Just what makes us who we are?
Autumn 2018, and American psychologist and geneticist Robert Plomin’s new book Blueprint: How DNA Makes Us Who We Are exploded onto the scene, to headlines of 'Parents Matter But they Don’t Make a Difference', 'How DNA Dictates Who We Are', and 'So is it Nature not Nurture After All?'.
Press coverage has come down mostly on the side of genetic causality, accepting Plomin’s case that DNA is the major systematic force which makes us who we are, and that the environmental events which make a difference are random and unpredictable.
Reading Plomin’s book as an applied psychologist, there is a sense of perhaps defensive disbelief… so much of what he writes runs counter to accepted psychological perspectives. Parenting and nurturing don’t 'make a difference'? Crucial environmental events in our lives are down to chance? This challenges the very core of psychology, and yet, as most of us are not behavioural geneticists, assessing the validity of these claims – or even understanding what they are based on – isn’t straightforward.
How Does DNA Make Us Who We Are?
Plomin’s book is a compelling story of the evolving field of behavioural genetics. It's a journey from the adoption studies of the 1970s – where, due to the system of mother and baby homes in the US, Plomin was able to assess pregnant mothers and then follow their babies as they grew up with adoptive families – through the twin studies of the 1990s and early 2000’s, onto the mapping of the human genome and the subsequent massive Genome-Wide Association studies, enabling researchers to carry out genetically sensitive studies without twin and adoption designs.
We read of the early days of looking for ‘candidate genes’, through to the realisation that for the vast majority of behavioural traits, multiple genes work in combination with each making only a tiny contribution. The days of looking for the ‘gene for autism’ are over, researchers are now calculating ‘polygenic risk scores’ – predictive scores made up of thousands of genetic markers.
Plomin suggests that genetic research is transforming how we understand not just the genome, but the environment too. He argues that much of what we think of as environmental – and therefore independent of the genome – is in fact heritable. Life events, for example: twin studies suggest that 30 per cent of the variance is accounted for by genetic differences.
Professor Plomin explained this when I interviewed him over email. 'Research on the "nature of nurture" shows that most of our measures of psychological environments show genetic influence because what we call the environment is not the environment "out there" independent of who we are. Many experiences assessed on measures of life events, such as conflicts with others, don’t just happen to us as hapless bystanders. Blueprint describes a new way of thinking about the environment, as experiences that we actively select, modify and create in part on the basis of our genetic tendencies.'
So individuals (and their genes) are active players in creating their environment. A calm and contented baby gets different parenting to a colicky one, a sociable child is likely to have more friends (and therefore interactions) to one who prefers to play alone.
This implies that no association between an environmental measure and later outcomes can be assumed to be causal, unless genes have been accounted for. Parents who frequently read to their children could be responding to a genetic predisposition in their children which means that as young children they like to be read to, and as older children they like to read. We might assume that it is the parental behaviour which creates the love of reading – but that behaviour is in itself influenced by the child, and their genes.
All human behavioural traits are heritable. This is often hard for psychologists to accept, but is uncontroversial in the world of behavioural genetics – so much so that Eric Turkheimer called it the First Law of Behaviour Genetics back in 2000. Studies have found, for example, that marital status and amount of TV watched are heritable.
And genes affect our environment because our behaviour affects our environment, which makes the environment heritable as well. Plomin explained the different mechanisms for this, using the example of adverse childhood events. 'For children whose adverse life events involve their parents, passive gene-environment correlation could mediate such correlations because parents and their children are 50 per cent similar genetically. For example, a correlation between parental physical punitiveness and children’s antisocial behaviour might be due to genetic influence on aggressiveness shared by parents and their children. Reactive or evocative gene-environment correlation comes about because people respond to children in part on the basis of their genetic propensities. For example, hostile experiences could reflect children’s oppositional defiance. Active gene-environment correlation refers to children selecting, modifying and creating experiences in part on the basis of their genetic tendencies. For example, aggressive children might seek out aggressive peers who enhance their aggressiveness.'
Plomin argues that DNA is the only stable, systematic and long-lasting source of who we become – and that the environmental events which do make a difference are 'unsystematic and unstable – in a word, chance'. This is where the headline 'Parents don’t make a difference' comes from. He bases this on the absence of evidence for systematic effects of nurture, or in the terminology of behavioural genetics, ‘shared environment’: that which is shared between children in the same family and which is assumed to make them similar. (Imagine a pair of identical twins raised in the same family: any differences between them would be down to non-shared environment, since they share the same genes and shared environment.)
Plomin’s studies have found that identical twins raised in different families are just as similar as those raised in the same family, leading him to conclude: 'If you had been adopted at birth, grown up in a different family and gone to a different school, you would be similar to who you are now in terms of your personality, mental health and illness, and cognitive abilities and disabilities. In fact, you would be as similar as identical twins who grow up in the same home.'
Parenting style and social economic status are counted as shared environmental factors. If they don’t make a difference to what we become, then what does? Plomin’s answer is chance or random events – but this feels unsatisfactory for many psychologists. Why would our brain respond differently (and in a more permanent way) to chance events than to many hours of nurture? How could this actually work?
What do Polygenic Scores actually tell us?
Polygenic scores in themselves do not give us any information about how the genotype becomes the phenotype. They are simply numbers with predictive power. When I asked, Plomin said that it was unlikely we would ever understand the mechanisms.
'It will be extremely difficult to do this because of two principles of genetics: pleiotropy (each gene affects many traits) and polygenicity (each trait is affected by many genes). Genome-wide association (GWA) studies, which provide the foundation for creating polygenic scores, have revealed that the heritability of complex traits is caused by thousands of DNA differences of minuscule effect size, which is why polygenic scores have become predictive only after GWA studies reached the statistical power needed to detect such tiny effects.”
