Data from: What caused over a century of decline in general intelligence? Testing predictions from the genetic selection and neurotoxin hypotheses

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 η^2 = .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 η^2 = .253, p <
.05); however, changing levels of neurotoxins did not significantly
predict variance in g.h net of time (partial η^2 = .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).