The heritability of intelligence

20 min read

The field of behavioural genetics attempts to identify aspects of human behaviour that are heritable. This line of research can be traced back to the nineteenth-century researcher, Francis Galton (1822-1911). Galton was a cousin of Charles Darwin and was inspired by Darwin’s Origin of Species (1859), this led him to investigate heredity. In particular, he was interested in the heritability of ability and intelligence. In 1869 he published Hereditary Genius. In this work,  he set out his method of historiometry, where he examined the achievements of relatives of eminent men. He observed that amongst more distant relatives there were fewer eminent people and he concluded that ability is heritable. But Galton did acknowledge the limitations of his research and he anticipated the study of twins as an improvement. Twin studies were an important part of behavioural genetics from the 1920s. I come to this research shortly. But casting aside any doubt about his own research, Galton believed in improving the genetics of human society. He coined the term eugenics and talked about race improvement.

I think that stern compulsion ought to be exerted to prevent the free propagation of the stock of those who are seriously afflicted by lunacy, feeble-mindedness, habitual criminality, and pauperism, but that it is quite different from compulsory marriage .

Galton’s critics were understandably concerned with the idea of selective breeding in order to improve the human race. For Galton, these criticisms appeared to be an overreaction. After all, he said, he was not looking to manufacture compulsory unions, it was simply a matter of restraining “ill-omened marriages” (ibid.). Importantly, it appeared to Galton that it was science and mathematics that had led to the derivation of the facts about heritability. And that we must, for the sake of society, ensure that democracy is “composed of able citizens” (ibid.) and that we must be aware of “the true state of things”. This is regardless of the fact that his own methods were inconclusive.

This is a prime example of how Enlightenment thinking can lead to folly, where blind faith in science – and the scientific method –  leads to dangerous conclusions and unethical consequences. Social science is not a science, it is political and a moral philosophy. It can draw on studies based on the scientific method, but we are in error to believe that social science, such as educational research, is a science. Social science does not lead to facts, it leads to the potential to make individual and collective judgements that are informed by theory and evidence. Decisions and judgements leading to evidential claims are based on power and moral choice not on absolute truth.

Undoubtedly Galton was an accomplished individual, he is generally characterised as a polymath. His contributions are startling and impressive. Galton developed the idea of regression to the mean and standard deviation. He also pioneered the use of questionnaires. What he seems to have been unaware of, was the real causes of the conditions of society. While Galton assumed that inequality in society was natural selection – the cream rising to the top – Karl Marx was explaining the existence of poverty as a result of the failings of liberal economics. The free market kept the rich rich and the poor poor and exploited. The conditions of the poor ensured that they were starved, overworked, poorly housed and consequently they were wretched examples of humanity. While Darwin’s natural selection takes place over thousands of generations, the conditions of the working poor in Victorian Britain had developed within a few generations. There is nothing natural about what Galton observed of lunacy, feeble-mindedness, habitual criminality and pauperism. These things were entirely man made.

Galton – and it is pretty much unforgivable – gave those who benefitted from the economic status quo scientific ‘facts’ to justify and explain the tracts of squalor and depravity across industrialised Britain. It was, they could say, just a matter of heredity. That the well off are well off because of their genetic superiority and the poor are that way because of their inferiority. But it becomes more sinister. There were programmes of sterilisation in some European countries and some states in America in the early 1900s. Adolf Hilter was inspired by eugenics; consequently, the Nazis killed thousands of disabled people in the 1930s. The Holocaust was the ultimate in racial cleansing with the gassing of millions of Jews during the Second World War.

Perhaps there is a case for eugenics: Toby Young. Perhaps his father, Michael Young, should have been made aware of the possible consequences of an “ill-omened marriage”. We could have avoided a retrograde step such as Toby Young. But we are all wise after the fact. And as a matter of principle, as you will no doubt have gathered, I am opposed to eugenics. But Young Junior, with his characteristically ill-informed gobshitery, argues for ‘progressive eugenics’ . Young rehashes many of Galton’s original arguments for eugenics with little smatterings of evidence, partial readings and partial understandings. Blah, the best people have the best IQs, blah. I am suddenly struck by the immensity of Galton; he was mistaken and the consequence of his work was the death of millions, but he was no second-rate right-wing establishment bum licker, he was an original thinker. What Young tries to do is to input into his ‘bold’ progressive eugenics some fresh thinking – poor people should be allowed access to genetic manipulation to improve their babies when the technology comes available. Eugenics remains abhorrent and an unacceptable form of social engineering, even if we do prefix it with ‘progressive’ and the state funds designer baby programmes to those on benefits and low incomes.

