses

Evolutionary Psychological Science volume 4, pages 272–284 (2018)Cite this article

Abstract

Several converging lines of evidence indicate that general intelligence (g) has declined in Western populations. The causes of these declines are debated. Here, two hypotheses are tested: (1) selection acting against genetic variants that promote g causes the decline and (2) the presence of neurotoxic pollution in the environment causes the decline. A linear mixed model was devised to test (1) and (2), in which the secular decline in a “heritable g” (g.h) chronometric factor (comprised of convergent indicators of simple reaction time, working memory, utilization frequencies of high difficulty and also social-intelligence-indicating vocabulary items and per capita macro-innovation rates) was predicted using a neurotoxin chronometric factor (comprised of convergent secular trends among measures of lead, mercury and dioxin + furan pollution, in addition to alcohol consumption) and a polygenic score chronometric factor (comprised of polygenic score means for genetic variants predictive of g, sourced from US and Icelandic age-stratified cohorts). Bivariate correlations revealed that (other than time) only the polygenic score factor was significantly associated with declining g.h (r = .393, p < .05 vs. .033, ns for the neurotoxin factor). Using a hierarchical linear mixed model approach incorporating 25 year lags between the predictors and g.h, time period, operationalized categorically as fifths of a century, accounted for the majority of the variance in the decline in g.h (partial η2 = .584, p < .05). Net of time period and neurotoxins, changing levels of polygenic scores also significantly predicted variance in the decline in g.h (partial η2 = .253, p < .05); however, changing levels of neurotoxins did not significantly predict variance in g.h net of time (partial η2 = .027 ns). Within-period analysis indicates that the independent significant positive effect of the polygenic score factor on g.h was restricted to the third fifth of a century period (β = .202, p < .05).

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Fig. 1

Notes

  1. The term appears to have been coined in 2015 by Charles Murray. Sarraf (2017, p. 239), however, first defined Woodley effects as “secular trends that plausibly result, in part or in whole, from population-level degradation of the integrity of genetic factors that underlie g.”

  2. Demeneix’s (2014, 2017) somewhat confused qualitative inferences might result in part from an apparent lack of familiarity with the literature on secular trends in intelligence. In discussing the Flynn effect (Demeneix 2017, pp. 75–91), for example, she wrongly maintains that Woodley et al.’s (2013) finding of a decline in g is at odds with studies revealing Flynn effects. However, Woodley et al. (2013, 2014) made clear that the Flynn effect and dysgenesis on g can co-occur as they involve different phenotypic variance components of intelligence.

  3. Without distinguishing the two, giving the charge a pleonastic quality.

  4. This is perfectly consistent with environmental determinist stratagems used to trivialize or eliminate the role of genes in generating differences in intelligence (see Brand 1999).

  5. One example of these can be seen in her effort to explain IQ differences among individuals as a function of “modulat[ed] gene expression” related to “prenatal placental blood supply and nutrition” (Demeneix 2017, p. 87).

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Affiliations

  1. Center Leo Apostel for Interdisciplinary Studies, Vrije Universiteit Brussel, Brussels, Belgium

    Michael A. Woodley of Menie

  2. Unz Foundation, Palo Alto, CA, USA

    Michael A. Woodley of Menie

  3. University of Rochester, Rochester, NY, USA

    Matthew A. Sarraf

  4. Department of Psychology, University of Arizona, Tucson, AZ, USA

    Mateo Peñaherrera-Aguirre & Heitor B. F. Fernandes

  5. Department of Psychology, Technische Universität Chemnitz, Chemnitz, Germany

    David Becker

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  1. Michael A. Woodley of Menie
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  2. Matthew A. Sarraf
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  3. Mateo Peñaherrera-Aguirre
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  4. Heitor B. F. Fernandes
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  5. David Becker
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Correspondence to Michael A. Woodley of Menie.

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Matthew A. Sarraf and Mateo Peñaherrera-Aguirre: Joint second authors

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Woodley of Menie, M.A., Sarraf, M.A., Peñaherrera-Aguirre, M. et al. What Caused over a Century of Decline in General Intelligence? Testing Predictions from the Genetic Selection and Neurotoxin Hypotheses. Evolutionary Psychological Science 4, 272–284 (2018). https://doi.org/10.1007/s40806-017-0131-7

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Keywords

  • Directional selection
  • Dysgenics
  • General intelligence
  • Neurotoxins
  • Polygenic scores