Read more: “Instant Expert: Intelligence“
Over the past century, each successive generation has answered more IQ test items correctly than the last, the rise being equivalent to around 3 IQ points per decade in developed nations. This is dubbed the “Flynn effect” after the political scientist James Flynn, who most thoroughly documented it. Are humans getting smarter, and if so, why?
One possible explanation is that today’s world supports or demands higher levels of intelligence. Flynn himself suggests that intelligence has risen in part because we view life more analytically, through “scientific spectacles”. However, the idea that cultural environments have potent and widespread effects on how smart we are does not square with what we know about the high heritability of intelligence. Environmental variation contributes relatively little to the IQ differences in a birth cohort as its members mature over the decades. How, then, could it create such big IQ differences across successive birth cohorts living in the same era?
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Another theory puts rising IQ down to physiological changes. In the past century human height has been increasing in tandem with IQ throughout the developed world. Better public health measures have reduced the need for our immune systems to consume resources to combat infectious disease, leaving us able to spend more on growth – and larger, smarter brains may be just one consequence. Not only that, as more people travelled and married outside their local group, populations may have benefited genetically from hybrid vigour. Inbreeding is known to lower intelligence, and outbreeding can raise it.
It is also possible that the Flynn effect does not in fact reflect a rise in general intelligence, or g. After all, can the average IQ of adults at the end of the second world war really have been 20 points less than today? That would put them in the bottom 10 per cent of intelligence by current standards, making them legally ineligible to serve in the US military on grounds of poor trainability. It defies belief.
Instead of an overall increase in g, perhaps just certain biologically rooted cognitive abilities are increasing. An IQ test comprises a series of subtests, and it turns out that scores in some of these have increased a lot – including our ability to identify similarities between common objects – whereas others have not increased at all – such as scores in the vocabulary and arithmetic subtests. That would imply changes in specific brain regions rather than the whole brain.
The inter-generational rise in IQ test scores is a brain-twister for researchers trying to figure out what it means. Nevertheless, it does not undermine the use of IQ tests within generations. Current IQ tests are not intended to give an absolute measure of intelligence akin to grams and kilograms, but only to rate an individual’s intellectual capacity relative to others born in the same year – no matter what the cohort, the mean score is always set at 100. As for the variation in g that IQ tests measure, it seems as wide and as consequential as ever.
Read more: “Instant Expert: Intelligence“
