Mean
polygenic score of Icelanders by year of birth (Kong et al. 2017, Fig. 2)
Three
polygenic studies have shown that cognitive ability declined among European
Americans, British people, and Icelanders during the 20th century. The decline briefly
stopped during the postwar baby boom and again with the liberalization of
abortion laws.
How
can we measure the genetic component of cognitive ability? For a long time, the
only way was to administer an IQ test, but the result would inevitably be
influenced by the test-taker’s environment—not only culture and life history
but also familiarity with taking tests and answering questions in rapid-fire
succession. Yes, twin studies and adoption studies suggest that genetic factors
largely explain variation in IQ between individuals. But the same is not
necessarily true for variation in IQ between populations. That could be 100%
environmental.
Recent
years have seen the advent of a direct measure of innate cognitive ability: the
educational attainment polygenic score. It’s a summation of the predicted
effects of genetic variants that together explain 11-13% of the variance in
educational attainment among individuals (Lee et al. 2018). It’s especially
suited for predicting the mean IQ of a population—the correlation is 98% with the
actual mean IQ (Piffer 2019). Polygenic data
predict a mean IQ of about 85 for sub-Saharan Africans, 100 for Europeans, and
105 for East Asians. There is also variation within each of those geographic
groups. Among Europeans, predicted IQ varies from 97 for southern Europeans to
102 for Finns and 110 for Ashkenazi Jews. Among sub-Saharan Africans, it seems
to be higher among groups who were more advanced during precolonial times,
particularly those, like the Igbo, who lived along the Niger and took part in
trade between the coast and the interior (Frost 2022).
The
above predictions should be viewed with some caution. Because the genetic
variants have been identified only in people of European descent, the polygenic
score is less valid for non-Europeans, particularly those of sub-Saharan
African descent. It thus predicts the IQ of African Americans with five times
less accuracy than that of European Americans (Lasker et al. 2019).
Generational
change
In
addition to predicting differences in mean IQ across space, we can do the same
across time. That begs the question: were past generations as intelligent as
the latest one? We can answer that question by examining past generations who
are still alive. That approach, however, raises the issue of survivorship bias:
people who live to an old age are generally smarter than those who do not
(Gottfredson and Deary 2004).
Beauchamp
(2016) deals with this issue point by point when he discusses polygenic data
from the Health and Retirement Study (HRS), a longitudinal study of 20,000
Americans shortly before and during retirement:
·
The
HRS participants are people who have lived to the age of 50; however, about 10%
of American women and 15% of American men born in 1940 were already dead by the
age of 50.
·
Thus,
in Beauchamp’s sample, 85% of the original participants were still alive in
2008, 69% were asked to be genotyped, and 59% consented to be genotyped.
·
Nonetheless,
no important differences emerged when he compared the genotyped participants
with the total sample.
European Americans
Beauchamp
found evidence that alleles associated with high educational attainment had
declined in frequency among European Americans between the 1931 and 1953 birth
cohorts. “[M]y results strongly suggest that genetic variants associated with
EA have slowly been selected against among both female and male Americans of
European ancestry born between 1931 and 1953.”
That
decline is attributed to differences in fertility: “individuals with high EA
typically have children at a more advanced age, which may further reduce their
fitness.”
Icelanders
The
above findings have been replicated by Kong et al. (2017) in their study of
Icelanders born since 1910. Although survivorship bias is still a problem, it
does not easily explain the decline in cognitive ability between the last two
cohorts, i.e., Icelanders born in the 1970s and those born in the 1980s. In
that decline, any survivorship bias would be due to deaths of people less than
44 years old, and in most cases less than 36 years old. “The samples studied
here were collected between 1998 and 2014, with a majority (68%) ascertained
before 2006” (Kong et al, 2017, p. E729).
Iceland’s
cognitive decline had two “pauses”: one in the 1950s and another in the 1970s.
