Yet another paper seems to show that our educational attainment is written in our genes. It claims that “individual differences in educational achievement are substantially due to genetic differences (heritability) and only modestly due to differences between schools and other environmental differences”. It’s been widely reported as a claim that exam grades are down to nature, not nurture.
This is based on comparisons of the figures for identical and non-identical twins. The reason why people use twin studies is because they believe that our personal characteristics are determined genetically, and so that studies of twins will confirm this. That is bad science. You’re supposed to design research so that it can disprove the proposition under test, and twin studies can’t do it. What a twin study could show, in principle, is that where monozygotic (genetically similar) twins are different, that difference cannot be genetic. That is not, however, what any of them try to do.
Genes are not blueprints for later development; the genetic structure (genotype) has to interact with the environment (phenotype). Your height depends on your genes, but it is not determined at birth; if you are starved you may be stunted. (The increase in height in successive generations has to be largely environmental, because the gene pool changes only very slowly.) If there is a genetic link between common genes and attainment, it does not necessarily mean that the level of attainment is determined by genes – it only predicts similar patterns of attainment within a given environment. So it is not possible to show that any level of GCSE scores is down to genes – it’s all about whether people from the same family, with the same home background, with the same school (and often with the same teacher), with the same experience in early years and of the same age will achieve similar results. Put that way, it would be surprising if the results weren’t very similar – the more so because the sample has been selected to exclude twins where one of them is disabled.
Heritability is supposed to examine the extent to which varability in the phenotype is attributable to the genotype. As there is no direct exploration of genes or genetics in most of these studies, what they actually look at is the way that similar characteristics occur in families, and those are attributed to the underlying genetic structure. There are lots of reasons besides genes why educational attainment might run in families – among them culture, lifestyle, language, common experiences, and so forth. The authors suppose that identical and non-identical twins all have similar home backgrounds, so that the differences between them must be down to the issue of whether they’re identical or not. That however depends on the proposition that identical twins are not treated more like each other than non-identical twins are, and that seems implausible. For example, non-identical twins may have gender differences, and children of different genders are liable to be treated differently.
While the study attributes the differences in performance to DNA, DNA was not usually examined. The results are supposed to be about the differences between monozygotic and dizygotic twins, but it makes no serious attempt to determine whether the twins it is studying are either. It states instead that “Zygosity was assessed through a parent questionnaire of physical similarity, which has been shown to be over 95% accurate when compared to DNA testing.” So what the study actually finds is that if parents think their children are really like each other, those children get more similar educational results than they do if their parents think they are different. The authors assume that the explanation for those similarities between twins must be their DNA – and not, for example, whether parents talk to them and treat them in the same way.
Having said that, there is one finding in this paper that brought me up short, and I think it does reflect on policy. The argument is that as the curriculum has become more standardised, less and less variation between results is attributable to the school, and more and more to ‘heritability’ – which really, in this case, means the home background and early years. That has deep implications for educational equality.