Paradoxical features of the genetics of intelligence
... The recent burst of interest in IQ, sparked first by Dominic Cummings (Michael Gove’s adviser), and then by Boris [Johnson], has been encouraging in one sense. As Robert Plomin, probably the world’s leading expert on the genetics of intelligence, put it to me, there used to be a kneejerk reaction along the lines of “you can’t measure intelligence”, or “it couldn’t possibly be genetic”. This time the tone is more like: “Of course, there is some genetic influence on intelligence but . . .”
The evidence from twin studies, adoption studies and even from DNA evidence is relentlessly consistent: in children, in Western society, the heritability of IQ scores is about 50 per cent. The other half comes equally from family (shared environment) and from unshared individual experiences: luck, teachers, friends.
This numerical precision easily misleads us into thinking genes and environment struggle against each other. In fact, they are like two pillars supporting an arch: nature makes you seek out nurture, which brings out your nature. But here is where things get interesting. The acceptance of genetic influence on intelligence leads to some surprising, even paradoxical implications, some of which turn the assumptions of both the Right and the Left upside down.
First, if intelligence was not substantially genetic, there would be no point in widening access to universities, or in grammar schools and bursaries at private schools trying to seek out those from modest backgrounds who have more to offer. If nurture were everything, kids unlucky enough to have been to poor schools would have irredeemably poor minds, which is nonsense. The bitter irony of the nature-nurture wars of the 20th century was that a world where nurture was everything would be horribly more cruel than one where nature allowed people to escape their disadvantages.
The Left, which has championed nurture against nature, is learning to take a different view — over homosexuality, for example, or learning disability, genetic influence is used as an argument for tolerance. A recent Guardian headline criticised Boris by saying “gifted children are failed by the system”, which presupposes the existence of (genetically) gifted children.
The second surprise is that genetic influence increases with age. If you measure the correlation between the IQs of identical twins and compare it with that of adopted siblings, you find the difference grows dramatically as they get older. This is chiefly because families shape the environments of young children, whereas older children and adults select and evoke environments that suit their innate preferences, reinforcing nature.
[See the new paper by Briley, D. A. , & Tucker-Drob, E. M. (in press). Explaining the increasing heritability of cognitive ability over development: A meta-analysis of longitudinal twin and adoption studies. Psychological Science.]
It follows — the third surprise — that much of what we call the “environment” proves to be itself under genetic influence. Children who are very good at reading are likely to have parents who read a lot, schools that give them special opportunities and friends who recommend books. They create a reading-friendly environment for themselves. The well-documented association between family socio-economic status and IQ, routinely interpreted as an environmental effect, is, writes Professor Plomin and colleagues, “substantially mediated by genetic factors”. Perhaps intelligence is an appetite, at least much as an aptitude, for learning.
The fourth surprise is that the better the economy, education, and welfare are, the more heritable IQ will be. Just as having extra food will make you brighter if you are starving, but not if you are plump, so the same applies to toys, teachers, books and friends. Once you have enough of any of these things, having more will not make as much difference.
Christmas shopping hint: Children tend to disagree that enough is enough when it comes to toys.
So differences due to environment will fade. In a world when some are starving and some are kings, the differences would be mainly environmental. In a world where all went to Balliol, the main difference remaining would be genetic. Social reformers rarely face this fact — the more we equalise opportunity, the more the people who get to the top will be the genetically talented.
For example, English aristocrats like Ridley used to be dramatically taller on the whole than the national average. Not so much anymore. Now height is more genetic.
And this brings a final paradox: a world with perfect social mobility would show very high heritability. The children of Balliol parents would qualify for Balliol disproportionately, having inherited both aptitude and an appetite for evoking the environments that amplified that aptitude. Far from indicating that parents are giving their children unfair environmental advantages, a high correlation between the achievements of parents and offspring suggests that opportunity is being levelled, albeit slowly and patchily. In Professor Plomin’s words: “Heritability can be viewed as an index of meritocratic social mobility.”
Moreover, assortative mating is probably reinforcing the trend. That is to say, 50 years ago, when women were not often allowed near higher education, Professor Branestawm chose to marry the girl next door because she was good at ironing his shirts, whereas today he marries another professor because she writes gorgeous equations about quantum mechanics, and they have children who are professors squared.
We are a long way from equality of opportunity, but when we get there we will not find equality of outcome. Already IQ — for all its flaws as an objective measure of intelligence — is good at predicting not just educational attainment, but income, health and even longevity remarkably well.
Do we reconcile ourselves to inequality, then? No! Just because capability is inherited does not mean it is immutable. Hair colour and short sight are highly heritable, but both can be altered. Education is not just about coaxing native wit from the gifted, but also coaching it into the less gifted.