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The nature of nurture: How much genes determine identity

Identical twins share 100% of their DNA, so comparing twins raised together with those raised apart can reveal the relative powers of “nature” and “nurture”. Photo/Getty

A startling new book reveals how much genes determine identity.

It is 2035. The month-old baby boy lies blinking in his bassinet. His mother glances at him nervously. The letter from Genetic Services has arrived.

Inside a discreet envelope is a short letter headlined “Scores are probabilistic, not certain.” Underneath is the number of a free counselling service.

The letter explains that personality, looks and health risks are influenced by many different genes, each contributing their own tiny chunk of variability. A polygenic risk score has analysed and weighted these pinpricks into a single predictive figure for each trait. The child’s genetic profile, containing many of these polygenic scores, has been made from a swab of his saliva, taken the day he was born.

The letter contains a string of numbers, like an IRD code: the password to the genetic file that will follow him all his life. As the genetic information of millions of other growing humans is absorbed into the database, his profile will be continually refined.

His mother finds the website and enters her password, and then, heart hammering, her son’s.

American psychologist Robert Plomin has been called an “uncritical DNA booster” by one distinguished academic, and “the doyen of behavioural genetics” by another.

Both were reviewing his new book, Blueprint (Penguin Random House, $40), a read so explosive The Spectator called it “a counterintuitive mind-melt”. 

The book explores the most fascinating questions in psychology: What forms identity? And why are we so different from one another? 

You get the impression that the prof has been ready to blow for a while. Or as he puts it, “I have been waiting 30 years to write Blueprint.”

Plomin is famous in academic circles for longitudinal studies of twins and adopted children, including the Twins Early Development Study (TEDS) of all twins born in England and Wales from 1994 to 1996, and his Colorado Adoption Project (CAP). Together, these have produced thousands of research articles, influential enough to earn the amiable American his ranking as the 71st most-cited psychologist of the 20th century.

Identical twins share 100% of their DNA, so comparing twins raised together with those raised apart can reveal the relative powers of “nature” and “nurture”. In Plomin’s adoption studies, children are compared with their adoptive and their birth parents. If a family’s lifestyle explains why weight runs in families, for example, adopted children should look more like their adoptive parents. (In fact, research shows the reverse. Across large data sets, children resemble the weight of their birth parents more than that of the biologically unrelated parents who feed them and oversee how much exercise they get every day. This is powerful evidence that our DNA has a big effect on our weight.)

Plomin is bursting to tell us about discoveries like this – but first, he surveyed public attitudes to see how much incoming fire Blueprint would need to withstand. Would his revelations require a tank of armour-plated references to protect his book, or could it stroll out in civvies?

The results of his poll, he says, gave him the confidence to publish a book aimed at the general reader, one unarmed with, for instance, a painstaking review of some 20,000 papers published in the past five years on this meltingly hot topic.

“It would be boring to condense this research because the bottom-line message is similar for all areas of psychology. Psychological traits are all substantially heritable, about 50% on average.”

And what accounts for the rest? You might not like Plomin’s answer.

“Nurture” has long been considered the most important single factor in our mental development. But so much of “nurture” turns out to be infused with a genetic influence Plomin refers to “the nature of nurture.” For example, he says, “In the tumult of daily life, parents mostly respond to genetically driven differences in their children.”

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After decades of work, the astonishing fact remains that no systematic environmental predictors of human traits have been identified after controlling for genes, Plomin says – even parenting. The rest of our differences are due to unstable, unsystematic effects that are difficult to study, such as chance, the tendrils of which affect even our dividing embryonic cells.

So you can throw away the parenting books that promise to convert your child into an athletic brainbox with a future in medicine and/or the All Blacks. Tiger Mom-style promises of developmental outcomes, says Plomin, are “peddling snake oil”. In the vast majority of households, parents’ influence on their offsprings’ neuroses come from the genes they bequeath, not the number of times they read their children The Very Hungry Caterpillar.

