DNA heritability

This article is a preview from the Winter 2018 edition of New Humanist

For a few months this summer, my five-year-old son had a slight cough. Or at least I thought it was a cough until I noticed there were long periods when the cough disappeared, returning later. A doctor suggested that it was probably a tic. Clearing his throat reassured him, becoming a temporary habit, like thumb-sucking. I thought at once of my father, who has a tic of his own (a nod of the head when he’s driving). He must have got it from him, I thought. Raising children is this game of inheritance-spotting. We expect the apple to fall near the tree and we can’t help but guess how near. Will he be confident or shy? Sporty or academic? He’s just like you, my mother says with a smile. He’s just like you, my mother-in-law informs my husband.

However much we like to think of ourselves as unique individuals, we somehow always come back to this. In an age when family dynasties continue to rule the world and Hollywood is stuffed with the actor children of famous actors, we still imagine that this is how things shake out naturally. Brilliance begets brilliance, and by extension, losers beget losers. It is this kind of logic that has always put the study of inheritance in dangerous territory. Heredity formed the backbone of the early 20th-century field of eugenics, which sought to control reproduction based on what were believed to be immutable characteristics passed down through families. First, those seen as feeble-minded or criminal types were sterilised – a practice that continued in the United States and Japan well into the second half of the century. For the National Socialists in Germany, it became an excuse for exterminating races.

Scientists have wisely steered clear of the topic ever since. But now, as seems to be the case with all dangerous ideas, it’s making a comeback. Heritability has become fashionable, propelled partly by widespread consumer genetic ancestry testing, but more deeply by that inexorable desire we have to know what makes us who we are.

A string of new science books examine the evidence across psychology, genetics and neuroscience to get to the heart of this nature-versus-nurture conundrum. In his comprehensive and balanced account of the history of heredity, She Has Her Mother’s Laugh, the New York Times science writer Carl Zimmer writes about how we live in “a society that practically worships DNA”. We are as essentialist as we have ever been. Over the years, the notion that we’re born blank slates has been steadily undermined by good science. Today it’s accepted that at least some aspects of our personalities and abilities are influenced by our biology. But if we aren’t blank slates, just how un-blank are we? Are we little more than vehicles for our genetic codes, subconsciously playing out lives determined by our DNA?

In Innate: How the Wiring of Our Brains Shapes Who We Are, neurogeneticist and blogger Kevin Mitchell, based at Trinity College Dublin, presents an alternative. Genes, he argues, aren’t the only factor in the biological portion of our personalities and abilities. The ongoing, random process of brain development through childhood and beyond also shapes who we become. The effects aren’t entirely genetic, but they are intrinsic, with a probabilistic relationship to our genetic makeup. This is perhaps one reason why everyone is different, and even siblings can be polar opposites in personality.

We are who we are, Mitchell implies, but who we are is not entirely decided by our genes. It’s a subtle but important shift. He moves us away from the old-fashioned idea that dull parents necessarily produce dull kids, or that criminal parents produce criminal kids. We are all capable of being exceptions. But at the same time, he cedes no ground to the blank slaters. It may be possible for us to be very different from our parents, but a large part of who we are is decided by our biology, and this is something we cannot escape.

In the most controversial portion of his book, Mitchell suggests that children who are maltreated by their parents may in fact be the victims of their own biological shortcomings. “It is certainly conceivable that a child who is, say, naturally aggressive or has behavioural ‘problems’… might attract more negative treatment from parents.” Alternatively, he writes, perhaps both parents and children are manifesting shared genetic effects, leading to a child being badly treated. Let’s hope that never makes it into a court of law as a defence of child abuse.

While Mitchell clearly believes that much of who we are is innate, his overall view is that our DNA is not the be-all and end-all, defining our personalities and intellectual capacities with no wiggle room. “It is not mapped out there like a blueprint”, he concludes. This brings us neatly to Blueprint: How DNA Makes Us Who We Are by psychologist Robert Plomin at King’s College London. Like Mitchell, Plomin strongly believes that who we are is in large part determined by our biology. He even repeats the controversial suggestion that children who are treated negatively by their parents “might be treated differently because they differ genetically”. But he reaches a more emphatic conclusion: “The systematic, stable and long-lasting source of who we are is DNA.”

Heritability is a topic about which, for many decades, some scientists have been very comfortable making bold proclamations. But the science remains remarkably unclear. The genomics revolution of the last couple of decades has turned up depressingly little. There are no single genes to explain intelligence or personality traits. Rather, most psychological traits have turned out to be polygenic, involving thousands, sometimes even tens of thousands, of genes, the vast majority of which are yet to be identified. The even harder job of fleshing out mechanisms, linking these mostly undiscovered genes to pathways and processes in the body or the brain, is a distant dream.

