Soviet-era
stamp. In Estonia, cranial volume shrank between the cohort of girls born in 1937
and those born in 1962, apparently because the intellectually gifted were more
likely to pursue higher education and postpone childbearing.
Is
the genetic basis for intelligence declining from one generation to the next?
That’s the conclusion of several recent studies on alleles associated with high
educational attainment. By adding up such alleles over the genome, we can get a
person's "polygenic score." By calculating the mean polygenic score
for each generation, we can then find out whether this genetic basis is
declining.
The
polygenic score has declined among Icelanders since the cohort born in 1910 and
among Euro Americans between the 1931 and 1953 cohorts (Beauchamp 2016; Kong et
al. 2017). The Icelandic study is especially interesting because that country
took in very few immigrants during the period under study. The decline was thus
driven by internal factors. One reason seems to be the tendency of
university-educated people to delay reproduction and have fewer children. But
that's not the whole story. Even among Icelanders who didn’t pursue higher
education, fertility was lower among the intellectually gifted, apparently
because their intelligence was associated with a desire to plan for the future
and delay gratification.
Before
the twentieth century, such forward-looking people were reproductively
successful. They were the ones who had enough resources to survive disasters of
one sort or another: famine, disease, the Little Ice Age, etc. Today, such
disasters are a lot less deadly, and it no longer matters so much whether one
is a grasshopper or an ant.
Moreover,
because of demographic and cultural changes during the twentieth century, it’s
no longer possible to count on the same degree of assistance for child-raising
from relatives and grandparents. With childbearing at later ages, grandparents
are either dead or too frail to help. With people moving around more, not all relatives
live nearby. If you’re the sort of person who plans for the future and delays
gratification, you may be a lot more intimidated than your forbears by the
costs of raising a family.
Shrinking cranial
volume in Estonians
Cranial
volume correlates with IQ and with educational attainment, albeit imperfectly
(see Frost 2020). Has it been declining in tandem with the decline in alleles
for educational attainment?
In
Soviet-era Estonia, cranial volume was one of several anthropometric traits
that were measured in girls born between 1937 and 1962. Because the
measurements were mandatory, there was no volunteer basis; mortality bias was
minimal because all the participants were younger than 20. In this respect, the
study is better than Western biobank studies. On the other hand, the results
may be less applicable to Western populations, given the differences in
demographic history. Estonia had no postwar baby boom. Fertility then rose from
the late 1960s until the breakup of the Soviet Union. By the late 1980s, fertility
was actually higher in Estonia than in any other major region of Europe.
Nonetheless,
there were demographic similarities between Soviet Estonia and the West,
particularly the rising prevalence of single mothers and the influence of
education on fertility:
-
Divorce rates began to rise during the interwar years, equalling or exceeding
those of Scandinavia from the 1970s onward.
-
Throughout the twentieth century, Estonian women with only primary education
bore 0.5 to 0.75 more children on average than women with tertiary education.
In the population under study, taller children and those with larger crania
were more likely to go on to secondary and/or tertiary education, independently
of sex, socioeconomic position, and rural vs urban origin (Valge et al. 2019).
The
second factor seems to explain why cranial volume declined from the older
cohorts to the younger ones:
[...]
the majority of selection for smaller cranial volume acted indirectly via
educational attainment, whereas the direct path of selection in the SEM model
was non-significant (Figs. 2 and 4). In other words, consistent with our prior
expectations, girls with larger heads were selected against because they were
more likely to obtain higher education than girls with smaller heads. Lower
education (Tiit, 2013) and rural origin (Kulu, 2005) have been independently
and additively associated with higher fertility in Estonia throughout the past
century. The reason for the link between education and fertility is that early
reproduction, a major determinant of LRS, is not compatible with schooling for
both cultural and genetic reasons. (Valge 2020)
It
is doubtful that this decline is due to ethnic change. All of the girls were
from Estonian schools (Russian-speakers had their own schools). Nonetheless
some of them were of mixed background. According to a personal communication
from the corresponding author, 84% of the fathers and 93% of the mothers were
Estonian. Ideally, the study should be redone without individuals of mixed
parentage. The problem here is not only that one of the parents was
non-Estonian but also that such individuals were disproportionately economic
migrants who had trouble finding suitable work elsewhere in the Soviet Union.
