28 THE SCIENTIST | the-scientist.com
© DUNG HOANGI
n 1987, political scientist James Flynn of
the University of Otago in New Zealand
documented a curious phenomenon:
broad intelligence gains in multiple human
populations over time. Across 14 countries
where decades’ worth of average IQ scores
of large swaths of the population were avail-
able, all had upward swings—some of them
dramatic. Children in Japan, for example,
gained an average of 20 points on a test
known as the Wechsler Intelligence Scale for
Children between 1951 and 1975. In France,
the average 18-year-old man performed 25
points better on a reasoning test in 1974 than
did his 1949 counterpart.
Flynn initially suspected the trend
reflected faulty tests. Ye t in the ensuing
years, more data and analyses supported the
idea that human intelligence was increas-
ing over time. Proposed explanations for
the phenomenon, now known as the Flynn
effect, include increasing education, bet-
ter nutrition, greater use of technology, and
reduced lead exposure, to name but four.
Beginning with people born in the 1970s,the trend has reversed in some Western
European countries, deepening the mystery
of what’s behind the generational fluctua-
tions. But no consensus has emerged on the
underlying cause of these trends.
A fundamental challenge in understand-
ing the Flynn effect is defining intelligence.
At the dawn of the 20th century, English psy-
chologist Charles Spearman first observed
that people’s average performance on a vari-
ety of seemingly unrelated mental tasks—
judging whether one weight is heavier than
another, for example, or pushing a button
quickly after a light comes on—predicts our
average performance on a completely differ-
ent set of tasks. Spearman proposed that a
single measure of general intelligence, g, was
responsible for that commonality.
Scientists have proposed biological
mechanisms for variations among individ-
uals’ g levels ranging from brain size and
density to the synchrony of neural activ-
ity to overall connectivity within the cor-
tex. But the precise physiological origin of
g is far from settled, and a simple explana-tion for differences in intelligence between
individuals continues to elude researchers.
A recent study of 1,475 adolescents across
Europe reported that intelligence, as mea-
sured by a cognitive test, was associated with
a panoply of biological features, including
known genetic markers, epigenetic modifi-
cations of a gene involved in dopamine sig-
naling, gray matter density in the striatum
(a major player in motor control and reward
response), and the striatum’s activation in
response to a surprising reward cue.
Understanding human smarts has been
made even more challenging by the efforts
of some inside and outside the field to intro-
duce pseudoscientific concepts into the mix.
The study of intelligence has at times been
tainted by eugenics, “scientific” racism, and
sexism, for example. As recently as 2014, for-
mer New York Times science writer Nicholas
Wade drew fire for what critics character-
ized as misinterpreting genetics studies to
suggest race could correlate with average dif-
ferences in intelligence and other traits. The
legitimacy of such analyses aside, for today’sImaging, behavioral, and genetic data yield clues
to what’s behind effective thinking.BY SHAWNA WILLIAMS