Are our life chances determined by our DNA?

In less than two decades, the bid to read the human genome has shrunk from billion-dollar space-race project to cheap parlour game. In 2000, President Bill Clinton and Tony Blair, then UK prime minister, jointly announced that scientists had elucidated the three billion letters of the human genome — or discovered “the language in which God created life”, as the US president solemnly phrased it.

In 2018, prompted by opposition goading, the Democrat US senator Elizabeth Warren took a consumer DNA test to prove a strain of Cherokee ancestry. Flashing a sliver of exotic bloodline for political advantage turned out to be a calamitous misjudgment: her actions upset Native Americans, who regard identity and culture as more than a matter of DNA.

Science too is engaged in the same enterprise: to reduce the complexity of human identity to genetics. While we have long known that genes build our bodies — determining eye and hair colour, influencing height and body shape — there is a growing conviction that genes also sculpt the mind. As the cost of gene-sequencing technology has plunged to a few hundred dollars, millions of people have had their DNA sliced and diced by scientists seeking to quantify the genetic contribution to personality, intelligence, behaviour and mental illness.

This is the dark and difficult territory explored by three important books that embody a new zeitgeist of genetic determinism. If DNA builds the brain and mind — the puppetmasters pulling our behavioural strings — then selfhood becomes circumscribed largely by our genes. The idea that we are little more than machines driven by our biology raises a profound conundrum: if the genes we inherit at conception shape personality, behaviour, mental health and intellectual achievement, where is the space for society and social policy — even parents — to make a difference? What of free will?

As might be guessed from its klaxon of a title, Blueprint is unequivocal in stating the supremacy of the genome. “Genetics is the most important factor shaping who we are,” opens Robert Plomin, a behavioural geneticist at King’s College London recognised globally (and reviled by some) for his research into the genetics of intelligence. “It explains more of the psychological differences between us than everything else put together,” he writes, adding that “the most important environmental factors, such as our families and schools, account for less than 5 per cent of the differences between us in our mental health or how well we did at school.”

For decades, Professor Plomin has been using twin and adoption studies to tease out the relative effects of genes and environment. Identical twins share 100 per cent of their DNA; in non-identical twins this drops to 50 per cent (the same genetic overlap as regular siblings). Adopted children share a home environment, but no DNA, with their adoptive parents; and 50 per cent of their DNA, but no home environment, with each of their biological parents.

A careful study of these permutations can point to the “heritability” of various characteristics and psychological traits. Body weight, for example, shows a heritability of about 70 per cent: thus 70 per cent of the differences in weight between people can be attributed to differences in their DNA. Identical twins tend to be more similar than non-identical, fraternal twins; adopted children are more like their biological parents than their adoptive parents.

Breast cancer, widely thought of as a genetic disease, shows a heritability of only 10 per cent. In contrast, it is 50 per cent for schizophrenia; 50 per cent for general intelligence (reasoning); and 60 per cent for school achievement. Last year Plomin claimed that children with high “polygenic scores” for educational achievement — showing a constellation of genetic variants known to be associated with academic success — gained good GCSE grades regardless of whether they went to non-selective or selective schools. His conclusion was that genes matter pretty much above all else when it comes to exam grades.

Even the home, the very definition of “environment”, is subject to genetic influence, he says. If kids in book-filled homes exhibit high IQs, it is because high-IQ parents tend to create book-filled homes. The parents are passing on their intelligence to their children via their genes, not their libraries: “The shocking and profound revelation . . . is that parents have little systematic effect on their children’s outcomes, beyond the blueprint that their genes provide.” His conclusion is that “parents matter, but they don’t make a difference”.

That is not the only seemingly contradictory message. Plomin describes DNA as a “fortune-teller” while simultaneously emphasising that “genetics describes what is — it does not predict what could be”. This caveat is odd, given his later enthusiasm for using genetic testing predictively in almost every aspect of life: in health, education, choosing a job and even attracting a spouse. He suggests, for example, that we could use polygenic scores for schizophrenia “to identify problems on the basis of causes rather than symptoms”.

This vision sounds worryingly like pre-medicalisation. Plomin proclaims himself a cheerleader for such implications but is disappointingly light on the ethical issues. A predisposition might never manifest as a symptom — and besides, “possible schizophrenic” is not the kind of descriptor I would want to carry around from birth.