Almost everything we know about heritability comes not from genetics or neuroscience, but from the notoriously error-prone field of psychology, and in particular from studies of twins. The more similar twins are in their behaviour and habits, the more researchers believe that the roots of these behaviours and habits are innate. Plomin’s own work relies heavily on twin and adoption studies. General intelligence – one of the main focuses of his research – is 50 per cent heritable, he states. What’s more, it becomes more heritable as we age, partly due to what he calls the “nature of nurture”, our tendency to shape our environment to suit our genetic predispositions. A naturally smart child will choose to read more, go to the library and stretch her ability even further, he argues.

“I like the idea that we grow into our genes,” writes Plomin. As yet, there’s no proof that we do. Twin and adoption studies have been criticised for studying mainly healthy, well-cared-for children in developed countries. What Plomin fails to mention is that for children in the lowest socioeconomic groups, heritability of intelligence falls to zero. In other words, genes make no difference. Heritability is not a hard and fast percentage, the same for everyone, but one that depends heavily on circumstance. The 50 per cent heritability of intelligence, if that is indeed a reliable estimate (and it is important to remember that this estimate has been forced down over the years by better data), applies only to those of us living in comfortable environments where our basic needs are well taken care of. For anyone deprived, under-fed or neglected, it doesn’t apply.

Plomin argues that, given the high degree of heritability and its effects, it doesn’t matter which school your child goes to, because they’re likely to achieve the same results regardless. The obvious implication is that even kids in expensive private schools get better grades because they come from superior genetic stock. Another is that parents have little effect on their children’s life outcomes except in providing them with their genes. You might think that all those museum trips are affecting little Sonia and Steven’s life chances, but in reality, they will just “bounce back to their genetic trajectory,” he warns.

It’s hard to imagine a book more genetically deterministic than Blueprint. Plomin seems so sure of himself. And yet, I have to keep reminding myself as I read, almost all of this is based on nothing more scientific than twin studies. Plomin fills the gap between knowledge and speculation with a prediction: “Although twin studies support this model, the DNA revolution will provide definitive results.” But how does he know? How can this one psychologist know what hard scientific research hasn’t yet been able to definitively show us? And even then, if estimates of heritability of many psychological traits sit at below or around 50 per cent for those raised in healthy environments, surely that still leaves plenty of room for manoeuvre?

What is odder still is that both Mitchell and Plomin feel they can discuss this topic at all without reference to history, to culture, even to politics, except to dismiss them. There “clearly are male and female brains in the same way that there are male and female faces”, writes Mitchell, arguing with outdated science that a sexual division of labour throughout our evolutionary history may have driven these differences. He adds later, “After all, cultural expectations don’t come from nowhere.”

He’s right, they don’t come from nowhere. But does this mean they necessarily come from biology? Was a woman in the 19th century playing out her biological destiny when she didn’t vote in a country that wouldn’t allow her to? When she didn’t take up a certain job because nobody would employ her? To imagine that we shape the world around us to suit our innate capacities, that we make choices driven only by our biology, ignores the fact that most people have been powerless throughout history to do so and many still are. As Zimmer notes, inheritance does indeed impact people’s life outcomes, but often in ways that have nothing to do with genes. The disproportionate wealth accumulated by white Americans over many generations, he explains, gives their children an inevitable advantage over black American children.

The problem with the belief in biological innateness is that it ignores so much. This is a problem with science more widely. There is a wilful ignorance of the social sciences, of the wider world and its messiness, cultural variability and social complications, as though these things are peripheral to studying human behaviour rather than central to it. To treat every person as a unique individual, to understand us as complex beings with all our cultures and experiences as well as our biology, is difficult. It is easier to model people as you would a lab rat, to imagine that we are all playing out our biological urges in the same, predictable ways.

In his final chapters, Plomin asks us each to abandon the urge of becoming our dream selves, of sitting on “an impossibly tall pedestal” and instead try “to look for your genetic self and to feel comfortable in your own skin”. There is no point trying to be a rocket scientist if your genes have always decided that it’s impossible. Don’t reach for the stars, you’ll only fail. This may sound like sage advice from a wise uncle, but the question I found myself asking was: How do you know what your genetic self is? How do you know what your genes are trying to tell you? How can you see this genetic blueprint, what he describes without a hint of sarcasm as the “DNA fortune teller”?

The answer, he believes, is to use polygenic scores – a rough tally of the genetic features known to be correlated with certain traits, based on what little (and it is little) data we have. Applying these scores to himself, Plomin estimates that he is in the 94th percentile for educational attainment. How fortunate for him. His score for schizophrenia puts him in the 85th percentile, although he is satisfied that since he hasn’t yet shown any symptoms of it, he probably never will. So then what use was knowing his polygenic scores at all? Surely his academic record and an absence of any signs of schizophrenia would have been just as good, if not more reliable?

I will need harder molecular evidence, not to mention a consideration of social and cultural factors, before I tell my son to abandon his dreams in favour of something more modest in keeping with his genetic “blueprint”. To suggest that we can define who we are and what we can be based on genetics or neuroscience when we still don’t have much data just isn’t accurate. And if science is anything, shouldn’t it at least be that?

“She Has Her Mother’s Laugh” is published by Picador
“Innate” is published by Princeton University Press
“Blueprint” is published by Allen Lane