Other
anthropomorphic changes
Height
also declined. Unlike cranial volume, this decline was not wholly explained by
educational/socioeconomic differences:
Notably,
higher reproductive success of shorter girls in Estonia could not be entirely
ascribed to indirect selection via educational attainment, nor via other
measured socioeconomic variables such as rural/urban origin, although indirect
selection via education did account for a large portion of total selection
(Fig. 4). The finding that selection against height remains after controlling
for education or income (that favours less-educated individuals who are
generally shorter than highly-educated ones) is consistent with findings of
studies reviewed by Stulp and Barrett (2016).
Female
hips and female jaws became narrower even after controlling for educational/socioeconomic
differences. There seems to have been selection for rounder female faces, but
this selection is significant only if one allows for nonlinear effects. Finally,
there was no direct selection on two markers of overall health and nutritional
status: handgrip strength and lung capacity.
In
general, "direct selection favoured shorter, slimmer and lighter girls
with smaller heads, more masculine facial and body shapes and slower rates of
sexual maturation."
Conclusion
The
genetic basis for intelligence has declined in European populations, apparently
since the early twentieth century. This decline is attested by two
"hard" measures: 1) alleles associated with educational attainment;
and 2) cranial volume. Furthermore, it is attested in two relatively homogenous
societies, i.e., Iceland and Estonia.
In
Estonia, the decline seems entirely due to the intellectually gifted going to
university and postponing family formation. In Iceland, this factor explains
only part of the decline: the intellectually gifted chose to postpone family
formation even when they didn't go to university. Perhaps the Soviet system was
better at steering gifted individuals into higher education.
On
a final note, this problem will not go away on its own. If we wish to have
large numbers of intellectually gifted people who plan for the future and delay
gratification, we will need to reverse certain social and cultural changes of
the twentieth century.
Comments by Peeter Horak
In an email, Peeter pointed out that the decrease in height due to natural selection might be offset by an increase in height due to lower pathogen load (as a result of vaccination and antibiotics, see Hõrak and Valge 2015). In addition, we currently don't know the direction of
selection on boys. It may entirely cancel out natural selection on girls if men's income and education correlate positively with their
reproductive success. In the sample under study, taller
boys and those with larger heads went on to obtain more education; if they were reproductively successful, there would be sexually antagonistic
selection: selection would favor larger boys and smaller girls at the same
time.
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://www.ncbi.nlm.nih.gov/pmc/articles/PMC4948342/
Frost,
P. (2020). Did women jumpstart recent cognitive evolution? Evo and Proud, July 1 https://evoandproud.blogspot.com/2020/07/did-women-jumpstart-recent-cognitive.html
Hõrak, P., and M. Valge. (2015). Why did children grow so well at hard times? The ultimate importance of pathogen control during puberty, Evolution, Medicine, and Public Health, 2015 (1): 167–178, https://doi.org/10.1093/emph/eov017
Kong,
A., M.L. Frigge, G. Thorleifsson, H. Stefansson, A.I. Young, F. Zink, G.A.
Jonsdottir, A. Okbay, P. Sulem, G. Masson, D.F. Gudbjartsson, A. Helgason, G.
Bjornsdottir, U. Thorsteinsdottir, and K. Stefansson. (2017). Selection against
variants in the genome associated with educational attainment. Proceedings of the National Academy of
Sciences 114(5): E727-E732 https://core.ac.uk/download/pdf/154416179.pdf
Valge,
M., P. Horak, and J.M. Henshaw. (2020). Natural selection on anthropometric
traits of Estonian girls. Evolution and
Human Behavior in press. https://doi.org/10.1016/j.evolhumbehav.2020.07.013
Valge,
M, R. Meitern, and P. Horak. (2019). Morphometric traits predict educational attainment
independently of socioeconomic background. BMC
Public Health 19: 1696. https://link.springer.com/article/10.1186/s12889-019-8072-7
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