The best evidence for genetic selection against intelligence, also called dysgenic selection
or simply dysgenics, comes from a paper by Kong et al. (2017; further information about
dysgenic selection is provided in Chap. 8). This study discusses molecular genetic findings
indicative of selection against “educational attainment” (which is very highly genetically correlated
with intelligence) in the Icelandic population, derived using a model that estimates
the joint effects of both the timing and quantity of fertility on the decadal change in polygenic
scores (i.e. indexes of genetic variants associated with a phenotype or outcome)
associated with educational attainment. In a similar vein, Rindermann’s (2018) simulation of
changes in intelligence over time, which also factors in data on the quantity and timing of
fertility by cognitive ability level, reaches an estimate of a decline in average IQ of about 17
points in two centuries (or 0.87 points per decade), and a loss of 28 points over the same
time span for those at the 95th percentile of intelligence (or 1.4 points per decade; p. 427).
A reanalysis of Kong et al.’s (2017) data by Woodley of Menie, Figueredo, et al. (2017),
in which Kong et al.’s “conservative” estimate of the additive heritability of educational
attainment (0.30) was replaced with a biometric structural equation model behavior-genetic
estimate of the adult narrow-sense heritability of g (0.86), yielded an estimate of 0.87 points
of g lost per decade.