Kevin Mitchell, Ph.D.


No Clones in the Classroom

Differences in children's psychology can be amplified by experience.

Posted Jun 10, 2019

Any parent with more than one child knows that they don’t come out of the womb as blank slates. “They’re like chalk and cheese” is a common refrain, emphasising the innate differences between children and how refractory their individual natures are to parental influences. Teachers, too, are well aware of differences in their young charges – in their personalities, their interests, and their aptitudes.

Human nature is encoded in our DNA

If we think about human nature in general – the range of behavioral tendencies and capacities that characterise us as a species – it becomes obvious that this must somehow be encoded in our DNA. The human genome in any newly fertilised egg cells must contain the instructions to build a human being, with a human brain, that endows that being with human traits.

We know quite a bit about how that works – the building a human being bit at least. Developmental biologists have worked out many of the principles and processes by which a developing embryo becomes patterned, with a head at one end, a tail at the other, and a heart in the middle, etc., as well as the molecular mechanisms by which individual cells decide to become muscle cells or skin cells or bone cells. 

When it comes to the human brain, this gets much more complex. It is made up of hundreds of distinct regions and subregions, each with scores of specialised types of nerve cells. And those cells have to be arranged and connected with exquisite specificity to carry out the particular computations that underlie our various cognitive functions. All of this requires the actions of thousands of different genes, which each encode a specific protein, which interacts with other proteins within or between cells to drive all these developmental processes.

The details of that aren’t important here – what’s important is that variation in the sequence of DNA that comprises all those genes can affect the outcome. The human genome is a string of 3 billion chemical bases or “letters” of DNA, in a specific sequence. If you compare humans to chimps, 98.7 percent of those letters are identical. The 1.3 percent that is different – about 39 million differences in total – are responsible for the differences between humans and chimps, including in their brains and their respective natures.

The same principle applies to differences between individuals within any given species. The genomes of any two humans are 99.9 percent identical, meaning there are about 3 million differences in total between them. As with the differences between humans and chimps, many of the differences between individual humans affect the genes controlling brain development and will cause differences in the outcome, manifesting as differences in our respective natures.

Family resemblances

If genetic differences cause variation in our traits, then people who are more related to each other should be more similar to each other for those traits. This is exactly what is observed. Advances in neuroimaging let us examine in detail the structure and function of the brain, the way different regions are laid out and the way they are interconnected. These analyses reveal a unique "neural fingerprint" that characterises each of our individual brains. That pattern is much more similar between identical twins, who carry all of the same genetic variants, versus fraternal twins or siblings, who share only 50 percent of those genetic variants in common. The same effect is seen even across distantly related people in the general population (ruling out concerns that twins are somehow a special case).

When we look at psychological traits we see the exact same pattern. For pretty much any psychological trait than can be measured and thus compared between people – including cognitive traits like intelligence, memory, or quantitative reasoning, or personality traits like extraversion, neuroticism, or conscientiousness – people who are more closely related to each other are more similar for those measures. The same is true for the incidence of psychiatric conditions, including autism, dyslexia, ADHD, and others – these are all highly genetic conditions. By contrast, it is consistently found in twin and adoption studies that growing up in the same family environment shows a surprisingly small, often negligible, effect on our psychological traits.

Our genes thus literally direct the wiring of our brains in ways that impact on our psychology, endowing us with innate predispositions and tendencies. However, the effects of our genes are not fully deterministic. The genome does not encode a specific outcome of development, only the rules by which the processes are governed. The way that program plays out will vary from run to run. So even identical twins – while much more similar to each other than unrelated people or even regular siblings – are not fully identical in their brain structure or in their psychological make-up. Chance events during development ensure that by the time they are born they are already unique.

The interplay of nature and nurture

Our stories, as individuals, do not start with a blank page. But of course, they also do not end with the first page or the first chapter. Nature and nurture are typically set in opposition to each other but in reality there is an intimate interplay between them. Humans – especially young children – are learning machines. Our protracted period of development and maturation – taking much longer to reach adulthood and independence than other primates – gives us the opportunity to learn from our experiences and adapt our behaviour accordingly. Given that, you might expect that experience would tend to flatten out or override our innate predispositions. In fact, exactly the opposite tends to happen – our experiences more often amplify these innate differences.

There are several important reasons for this. First, our experiences do not just happen to us – they are also influenced by our genetics. In particular, if personality traits are shared between parents and children, this interaction may amplify the traits of the child. For example, if a naturally cautious child also has over-protective parents, this will reinforce the child’s initial temperament. While a naturally aggressive child who also has aggressive parents will have that pattern of behaviour consolidated.

In addition, young children with different temperaments evoke different responses and reactions from their parents, peers, teachers, coaches, etc., which can similarly influence their development. A naturally gifted child – academically, musically, athletically – is likely to receive encouragement from parents and teachers in ways that can lead to a virtuous cycle of increased practice, achievement, praise, and motivation.

As we mature, we become more and more active, autonomous agents who increasingly select our own experiences. An outgoing child will choose to socialise more and develop more expertise in social skills, while a naturally shy child may lag behind in these skills, due to lack of practice. A child with dyslexia, for whom reading is effortful, will naturally tend to read less and fall farther and farther behind their peers without adequate specialized instruction.

Finally, we learn from reward or punishment – from things feeling good or bad. However, the neural circuits that mediate signals of reward or punishment, or that control what kinds of things we pay attention to or find salient, also differ between people. This means that even when two people are exposed to what looks objectively like the same environment or circumstances, their subjective experiences may be highly different. And it is the subjective experience that determines whether we learn from something and how it shapes our future behaviour.

Our innate predispositions are thus crucially important in influencing our habits in response to our environments and experiences. We all start out different from each other, and in many ways we become more so over time – more and more crystallised versions of ourselves.

For educators, recognising this diversity is crucially important. It can, in the first instance, lead to greater acceptance of the range of behaviours, abilities, aptitudes, and interests that individuals will present in a classroom. And it can help identify children who may benefit from intervention to counteract the vicious cycles that can amplify initial difficulties if left unchecked.  


Mitchell, K.J. (2018) INNATE – How The Wiring of Our Brain Shapes Who We Are (Princeton University Press)