Plomin admits that cowardice stopped him writing such a book before now; it probably also stopped him from addressing alleged racial differences in intelligence. This is a grave omission, as he is one of the few academics capable of authoritatively quashing the notion. James Watson, the 90-year-old DNA pioneer, recently restated his belief that blacks are cognitively inferior to whites. Those, like Plomin, responsible for fuelling the resurgence in genetic determinism have a responsibility to speak out — and early — against those who misuse science to sow division. (Plomin is writing an afterword for future editions.)

Neuroscientist Kevin Mitchell believes that genes conspire with a hidden factor — brain development — to shape psychology and behaviour. Neural development, he contends persuasively in his book Innate, adds random variation to the unfurling of the genetic blueprint, ensuring individuality, even among identical twins. These special siblings, though clones, rarely score identically for psychological traits. Genes are the ingredients but a lot depends on the oven: “You can’t bake the same cake twice.”

Mitchell, associate professor of neuroscience at Trinity College Dublin, explains: “It is mainly genetic variation affecting brain development that underlies innate differences in psychological traits. We are different from each other in large part because of the way our brains get wired before we are born.” Genetic relatives have brains that are wired alike. Thus, we should look to the cranium, not only to chromosomes, to learn how minds are shaped.

Indeed, each of us is a miniature study in how a genetic blueprint can quiver under the influence of random variation, like a pencil tracing that does not conform exactly to the original outline. The genes directing the development of each side of your body are identical — but you are still slightly asymmetrical (put a mirror down the middle of a mugshot and see how weird you look with perfect symmetry). Fascinatingly, identical twins do not always show the same handedness, despite shared DNA and upbringing.

What goes on in that oven, or the brain, cannot be described as environmental — the catch-all term for non-genetic factors — because it is intrinsic to the individual rather than shared. Mitchell labels it the “non-shared environment”, a crucial but overlooked component of innate traits. Once this factor is folded in, “many traits are even more innate than heritability estimates alone would suggest”.

This, he insists, does not close the door to free will and autonomy. Genes plus neural development pre-programme a path of possible action, not the action itself: “We still have free will, just not in the sense that we can choose to do any old random thing at any moment . . . when we do make deliberative decisions, it is between a limited set of options that our brain suggests.” Having free will, he adds, does not mean doing things for no reason, but “doing them for your reasons.” Those include wanting to conform to social and familial norms; unlike Plomin, Mitchell recognises the reality that societies and families can and do make a difference.

While both discuss heritable conditions such as autism and schizophrenia in terms of defective genes, Randolph Nesse turns this thinking on its head. In Good Reasons for Bad Feelings, he asks: why do such disorders persist in the human population, given that natural selection tends to weed out “bad” genes?

Mental illness and psychological ill-health, he theorises, could be the collateral damage caused by the selection, over evolutionary time, of thousands of genes for survival and fitness. Autism, for example, has a well-documented genetic overlap with higher cognitive ability: some biologists now regard autism as a disorder of high intelligence. Once, only the clever survived.

Nesse, who runs the Centre for Evolution and Medicine at Arizona State University, can also explain why life offers mental torment in abundance: “Natural selection does not give a fig about our happiness. In the calculus of evolution, only reproductive success matters.”

Charles Darwin was one of the first to see the similarity in facial expressions between humans and other animals: these hint at a shared evolutionary heritage when it comes to emotions. Jealousy and fear, for example, are thought to promote genetic survival: a jealous man who controls his partner is more likely to end up raising his own genetic offspring, according to the evolutionary scientist David Buss; fear makes us cautious and keeps us alive.

These are indeed good reasons for bad feelings. But extreme jealousy can lead to murder; extreme fear can become debilitating phobia. Panic attacks — an exceedingly common experience — mirror the fight-or-flight response. Anxiety, meanwhile, works on the smoke detector principle: “a useful response that often goes overboard”.

Nesse’s book offers fresh thinking in a field that has come to feel stagnant, even if new therapeutic avenues are not immediately obvious. The prevailing orthodoxy that each mental disorder must have its own distinct cause, possibly correctable through chemicals, has not been wholly successful over the decades. Biologists have also failed to uncover tidy genetic origins for heritable conditions such as schizophrenia and autism, instead finding the risk sprinkled across thousands of genes. Recasting our psychiatric and psychological shortcomings as the unintended sprawling by-products of evolution seems a useful way of understanding why our minds malfunction in the multiple, messy ways that they do. The UK’s Royal College of Psychiatry thinks so: it recently set up a special interest group on evolutionary psychiatry.

Given that natural selection is blind to organisms being happy, sad, manic or depressed, Nesse notes that things could have turned out worse: “Instead of being appalled at life’s suffering, we should be astounded and awed by the miracle of mental health for so many.”

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