Read more: “Instant Expert: Intelligence“
Intelligence tests are calibrated so that, at each age, the IQ average score is 100 and 90 per cent of individuals score between IQ 75 and 125. The typical IQ difference between strangers is 17 points and it is 12 between full siblings. Everybody accepts that intelligence varies. But what makes some people smarter than others? How do nature and nurture interact to create that variation as we develop? Are differences in g set at birth, or can we increase someone’s intelligence by nurturing them in the right environment?
Nature and nurture
Each of us is the embodiment of our genes and the environment working together from conception to death. To understand how these two forces interact to generate differences in intelligence, behavioural geneticists compare twins, adoptees and other family members. The most compelling research comes from identical twins adopted into different homes – individuals with identical genes but different environments – and non-kin adopted into the same home – unrelated individuals sharing the same environment. These and other studies show that IQ similarity most closely lines up with genetic similarity. More intriguingly, the studies also reveal that the heritability of intelligence – the percentage of its variation in a particular population that can be attributed to its variation in genes – steadily increases with age. Heritability is less than 30 per cent before children start school, rising to 80 per cent among western adults. In fact, by adolescence, separated identical twins answer IQ tests almost as if they were the same person and adoptees in the same household as if they were strangers. The surprising conclusion is that most family environments are equally effective for nurturing intelligence – the IQ of an adult will be the same almost regardless of where he or she grew up, unless the environment is particularly inhumane.
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“Intriguingly, the heritability of intelligence is less than 30 per cent before children start school, rising to 80 per cent among adults”
Why does the shared environment’s power to modify IQ variation wane and genetic influences increase as children gain independence? Studies on the nature of nurture offer a clue. All children enter the world as active shapers of their own environment. Parents and teachers experience this first-hand as their charges frustrate attempts to be shaped in particular ways. And increasing independence gives young people ever more opportunities to choose the cognitive complexity of the environments they seek out. The genetically brighter an individual, the more cognitively demanding the tasks and situations they tend to choose, and the more opportunities they have to reinforce their cognitive abilities.
Given that an individual’s ability to exploit a given environment is influenced by their genetic endowment, and given that “better” family environments tend not to produce overall increases in IQ, it is not surprising that attempts to raise low IQs by enriching poor school or home environments tend to disappoint. Narrow abilities can be trained up but g apparently cannot. This makes sense if g is an overall property of the brain. That does not mean intensive early educational interventions lack positive effects: among other things they may reduce rates of teenage pregnancy, delinquency and school dropout. Besides, even if we cannot boost low intelligence into the average range, we do know how to help all children learn more than they currently do and achieve more with the intelligence they have.
Older and wiser
The brain is a physical organ and no less subject than any other to ageing, illness and injury. The normal developmental trajectory is that aptitude at learning and reasoning – mental horsepower – increases quickly in youth, peaks in early adulthood, and then declines slowly thereafter and drops precipitously before death. The good news is that some important abilities resist the downturn.
Some IQ researchers distinguish between tests of fluid intelligence (gF) and crystallised intelligence (gC). The first assess on-the-spot learning, reasoning and problem solving; the second assess the crystallised fruits of our previous intellectual endeavours, such as vocabulary in one’s native language and broad cultural knowledge. During youth gF and gC rise in tandem, but they follow different trajectories thereafter. All gF abilities decline together, perhaps because the brain’s processing speed slows down with age. However, most people’s gC abilities remain near their personal peak into old age because they reside in the neural connections that gF has laid down over a lifetime of learning and practice. Of course, age-related memory loss will affect an individual’s ability to recall, but exactly how this affects intelligence is not yet known.
This has practical implications. On the positive side, robust levels of gC buffer the effects of declining gF. Older workers are generally less able to solve novel problems, but they can often compensate by calling upon their larger stores of experience, knowledge and hard-won wisdom. But gC can also disguise declines in gF, with potentially hazardous results. For example, health problems in later life can present new cognitive challenges, such as complex treatments and medication regimes, which individuals with ample gC may appear to understand when actually they cannot cope.
There are ways of slowing or reversing losses in cognitive function. The most effective discovered so far is physical exercise, which protects the brain by protecting the body’s cardiovascular health. Mental exercise, often called brain training, is widely promoted, but it boosts only the particular skill that is practised – its narrow impact mirroring that of educational interventions at other ages. Various drugs are being investigated for their value in staving off normal cognitive decline, but for now preventive maintenance is still the best bet – avoid smoking, drinking to excess, head injuries and the like.
Read more: “Instant Expert: Intelligence“
