Relax, dear parents – you can’t
really increase your child’s intelligence

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Although intelligence has been at the heart of a century-long “nature-nurture” debate, scientists and society now generally accept that genetics plays a role in its development. But people often don’t understand just how important inherited DNA differences are. In my book “Blueprint”, I outline research from my fifty years in the field that leads me to conclude that inherited DNA differences are the major systematic force responsible for individual differences in intelligence. This is an eye-opener for many people, especially parents who are led to believe that their parenting or their children’s schooling are the key factors in their children’s cognitive development.

A mountain of evidence from the last century from twin and adoption studies leaves little room for doubt that genetics contributes importantly to differences between children in their general cognitive ability (measured in the intelligence quotient [IQ]), their specific verbal (e.g., vocabulary) and nonverbal abilities (e.g., spatial ability), and their achievement at school. Here is some of the evidence: IQ scores of adopted children resemble their birth parents’ IQ, not their adoptive parents’ IQ. Identical twins reared apart from birth are almost as similar as identical twins reared together. Identical twins, who are genetically identical, are much more similar than fraternal twins, who, like other siblings, are fifty percent similar genetically.

Heritability increases throughout the lifespan

Together, such data from hundreds of twin and adoption studies indicate that about fifty percent of the differences between children in their cognitive abilities can be explained by genetic differences. This statistic is called heritability, which describes the extent to which differences between people can be explained by inherited DNA differences. It is remarkable that these two very different quasi-experimental designs converge on a heritability estimate of about fifty percent. Even more remarkably, we are now able to use DNA itself to predict intelligence and academic achievement, as described later.

Less well known than the substantial heritability of intelligence is a counterintuitive finding about how heritability changes during the lifespan. Across all studies, genetics accounts for 50 percent of the differences between individuals in cognitive abilities. However, when this research is analysed by age, heritability increases linearly from infancy (20 percent) to childhood (40 percent) to adulthood (60 percent) to later adulthood (80 percent). In other words, inherited DNA differences become increasingly important during the lifespan, contrary to the reasonable reverse assumption that intelligence is increasingly affected by Shakespeare’s “slings and arrows of outrageous fortune”.

«Across all studies, genetics accounts for 50 percent of the differences between individuals in cognitive abilities.»

The nature of nurture17

Why does heritability increase during the lifespan? The answer reveals another important finding from genetic research. The environment is not “out there” hitting us like passive bystanders. Instead, brighter children select, modify and create experiences correlated with their genetic propensities at home, at school, and throughout life. They read more, think more, and get more out of school. As children leave home and make their own way in the world, these genetically driven experiences snowball into ever bigger differences in intellectual capabilities. This phenomenon is technically called gene-environment correlation, or more colloquially, the nature of nurture.

What about the “other half”?

Another surprise from genetic research is the cause of the “other half” of differences between children in cognitive abilities. If half of the differences are due to nature, surely the other half are due to nurture, the family environment that parents provide for their children? That’s true to some extent in childhood. For example, the IQ scores of adoptive siblings (genetically unrelated children adopted into the same family) correlate 0,25, suggesting that 25 percent of IQ differences between children is due to growing up in the same family. However, this shared environmental influence declines in adolescence and disappears in adulthood as children grow up and make their own way in the world, even how well they do at university. So, in the long run, environmental factors that make a difference are not shared by children growing up in the same family.

What are these mysterious nonshared environmental factors that make children growing up in the same family different? After three decades of research trying to identify them, I conclude in a 2024 paper “Nonshared environment: Real but Random” that they are random in the philosophy of science sense of being unpredictable. I think that this randomness is noise that emanates from the complexity of biological systems. Throughout science, from quantum mechanics to biology to cosmology, randomness is supplanting the traditional hubristic belief in determinism. So, the nonshared environmental influences responsible for almost half of the differences in intelligence may be more a matter of chance than choice.

The DNA revolution

The most exciting development is the DNA revolution, which has made it possible to use DNA itself to predict intelligence and its correlate of academic achievement. The revolution began early this century when the three billion base pairs of DNA, the steps in the spiral staircase of DNA’s double helix called the genome, were sequenced. We learned that we all have the same DNA sequence for 99,9 percent of these three billion base pairs – that’s what makes us human. But the one in a thousand DNA differences that we inherit at the moment of conception is what makes us individuals. It’s amazing to think that each of us began life as a single cell with half our DNA from our mother and half from our father and that same DNA is in all the trillions of cells in our body.

The next major advance was the realisation that the heritability of complex traits like intelligence is caused, not by a single gene or several genes, but by thousands of DNA differences each with miniscule effects. It is possible to aggregate these thousands of tiny DNA differences to create an index, called a polygenic score, that can be used to predict differences between children. Polygenic scores can now predict 14 percent of IQ differences between children. The most powerful polygenic score prediction is for educational achievement. In the UK, at the end of compulsory schooling at age 16, all children are tested on a national exam, called the General Certificate of Secondary Education (GCSE). We can, after just a decade of DNA research, predict more than 17 percent of the differences between students in their GCSE scores just from their DNA. This means, for example, that only 25 percent of children with the lowest polygenic scores go to university, as compared to 75 percent of those with the highest 10 percent scores.

This 17 percent is a long way from 60 percent, which is the heritability of GCSE scores, so there is a lot of headroom for more powerful DNA prediction. Nonetheless, in psychology, 17 percent is a very powerful prediction. To put this in perspective, schools in the UK are ranked according to multiple indices of school quality. These school quality ratings account for only 4 percent of students’ GCSE scores, which is less than a quarter of the predictive power of DNA.

What can parents do?

Putting these findings together leads to the conclusion that parents have much less control than they think about their children’s intelligence and how well they do at school. DNA differences account for most of the systematic differences between children (heritability), environmental effects are random in the long run (nonshared environment), and what looks like systematic effects of parenting are genetic effects in disguise (nature of nurture).

In other words, parents have little systematic effect on their children’s cognitive development beyond the blueprint that their genes provide. No evidence exists beyond anecdotes that children’s success depends on parents giving their children “grit”, “growth mindset” or 10 000 hours of training. I am not advocating that parents should just let their children do whatever they want to do. Parents can and should control their children’s behaviour, for example, guiding their children’s activities. But controlling children’s behaviour does not change their cognitive development.

«Parents have little systematic effect on their children’s cognitive

development beyond the blueprint that their genes provide.»

Importantly, this conclusion is limited to parenting in the samples studied, which do not include parents who severely abuse or neglect their children. As Judith Rich Harris put it in her book “The Nurture Assumption”: “We may not hold their tomorrows in our hands but we surely hold their todays, and we have the power to make their todays very miserable.”

I hope these findings free parents from the illusion that their child’s success depends on how hard they push them. This is not to say that genes are destiny. It just seems more sensible, when possible, to go with the genetic flow rather than trying to swim upstream.

I suggest that parenting is not a means to an end. It is a relationship, one of the longest lasting in our lives. Help your children because you love them and want them to be happy, not because you want to make them into something you want them to be.

Relax and enjoy your relationship with your children. Part of this enjoyment comes from watching your children become who they are.