The father of modern genetics, Gregor Mendel (1822-1884), identified physical characteristics of pea plants that were heritable: plant height, the shape of the peapod, seed shape and colour, and flower position and colour. While biologists at the time believed that inherited traits were blended, Mendel’s experiments showed that there were dominant and recessive traits that are passed from parent to offspring. It was not until the beginning of the twentieth century that the significance of Mendel’s work was recognised. Heritable information is carried in genes which come in pairs and offspring inherit one gene from each parent. Genes are made from DNA (deoxyribonucleic acid). A gene is a length of DNA that codes for a specific protein. A DNA molecule can make copies of itself and it carries information for creating proteins. One gene will code for the protein insulin, which has an important role in controlling the amount of sugar in the blood; human beings have 20,000 to 25,000 genes1 In humans, observable heritable traits include height, hair colour, earlobe attachment, tongue rolling, dimples, handedness, freckles, curly hair, red/green colour blindness and hairline shape, for example. DNA codes for proteins that result in the development of these characteristics. Behavioural genetics assumes that if physical characteristics are inherited, then our inherited hardware and architecture can lead to the inheritance of higher-order characteristics such as intelligence and personality. But to what extent is our behaviour, our successes and failures, attributable to our environment and upbringing. Or is it already hard-wired into us genetically? Are we preloaded with certain capacities that can predict our outcomes?

Galton anticipated the use of twin studies to identify the genetic basis of psychological traits such as IQ and personality. The methodology exploits the fact that identical twins share the same genes and that non-identical twins share half their genes. Differences in the behaviours of identical and non-identical twins can be used to estimate the proportions of their behaviour that are inherited. Krapohl et al. , claim that academic achievement is a result of just over 60 per cent heritable characteristics. This, in turn, is a result of heritable intelligence, self-efficacy and personality. The study is based on a classic twin study involving 6,653 pairs of twins in the UK and using GCSE2General Certificate of Secondary Education. The examinations taken at the end of compulsory schooling in the UK at age 16. results. The assumption is that the similarities in the performance of identical twins are entirely genetic since identical twins have the same genes and they have been brought up in the same environment. Underpinning this assumption is the belief that the environments that identical and non-identical twins develop in are similar: each twin in both groups has a similar experience of the environment. This is referred to as the equivalent environment assumption. Yet, it is recognised that identical twins are generally treated in the same way as they grow up, much more so than non-identical twins. Therefore, identical twins’ behaviours may not be attributable to their genes, but to the way in which they were brought up and not treated as two individuals. The equivalent environment assumption is therefore unjustified: at best it means that findings from twin studies overestimate the heritability of psychological aspects, at worst it invalidates these claims altogether .

To counter this, behavioural geneticists have used studies of identical twins that had been separated at birth and who have subsequently been brought up in different environments . However,  Joseph argues that the comparisons between identical twins raised together and those raised apart still do not justify the equivalent environment assumption. He identifies the following similarities between identical twins raised together and those raised apart:

  • They are exactly the same age (birth cohort).
  • They are always the same sex.
  • They are almost always the same ethnicity.
  • Their appearance is strikingly similar (which will elicit more similar treatment from the social environment).
  • They usually are raised in the same socioeconomic class.
  • They usually are raised in the same culture.
  • They shared the same prenatal environment.
  • Most studied pairs spent a certain amount of time together in the same family environment, were aware of each other’s existence when studied, and often had regular contact over long periods of time  .

The study of identical twins raised apart has not provided a valid defence of the equivalent environment assumption. More recently a defence of behavioural genetics has come from genome-wide association studies . GWAS involves scanning markers across the complete sets of DNA, or genomes, of many people to find genetic variations associated with particular behaviours3 Effectively, this is a hunt for genes or sets of genes that lead to particular behaviours, intelligence or personality. However, it has not been possible to identify the sets of genes that contribute to intelligence and academic achievement. Krapohl et al.  found that sets of genes (genome polygenic scores, GPS4 explained ~2% of educational achievement.  And according to Joseph, this kind of molecular genetic research, is a result of a mistaken belief that the twin studies provide unequivocal evidence that genetic factors contribute to observed variation in behaviours; gene-finding exercises are unlikely to yield results .

