Education's marginal cognitive benefit does not reach a plateau until 17 years of education; those with low childhood intelligence derive the largest benefit of education

The influence of educational attainment on intelligence. Emilie Rune Hegelund et al. Intelligence. Volume 78, January–February 2020, 101419. https://doi.org/10.1016/j.intell.2019.101419

Highlights
•    Education has a positive influence on intelligence.
•    The marginal cognitive benefit of education does not reach a plateau until 17 years of education.
•    Individuals with low childhood intelligence derive the largest benefit from education.
•    Findings of relatively small cognitive benefits might be explained by selection bias.

Abstract: Education has been found to have a positive influence on intelligence, but to be able to inform policy, it is important to analyse whether the observed association depends on the educational duration and intelligence prior to variations in educational attainment. Therefore, a longitudinal cohort study was conducted of all members of the Metropolit 1953 Danish Male Birth Cohort who were intelligence tested at age 12 and appeared before the Danish draft boards (N = 7389). A subpopulation also participated in the Copenhagen Aging and Midlife Biobank (N = 1901). The associations of educational attainment with intelligence in young adulthood and midlife were estimated by use of general linear regression with adjustment for intelligence test score at age 12 and family socioeconomic position. Results showed a positive association of educational attainment with intelligence test scores in both young adulthood and midlife after prior intelligence had been taken into account. The marginal cognitive benefits depended on the educational duration but did not reach a plateau until 17 years. Further, intelligence test score at age 12 was found to modify the association, suggesting that individuals with low intelligence in childhood derive the largest benefit from education. Comparing the strength of the association observed among participants and non-participants in our midlife study, we showed that selection due to loss to follow-up might bias the investigated association towards the null. This might explain previous studies' findings of relatively small cognitive benefits. In conclusion, education seems to constitute a promising method for raising intelligence, especially among the least advantaged individuals.


