Disturbingly for all of us involved in education, it seems as if schools and teaching may matter a lot less than we would like to believe. Before setting out the arguments I want to make it clear that this is a struggle for me and I really don’t want it to be true. That said, being professionally sceptical requires that we doubt what we want to believe as much – more – than the stuff that’s obvious guff.

In order to understand what comes next, I’m going to take the liberty of providing a quick refresher on the mechanics of behaviour genetics. When we want to find out to what extent a trait like intelligence can be attributed to genes or the environment, we usually start by looking at identical twins because we know they share the same genetic material. Therefore, we assume that the extent to which twin pairs have similar IQ scores is due to heritability while the extent to which their scores are different can be put down to differences in the environment. Simple.

Or not so simple. Most identical twins are raised by the same parents in pretty much the same environment and yet, although they’re very similar, they’re not exactly the same. For instance, about a fifth of identical twins don’t share handedness – one will be left-handed while the other is right-handed, or sometime, twins can have different sexualities – with one being gay, the other straight. If both their genes and environments are the same, what could be causing these differences? The truth is, we don’t really know, but act as if we do. We label the cause of any differences between identical twins raised together as ‘non-shared environment’. 

In my last post I showed that shared environmental factors, like parenting, have pretty much no longterm effect on adults and that pretty much every human trait can be attributed fairly equally between heritability and non-shared environment.

For the past thirty years or so, researchers have decided that non-shared environment probably includes things like the unique experiences each twin has at school, or the different peer groups each belongs to.* This is plausible in most cases, but is harder to accept in the case of identical twins. Although I’m sure there will be many identical twin pairs who buck this trend, many – perhaps most – will end up in the same class at the same school with the same teacher and the same peer group. Of course each twin will have an ever increasing variety of unique experiences as they grow up, but can we plausibly conclude that these small and irregular differences really account for so much difference.

However, non-shared environment is, in reality, a slops bucket into which researchers stuff everything that isn’t nature or nurture. We don’t really know what it contains. This post by Scott Alexander suggests various possible factors that might make up ‘non-shared environment’ that aren’t anything to do with schools or peers. The factors he considers are measurement error, luck, and various small biological differences. Let’s briefly consider how each of these might contribute to the differences that show up when identical twins are tested for different traits.

Measurement error

Most human characteristics are tricky to measure and even stable ones like intelligence as measured by IQ are subject to predictable measurement problems. Alexander gives this example:

Imagine a world where intelligence is entirely genetic. Two identical twins take an IQ test, one makes some lucky guesses, the other is tired, and they end up with a score difference of 5 points. Then some random unrelated people take the test and they get the 5 point difference plus an extra 20 point difference from genuinely having different IQs. In this world, scientists might conclude that about 80% of IQ is genetic and 20% is environmental. But in fact in terms of real, stable IQ differences, 100% would be genetic and 0% environmental.

Most other traits are much harder to measure than intelligence and rely on much fuzzier tests, self-reports and other error-prone tools. As a result, it’s hard to know if what shows up as ‘non-shared environment’ is actually the effect of genes in disguise. This study apparently shows that when all these factors are taken into account, it looks like the effects of non-shared environment tend to average out at 15% rather than 50%.


It’s theoretically possible for heritability to account for all the differences between individuals and yet for each to be subject to the slings and arrows of outrageous fortune. Alexander gives the example of identical twins who get identical jobs in the same industry and work equally hard. One has a great boss who recognises his talents, the other has the bad luck to be supervised by an unprincipled fool. One gets promoted, the other doesn’t and any researchers studying their experiences might draw the conclusion that the unlucky twin may have done something to deserve his lack of promotions. They start looking and notice that the twins were in different Year 7 maths classes and decided one must have had an inspirational teacher. Or maybe they see one twin had a slightly different friendship group who were a little less academically inclined. Post hoc explanations are troublingly easy to find.

Biological differences

It turns out that identical twins don’t actually have 100% identical genetic material. Although they both come from the same zygote, each may be subject to slightly different random mutations. These tiny difference can magnify over time resulting in 100s of tiny genetic differences. Add to this the fact that all identical twins have subtle biological differences – fingerprints, freckles etc. – and you can imagine that these tiny deviations might add up to something greater than its parts. The point is, we don’t really know. We also don’t really know what difference epigenetics might make. The environment interacts with our genes in all sorts of subtle and profound ways. Identical twins may show substantially different epigenetic effects which can lead to genes being expressed in different ways. We’re on the fringes of discovering more about how all this effects heritability estimates – as we learn more we may find some of what we think of as environmental differences have as much to do with different gene expressions. Time will tell.

The point is, because we don’t know, we can’t really be sure whether the effects of schooling are as important as we want them to be. Just like parenting, it seems obvious that our experiences in school must account for some of the differences between us, but can we take that for granted? There are very obvious ways in which schooling and education matters: if you don’t learn to read then your life chances will be effected. Of course some children will find it harder to learn than others, but pretty much all children can, with sufficient support learn. I’ve argued before that if a child leaves school unable to read it is the school’s fault. The counter-intuitive truth is that as access to education becomes more equitable, children experience less and less environmental differences. In a truly equitable system, all of the differences between children would be due to heritability.

But beyond the brute force logic underpinning what we teach – if a child doesn’t know something he or she cannot then think about it – I wonder whether how we teach might be far less important to how children turn out as adults than we like to think. As I said at the outset, I’m not sure about all this and I certainly don’t want it to be true, and it’s not popular to go about attributing children’s success or failure to who they are rather than what they experience. That’s by the by: what we want to be true, or what’s popular should never be the foundation for our thinking.

To be clear, none of this is an argument for genetic profiling or eugenics or any other lunacy. Who we are is wonderfully unknowable and in some ways it’s reassuring to think we’re less at the mercy of our environments than is often supposed. maybe it’s a good thing our biology cannot be so easily derailed? Whatever the case, the universe is profoundly uninterested in what we want to be true. As ever, all we can do is our best; that will have to do.

* The arguments in favour of peer effects are made cogently and persuasively in Judith Rich Harris’s masterpiece, The Nurture Assumption