This comes with a caveat. Plomin cautions that genetic research is describing a normal range of variation. Neglect and abuse can have catastrophic effects on a child’s development. “But,” he says, “these devastating events are, fortunately, rare and do not account for much variance in the population. Parents and parenting matter tremendously – they are the most important relationship in children’s lives. Still, it is important they get the message that children are not blobs of clay to be moulded however they wish. The shocking and profound revelation for parenting from these genetic findings is that parents have little systematic effect on their children’s outcomes, beyond the blueprint that their genes provide.”

Plomin is at his most boosterish when he writes about the “polygenic score” referred to at the beginning of this column – a powerful new tool he has unleashed on his own genome. This is an individual prediction based on massive data sets called genome-wide association studies, or GWASs.

More than 3000 of these studies have explored over 1800 diseases and traits, crunching data to find associations. There is no single “gene” for height, for example, but many, each contributing often tiny variations. A person’s polygenic score for a trait weighs and analyses these variants to make a prediction. Plomin’s for height is in the 90th percentile – which means you would expect him to be tall. He is 1.95m (6ft 5in).

Imagine an unbiased tool that can predict if an infant is going to get “a PhD or a psychosis”, in Plomin’s words. Polygenic scores, he says, are the only way to predict traits such as intelligence in a newborn. It is marvellous and terrifying – and, whether we like it or not, promises to transform psychology.

It’s in your genes

How much are these traits influenced by genetics, according to the latest science?

  1. Personality 40%
  2. General intelligence (eg reasoning) 50%
  3. Spatial ability (eg navigation) 70%
  4. Remembering faces 60%
  5. Verbal ability 60%
  6. School achievement 60%
  7. Reading disability 60%
  8. Autism 70%
  9. Schizophrenia 50%
  10. Stomach ulcers 70%
  11. Breast cancer 10%
  12. Weight 70%
  13. Height 80%
  14. Eye colour 95%

    Your DNA, says Plomin, influences these more than any other single factor:
  1. Divorce*
  2. Kindness
  3. Time spent watching television
  4. Years spent in higher education
  5. Exam grades
  6. Grit
  7. Depression
  8. Shyness
  9. Hyperactivity
  10. Energy levels
  11. Emotionality
  12. Inattention
  13. Conscientiousness
  14. Callous-unemotional traits
  15. Optimism
  16. Neuroticism

* “The heritability of divorce is about 40% across studies,” writes Plomin. “It is a long way from 100%, meaning that non-genetic factors are also important. However, the major systematic factor affecting divorce is genetics. In contrast, no environmental predictors of divorce have been identified in research after controlling for genetics... Surprisingly, people are more likely to get divorced if they are joyful and engaged with life, emotional and impulsive. These are not bad aspects of personality.”

What does it all mean?

Myth: “Obesity is increasing, so it can’t be genetic.”

Fact: Twin studies show weight (and height) are highly heritable, except for countries with such bad nutrition that genetic effects are corked. Even with the same diet, genetic differences make people differ in weight.

What does heritability mean?

This has a precise meaning to a geneticist – and it is a curly one. It refers to the variation in a trait that has actually been reported between individuals in a population, and how much of this is due to DNA. This is why height and weight are more heritable in well-fed countries where nutrition allows DNA differences to reveal themselves. “DNA differences,” says Plomin, “need to be expressed to make a difference.”

The nature of nurture? What the hell is that?

From the days of Freud, the family environment was assumed to be critical in shaping our identity. Psychologists treated “nurture” with reverence, and genetics with a barge pole. Their studies were usually insensitive to the effects of DNA, says Plomin. Research now suggests DNA plays a far greater role in “environment” than previously accepted. “We actively perceive, interpret, select, modify and even create environments to correlate with our genetic propensities. This is a new way of thinking about experience.”

We have three copies of Plomin’s book to give away. To go in the draw, email your name and address to north&south@bauermedia.co.nz by 19 February 2019, with “BLUEPRINT” in the subject line.