Intelligence is a social construct, we judge intelligence based on observed behaviours. In parallel with the work of behavioural geneticists, psychologists have attempted to distil a measure of intelligence5An excellent summary of the history intelligence tests and IQ measures can be found here Yet, measures such as intelligence quotient (IQ) are measures of how good participants are at the IQ tests. IQ is confounded by socioeconomic status, the tests have a class bias and reflect the social and cultural capital of particular groups. Yet, I acknowledge also that intelligence involves cognitive processes as well as being a social construct. Intelligence has the following features (but not limited to these): perception and recognition, reasoning, the creative use of existing knowledge, strategic and tactical planning, and the capacity to act and adjust actions as information is updated and the context changes. Our intelligence, our reasoning and the decisions we make are sensitive to emotions and affect. Confidence, self-concepts and motivations have a profound effect on our attainments and will have an effect on any assessment of our intelligence. In addition, our physiological and affective states also have an impact on how intelligently we act. Intelligence is a complex psychosocial construct, it is unsurprising therefore that it continually eludes behavioural geneticists.

Philip Johnson-Laird, a cognitive psychologist and philosopher, explains human reasoning (the core of human intelligence) using mental models . He draws on a dual processing model of human reasoning. One type of reasoning is entirely conscious and can draw on logical analysis to derive solutions and construct action. Although, as Johnson-Laird points out, people are quite bad at logic and there are many situations where there is insufficient information to permit an entirely logical analysis. Information is missing, assumptions have to be made, gaps have to be filled and judgements made using synthetic mental models. This draws on past experience and relies on matching the situation at hand to situations and strategies. The second type of reasoning is almost entirely intuitive and subconscious. Rational conscious thought is demanding and consumes resources, it appears to be hard-wired into humanity to limit rational thought, probably because of the resources it requires and also to allow sensory resources to be available should something unusual come our way, such as a predator. Much of the time, in our day-to-day lives, and in many of the things we do routinely we use subconscious reasoning. In this we rely on shared cultural patterns of behaviours and shared mental models; we are at ease in our communities and families and have a sense of how others will act and respond in these contexts without the constant demand for conscious thought.

The complexity of human reasoning and intelligence is irreducible to a genetic marker, but genetics dictates the format of our central nervous system and our neurophysiology. Intelligence is the ability to reason and act effectively in different situations. Thus, a person with the quality of intelligence would be judged by an observer to have negotiated an obstacle, problem or context, efficiently and effectively. There has to be some degree of challenge in the problem, some complexity or novelty that requires reasoning. The challenge faced cannot be solved using a routine, method or algorithm. In other words, the distinction between a robot and an intelligent being is that the being has the capacity to use creative reasoning processes to solve problems. A robot or artificial intelligence is reliant on routines and algorithms to negotiate the situations it meets. Intelligence is primarily dictated by the way in which we learn to use our ‘hardware’. Experiences, relationships and the contexts in which we learn and how we learn really define how intelligent we become. I want to use the analogy of cinema.

Edison’s patented invention, the Kinetoscope, was introduced in 1891, the Lumière brothers’ first projection of films to a paying audience took place in 1895. Films create an illusion of continuous movement by passing a series of images in front of a light source enabling the images to be projected on a screen. The moving image as a form of collective entertainment spread in the form of photographic images printed on a semi-transparent celluloid base cut into strips 35 mm wide. This was devised by Henry M. Reichenbach for George Eastman in 1889 . Through the twentieth century, cinema technology evolved with the introduction of sound and colour. More recently cinema has used digital technology and computer-generated images. The hardware and technology have evolved, improving the quality of production. Cinema, as an art form, is dependent on the technology but relies on human creativity, reasoning and culture to create a narrative, a visual and auditory effect, a spectacle and artistic form. From the point we had the technology to project a feature film a century ago, the artistic content has not advanced significantly. Clearly, film makers have more sophisticated technology, but the limitation of the art form has been human creativity, society and culture. For example, according to the IMDb6 website, D W Griffith’s 1916 film Intolerance is rated 8 out of 10, while Kathryn Bigelow’s The Hurt Locker (2009) is rated similarly. Clearly, there is a considerable amount of high-quality contemporary output, much more than there was at the time of the release of Intolerance. However, while cinematic hardware has advanced dramatically the quality of the artistic output has progressed at a steadier rate.

Our genetics provide us with the hardware, analogous to cinematic hardware: our brain, central nervous system is like the projection equipment and the film. Human intelligence is like the film content, constructed and devised based on free will, knowledge and in congress with culture and society. Genetics and heredity are important, but only in giving us the hardware. It is our experience of society, knowledge, culture and ourselves that allow us to develop intelligence. Or, indeed a poverty of these things does not permit the development of intelligence.

