Great at reading or recognizing faces? You might not do so well on an IQ test. Source: Histoire
naturelle générale et particulière avec la Description du Cabinet du Roy (1749)
(Wikicommons)
The
English psychologist Charles Spearman was the first to argue that a single
factor, called "g," explains most of the variability in human
intelligence. When observing the performance of children at school, he noticed
that a child who did well in math would also do well in geography or Latin.
There seemed to be a general factor that facilitates almost any kind of mental
task.
Spearman
did, however, acknowledge the existence of other factors that seem more
task-specific:
[...]
all branches of intellectual activity have in common one fundamental function
(or group of functions), whereas the remaining or specific elements of the
activity seem in every case to be wholly different from that in all the others.
(Spearman, 1904, p. 284)
That
is where things stood for over a century. In recent years, however, we’ve begun
to identify the actual genes that contribute to intelligence. These genes are
very numerous, numbering perhaps in the thousands, with each one exerting only
a small effect. Many act broadly on intelligence in general and may correspond
to the g factor, which seems to be a widespread property of neural tissue,
perhaps cortical thickness or the integrity of white matter in the brain. Other
genes act more narrowly on specific mental tasks. The ability to recognize
faces, for instance, seems to have no relation at all to general intelligence.
You can be great at recognizing faces while being as dumb as rocks (Zhu et al., 2009).
One
way to locate these genes is through genome-wide association studies. We look
at the various alleles of genes whose locations are already known, typically
SNPs (single nucleotide polymorphisms), and see whether this source of variability
correlates with variability in a mental trait. If we find a significant
correlation, the genes for that trait must be nearby. The same kind of study can
also show us how narrowly or broadly these genes act. Do they merely influence
intelligence in general? Or do they provide more specific instructions? Such as
how to recognize certain objects or how to react to them?
A
genome-wide association study has recently shed light on various mental traits.
In most cases, a common factor seems to explain about half of the genetic variability.
This common factor is weakest for emotion identification, i.e., the ability to
identify the emotions of other people by their facial expressions. Emotion
identification actually correlates negatively with nonverbal reasoning (-0.25)
and only weakly with verbal memory (0.17) and spatial reasoning (0.26). The
highest correlation is with reading (0.40) and language reasoning. (0.45). Reading
and language reasoning are highly intercorrelated, perhaps because they share the same mental module (Robinson et al., 2014).
This
partial modularity has been confirmed by a recent twin study on reading and
math ability. If we look at the genetic component of either reading or math
ability, at least 10% and probably half affects performance on both tasks.
Conversely, the other half is specific to either one or the other (Davis et al., 2014).
An evolutionary
mystery?
But
how can reading ability have a specific genetic basis if people began to read
only in historic times? Indeed, history is said to begin with the first written
documents. Surely humans weren't still evolving at that point?
To
ask the question is to answer it. Not only were they still evolving, they were
actually doing so at a faster pace than their prehistoric ancestors. Humans
have undergone much more genetic change over the past 10,000 years than over
the previous 100,000 (Hawks et al., 2007). This is a difficult fact to swallow,
let alone digest, but we must learn to accept it and all of its implications.
The
new findings on reading ability are consistent with other ones. The human brain
has a special region, called the Visual Word Form Area, that is used to
recognize written words and letters. If it is damaged, your reading ability will
suffer but not your recognition of objects, names, faces, or general language
abilities. There will be some improvement over the next six months, but reading
will still take twice as long as it had previously. This brain region varies in
size and organization from one individual to another and from one human
population to another, being differently organized in Chinese people than in
Europeans (Frost, 2014; Gaillard et al, 2006; Glezer and Riesenhuber, 2013;
Levy et al., 2013; Liu et al., 2008).
Genome-wide
association studies may help us pinpoint the actual genes responsible for the
Visual Word Form Area. In fact, we may have already found one: ASPM. This gene influences brain growth
in other primates and has evolved in humans right up into historic times. Its
latest allele arose about 6000 years ago in the Middle East and proliferated
until it reached incidences of 37-52% in Middle Easterners, 38-50% in
Europeans, and 0-25% in East Asians. Despite its apparent selective advantage,
this allele does not improve performance on IQ tests (Mekel-Bobrov et al.,2007; Rushton et al., 2007). It is nonetheless associated with larger brain
size in humans (Montgomery and Mundy, 2010).
Its
Middle Eastern origin some 6000 years ago suggests this allele may have owed
its success to the invention of writing. Most people had trouble reading, writing,
and copying lengthy texts in ancient times, when characters were written continuously
with little or no punctuation. There was an acute need for scribes who could excel
at this task, and such people were rewarded with reproductive success (Frost, 2008; Frost, 2011).
Conclusion
Human
intelligence is modular to varying degrees, and much of this modularity seems
to have arisen during historic times. It is a product of humans adapting not
only to their physical environments but also to their more rapidly evolving
cultural environments.
While
there is such a thing as general intelligence, it seems to be only half of the
picture. Two people may have the same IQ and yet differ significantly in
various mental abilities. There may also be trade-offs between general
intelligence and more specific mental tasks. If you're great at abstract
reasoning, you may be lousy at decoding facial expressions. This may be because
the two abilities compete with each other for limited mental resources. Or it
may be that selection for abstract reasoning has occurred in an environment
where people can trust each other and have no need to scrutinize facial
expressions for signs of lying ... or imminent physical assault.
The
same applies to human populations. Two populations may have the same mean IQ,
and yet differ statistically over a large number of mental and behavioral
traits. Although these differences may be scarcely noticeable if we compare two
individuals taken at random from each population, their accumulative effect
over many thousands of individuals can steer one population along one path of
cultural evolution and the other along another. Furthermore, two populations
may arrive at a similar outcome via different paths of cultural evolution and
via different mental and behavioral packages. Europeans and East Asians have
both reached an advanced level of societal development, but this similar
outcome has been achieved in East Asian societies largely through external
mediation of rule enforcement (e.g., shaming, peer pressure, family discipline)
and in European ones mainly through internal means of control (e.g., guilt,
empathy).
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