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
Gaillard et al, 2006;
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 (
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 (
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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