George Orwell in his anthropological account of the British working class in the 1930s provides a unique insight into the conditions of society and how it impacts on working people living in poverty. He reflects on the intelligence of his boarding house landlord and landlady, the Brookers. Orwell, observes first hand how conditions and political economy crush intelligence and reason.

The most dreadful thing about people like the Brookers is the way they say the same things over and over again. It gives you the feeling that they are nor real people at all, but a kind of ghost for ever rehearsing the same futile rigmarole…But it is no use saying that people like the Brookers are just disgusting and trying to put them out of mind. For they exist in tens and hundreds of thousands; they are one of the characteristic by-products of the modern world. You cannot disregard them if you accept the civilisation that produced them. For this is part of what industrialism has done for us .

Thirty years previously Robert Tressell, observed a similar impact on the working class: conditions of poverty, limited opportunity, repetitive drudgery and exploitation that lead to the apparent absence of intelligence . Like Orwell, Tressell attributes conditions to political economy and the political choices of those who hold power and wealth.

When we consider intelligence we have to look at society, culture and political economy and not at genetics.


Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart. Science, 250(4978), 223–228.
Cherchi Usai, P. (1996). Origins and Survival. In G. Nowell-Smith (Ed.), The Oxford History of World Cinema (1745508883; p. 11). Oxford University Press; Screen Studies Collection.
Galton, F. (1908). Memories of my life. Methuen.
Johnson-Laird, P. N. (2006). How we reason. Oxford University Press.
Joseph, J. (2013). The Use of the Classical Twin Method in the Social and Behavioral Sciences: The Fallacy Continues. The Journal of Mind and Behavior, 34(1), 1–39.
Joseph, J. (2011). The crumbling pillars of behavioural genetics. GeneWatch, 24(6), 4–7.
Joseph, J. (2010). The Genetics of Political Attitudes and Behavior: Claims and Refutations. Ethical Human Psychology and Psychiatry, 12(3), 200–217.
Krapohl, E., Rimfeld, K., Shakeshaft, N. G., Trzaskowski, M., McMillan, A., Pingault, J.-B., Asbury, K., Harlaar, N., Kovas, Y., Dale, P. S., & Plomin, R. (2014). The high heritability of educational achievement reflects many genetically influenced traits, not just intelligence. Proceedings of the National Academy of Sciences, 111(42), 15273–15278.
Krapohl, E., Euesden, J., Zabaneh, D., Pingault, J.-B., Rimfeld, K., von Stumm, S., Dale, P. S., Breen, G., O’Reilly, P. F., & Plomin, R. (2016). Phenome-wide analysis of genome-wide polygenic scores. Molecular Psychiatry, 21(9), 1188–1193.
Orwell, G. (1986). The road to Wigan Pier. Penguin Books. (Original work published 1937)
Tressell, R. (1993). The ragged trousered philanthropists. HarperCollins. (Original work published 1914)
Young, T. (2015, September 7). The fall of the meritocracy. Quadrant.


  1. Thanks for this Steven. When you talk about science (para 3) do you think it would be useful to expand the idea of positivist science, the one everybody knows about, cause/effect “20 people who were asked said Gloop make their skin smoother” as one way to go about reducing uncertainty? I see people marginalising qualitative research as being “just anecdotal” without understanding that not all science is aimed at generalisation. It might help in people seeing social/educational research as both scientific and different. The people I have in mind are the ones who write rubbish about science. (I’m thinking about the place for critical realism and tentative truth rather than “hard facts” where the gap opens and the discussion becomes good/bad)
    Maybe we’ll meet sometime, I’d really like to chat over a cuppa.

  2. It’s interesting to read your views and arguments on the issue of intelligence. I think the definition of intelligence is one area that man struggles with and I say this because in the neo liberal world – intelligence seems to mean the ability to generate material gains and as such success in life or in anything is defined mainly along that route.
    Also, issues like environment, culture and political economy as you rightly said should be considered when talking about intelligence. However, I think it’s important to note that intelligence involves other areas of life like social, emotional, physical and psychological. For instance, if I was to move to a new country without having lived there enough to understand certain bodily reactions to issues (body language) I probably wouldn’t understand what reactions I’m getting from the locals, now that’s a physical process that I have to interpret visually and cognitively.
    Likewise through the use of language and tones in language at times one might be able to decipher the issue at stake and without having the requisite auditory intelligence associated with such a language one would probably struggle to appreciate the magnitude of the issue at stake to the native speakers of that particular language.
    In summary it was an interesting and engaging read that I think should be thrown open for further debates.

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