The first pause coincides with the postwar economic boom and a corresponding improvement
in the ability of middle class couples to start families early in life. The
second pause may reflect the passage in 1975 of Iceland’s abortion law, which,
while not allowing abortion on demand, did allow it for cases of rape, mental
disability of the mother, and “difficult family situation” (Wikipedia 2022)
Kong
et al. (2017) concluded that the cognitive decline was due only in part to more
intelligent Icelanders staying in school longer and postponing reproduction. In
fact, a high educational level, unlike a high polygenic score, was actually
associated with somewhat higher fertility among males. A high polygenic score
seems to reduce fertility independently of whether one pursues or does not
pursue higher education, perhaps because higher intelligence goes hand in hand
with greater ability to plan ahead and thus foresee, with trepidation, the
costs of raising a family.
British of
European descent
In
a study of British of European origin, using the UK Biobank, Hugh-Jones and
Abdellaoui. (2022) found that mean cognitive ability had declined between two
successive generations, particularly in lower-income groups. The median birth
year was 1950 for the second generation and unknown for the first. The authors
also looked at genetic variants that influence non-cognitive traits. In
general, the bulk of the population seems to be getting dumber, fatter, and nuttier.
The
authors nonetheless warn against excessive pessimism:
Many people would probably prefer to have
high educational attainment, a low risk of ADHD and major depressive disorder,
and a low risk of coronary artery disease, but natural selection is pushing
against genes associated with these traits. Potentially, this could increase
the health burden on modern populations, but that depends on effect sizes.
The
authors go on to argue that the effect sizes are “small,” although one wonders:
smaller than what? They then make the obvious point that generational change
can accumulate from one generation to the next: “Although effects on our
measured polygenic scores are small even after weighting, individually small disadvantages
can cumulate to create larger effects” (Hugh-Jones and Abdellaoui 2022).
Beauchamp (2016) makes a similar comment that seems reassuring on first
thought, and then not so reassuring on second thought: “natural selection has
thus been occurring in that population—albeit at a rate that pales in
comparison with the rapid changes that have occurred in recent generations.”
Finally,
Hugh-Jones and Abdellaoui (2022) point out that their data may suffer from
ascertainment bias. For instance, the first generation of their dataset is
composed of the parents of the second generation. The dataset thus excludes the
childless individuals of the first generation. Were they more intelligent or
less intelligent on average than the succeeding generation? Furthermore,
participation in the UK Biobank is voluntary. Could that factor also be a
source of bias? If so, in what direction?
Conclusion
As
time goes by, intergenerational genetic datasets will become more complete, and
the problem of survivorship bias will diminish. Ideally, we should conduct a
“genetic census” of each generation, perhaps by collecting a DNA sample from
everyone at the time of death. We will thus be able to see how we are evolving.
We
now have intergenerational polygenic studies from three different Western
countries: the U.S., the U.K., and Iceland. In all three cases, the genetic
component of cognitive ability declined during the 20th century, with the
exception of two pauses: one during the postwar baby boom and the other with
the liberalization of access to abortion.
If
we wish to halt the cognitive decline, we should push for the following
measures:
·
A
return to the protected high-wage economy that prevailed during the postwar
era;
·
Free
access to abortion, at least for cases of rape, mental disability of either
parent, and difficult economic circumstances;
·
Pro-natalist
measures to counteract the fear of not having the means to support a family.
This fear is particularly strong among people who like to plan and are oriented
toward the future.
References
Beauchamp,
J.P. (2016). Genetic evidence for natural selection in humans in the
contemporary United States. Proceedings
of the National Academy of Sciences 113(28): 7774-7779. https://doi.org/10.1073/pnas.1600398113
Frost,
P. (2022). Recent cognitive evolution in West Africa: the Niger’s role. Evo and Proud, April 30. https://evoandproud.blogspot.com/2022/04/recent-cognitive-evolution-in-west.html
Gottfredson,
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Wikipedia (2022). Abortion
in Iceland. https://en.wikipedia.org/wiki/Abortion_in_Iceland