4.2. Comparison with the existing literature
The finding of a positive association between educational attainment
and intelligence test scores after prior intelligence has been taken
into account is consistent with extant literature (Clouston et al., 2012;
Falch & Massih, 2011; Ritchie, Bates, Der, Starr, & Deary, 2013), including
a recent meta-analysis (Ritchie & Tucker-Drob, 2018). More
specifically, our results suggested an average increase in intelligence
test score of 4.3 IQ points per year of education in young adulthood and
1.3 IQ points per year of education in midlife. The effect estimate in
young adulthood is considerably higher than the effect estimate of 1.2
IQ points (95% confidence interval: 0.8, 1.6) reported in the metaanalysis
for the control prior intelligence design. However, in a simultaneous
multiple-moderator analysis, the authors report an adjusted
effect estimate of 2.1 IQ points (95% confidence interval: 0.8, 3.4),
taking into account the possible influence of age at early test, age at
outcome test, outcome test category, and male-only studies. Besides
these possible moderators, contextual factors might also account for the
contrasting findings between our study and the seven studies included
in the meta-analysis. However, it is important to notice that our effect
estimate in midlife is consistent with the findings of the meta-analysis,
suggesting that sample selectivity might have influenced the association
observed in midlife in our study and perhaps the associations observed
in the cohort studies included in the meta-analysis as well. This is
supported by findings of higher educational attainment among the individuals
in our study population who participated in the midlife study,
higher IQ at age 12 (IQ mean: 103.2 vs. 98.9) as well as our finding of
effect measure modification. If both educational attainment and intelligence
are positively associated with participation in studies and
individuals with low intelligence in childhood derive the largest benefit
from education, selection will bias the investigated associations towards
the null. This is probably the reason why the study with the least
selected sample in the meta-analysis is the one reporting the largest
effect estimate (Falch & Massih, 2011). Based on a male population who
were initially intelligence tested in school at age 10 and later at the
mandatory military draft board examination at age 20, the authors
found an increase in intelligence test score of 3.5 IQ points per year of
education (95% confidence interval: 3.0, 3.9), which is much more in
line with our results.
Our finding of a stronger association between educational attainment
and intelligence test scores in young adulthood compared with
midlife in the subpopulation of individuals who participated in both
examinations is consistent with the findings of the meta-analysis
(Ritchie & Tucker-Drob, 2018). One possible explanation for this
finding might be that schooling has a larger influence on intelligence
compared with vocational education or training, which mainly takes
place after the age of 18. However, the finding might also be explained
by the smaller time gap between the measurements of educational attainment
and intelligence, as the measurement of educational attainment
in midlife in most cases will reflect one's educational attainment
before the age of 30. The older the age at outcome testing, the larger the
time gap and thus more additional factors might influence the association,
such as the individual's occupational complexity and health
(Smart, Gow, & Deary, 2014; Waldstein, 2000).
In general, our finding of a positive association between educational
attainment and intelligence test scores should be interpreted with
caution. As previously written, it is difficult to separate the positive
influence of educational attainment on intelligence from the influence
of selection by prior intelligence, whereby individuals with higher intelligence
test scores prior to variations in educational attainment
progress further in the educational system. Although our results show a
strong positive association between educational attainment and intelligence
test scores after prior intelligence has been taken into account,
a hierarchical analysis of our data suggests that educational attainment
only increases the amount of explained variance in later IQ by
7% when IQ at age 12 is already accounted for (R2 = 0.46 vs
R2 = 0.53; p < .001). Therefore, our findings most likely not only
reflect the positive influence of educational attainment on intelligence,
but also a residual influence of selection processes which our statistical
analyses were not able to take into account.
To answer one of our specific aims, we also investigated whether the
increase in intelligence test score for each extra year of education depended
on the educational duration. Irrespective of whether intelligence
was measured in young adulthood or midlife, intelligence test
scores were found to increase with increasing years of education in a
cubic relation, suggesting that the increase in intelligence test score for
each extra year of education diminishes with increasing length of
education. This finding supports the hypothesis proposed by the authors
of a recent study, who, based on their own data and the existing literature,
suggest that the influence of educational attainment on intelligence
eventually might reach a plateau (Kremen et al., 2019).
However, where the authors of the previous study suggest that this
plateau might already be reached by late adolescence as their findings
show no significant association between educational attainment and
intelligence test score in midlife after IQ at age 20 has been taken into
account, we find no plateau until approximately 17 years of education.
In fact, replicating the previous study's statistical analyses we find an
average increase in intelligence test score in midlife of 0.8 IQ points per
year of education taking IQ at age 20 into account (Supplementary
Table 4). We speculate whether this contrasting finding might be explained
by the representativeness of the study populations as the previous
study is based on a selected sample of twins serving in the
American military at some point between 1965 and 1975. Thus, a study
based on the two Lothian birth cohorts, which like our study includes a
follow-up examination of a population-representative survey in childhood,
finds a weighted average increase in intelligence test score in late
life of 1.2 IQ points per year of education taking IQ at age 11 into
account (Ritchie et al., 2013). However, the contrasting finding might
also be explained by residual confounding due to the use of non-identical
baseline and outcome intelligence tests in our study. Nevertheless,
in our study, the strongest association between educational attainment
and intelligence in midlife was observed in upper-secondary school, i.e.
around 10–13 years of education. A possible explanation for this finding
might be that pupils up to and including upper-secondary school receive
general education, which improves exactly what the intelligence
tests included in our study most likely measure: General cognitive
ability. After upper-secondary school, individuals start to specialize in
different fields, which may explain why the increase in intelligence test
score for each extra year of education diminishes. However, the cubic
tendency was relatively weak and as written above the association
between educational attainment and intelligence did not reach a plateau
until approximately 17 years of education, corresponding to the
completion of a Master's degree program. As our study is the first to
investigate whether the increase in intelligence test score for each extra
year of education depends on the educational duration, replication of
our finding is needed – preferably in studies with access to the same
baseline and outcome test.
Finally, to answer another of our specific aims, we investigated
whether the increase in intelligence test score for each extra year of
education depended on the intelligence prior to variations in educational
attainment. The results showed that the increase in intelligence
test score for each extra year of education was higher in the group of
individuals with an IQ < 90 compared with the group of individuals
with an IQ of 90–109. Although this finding clearly needs to be replicated,
it is in line with the findings of a Danish study investigating
whether distributional changes accompanied the secular increases in
intelligence test scores among males born in 1939–1958 and
1967–1969 (Teasdale & Owen, 1989). According to the authors of that
study, a possible explanation of why individuals with low intelligence
in childhood derive the largest benefit from education is that the Danish
school system for the last seven decades mainly has focused on improving
the abilities of the least able (Teasdale & Owen, 1989).
Therefore, future studies are needed to investigate whether our finding
is peculiar to the Danish school system or whether it can be generalized
to school systems in other countries.