He sees this as the power of polygenic scores: that they can predict psychological and behavioural traits despite an absence of understanding how this works.
Do childhood experiences make a difference?
A few months before Plomin’s book was published, clinical psychologist Lucy Maddox's book Blueprint: How Our Childhood Makes Us Who We Are came out. The matching titles are coincidence, the single word difference intriguing. Maddox’s book is an accessible ride through the child development literature, taking in Piaget, attachment and classic experimental paradigms such as false belief tasks and infant habituation studies. She doesn’t mention polygenic scores or behavioural genetics at all, although there is a chapter on epigenetics. When we talked, I asked her what she thinks of Plomin’s book.
'He’s making a really convincing argument to say, hang on, genetics are really important. He’s a real heavy hitter in this field, he’s done some of the most important and elegant studies in genetics, and the effect of our genes are undoubtedly huge. But I don’t think they’re the only important factor to consider. I feel this book presents one side of the arguments really clearly but I feel that there’s a few bits missing – particularly epigenetics, but also a bit more about the way our environment can make a difference on an individual level.'
Maddox points out that she and Plomin think of who we are in different ways. Plomin’s studies rely on broad brush stroke measures by necessity, since his studies require such large numbers. For Maddox, what happens to us as individuals and the quality of our experience is important regardless of the effect on standardised measures. In fact, she chose not to discuss the main factors by which Plomin defines ‘who we are’ in her book at all, explaining to me that she finds the constructs of IQ and personality problematic and culturally bound. When I asked her about Plomin’s argument that we would be essentially the same people if brought up in a different family, she says 'Maybe you are the same on some of the measures he’s using – IQ, personality. Maybe these sorts of broad brush strokes measures are the same. But your personal experience, it makes a huge difference. And in terms of a child’s ability to do a range of different things and access a range of opportunities that makes a huge difference as well.'
Maddox herself works with foster parents who are caring for children who have experienced abuse and neglect, and so has direct experience of trying to intervene so that parenting can ‘make a difference’. She says that Plomin makes caveats which she felt were not small at all.
'He said of course he wasn’t talking about extreme abuse and neglect and severe maltreatment. Actually, I think we do have to talk about that. It’s not that unusual unfortunately. We know it to be extremely important. Childhood maltreatment does make a difference to outcomes later; to ignore that end of the spectrum I found problematic, because if we’re thinking about spectrums we have to include those things as well. As Plomin himself acknowledges, it’s not normal and abnormal, it’s a spectrum of experience. So that means that we can learn things from the extreme ends that can also be applied in the middle.'
It’s true that the behaviour genetic studies limit the range of environmental variation by excluding these children who are severely abused or neglected – and also because they have all been carried out in Western Europe or America. They can tell us nothing about the effects of the shared environment of culture. Is a child adopted from Korea to America also essentially the same person when they grow up, as similar to an identical twin left in Korea as they would have been if they had been raised in the same home? As Plomin says, these studies describe what is, not what could be – and they describe ‘what is’ within a sample whose environmental variation is limited.
Genes and environment, here we go again?
This isn’t a new debate. Back in 1983, Sandra Scarr was writing about gene-environmental interactions, and picking apart the detail of how this might work in development. Her approach acknowledges the role of both genes and environment, and comes up with clear predictions as to how they interact in development. She wrote: 'We propose that the genotype is the driving force behind development, because... it is the discriminator of what environments are actually experienced. The genotype determines the responsiveness of the person to those environmental opportunities.'
The difference with Plomin is that whilst Scarr sees the genotype as driving development, she did not think that this means that the important environmental factors are random or chance. She did, however, see that environment can never be independent of the genotype, because, as she puts it, 'Genes are components in a system that organises the organism to experience its world'.
It’s striking how, despite the arrival of polygenic scores and genome-wide association studies on the scene, we are still thinking about gene-environmental interactions in a similar way. Polygenic scores haven’t shifted the debate as much as you might think.
'People react in different ways that are very hard to predict'
When doing the research for this article, I came across Eric Turkheimer’s blog, in which he discusses (and disputes) some of Plomin’s conclusions. Turkheimer is the Hugh Scott Hamilton Professor of Psychology at the University of Virginia and a clinical psychologist. He is also a behaviour geneticist.
When we talked over Skype, Turkheimer was keen to point out that he doesn’t have a problem with polygenic scores, which he describes as an interesting new tool for research. He does, however, disagree with Plomin about their significance.
'I think he’s [Plomin] trying to sell the idea that the existence of polygenic scores have moved the general nature/nurture needle in a nature direction. That now that we can do polygenic scores, the world looks like a more genetic place than it used to. I don’t think that’s true. We’ve known that genotypic variation is correlated with phenotypic variation for pretty much everything since the twin study era of the 70s and 80s.'
Turkheimer explained how he thinks that the failure to find systemic environmental effects is paralleled by the earlier failure to find candidate genes. 'If you look at the environment, it accounts for 50 per cent of the variance, just like genes account for 50 per cent. We couldn’t figure out which gene was doing what to make that 50 per cent of the variance. We couldn’t figure out which [part of the] environment is doing what to make up that 50 per cent of the variance. So at that level, everything is parallel. But then something different happens in the genotype. The genotype and the environment are not exactly the same, especially in that the genotype is much more quantifiable than the environment.'
The difference he is referring to is that we can measure the genome at any time, calculate a polygenic score made up of thousands of genetic markers, and make predictions with that even though we have no idea what any of the genes are doing. This is simply not possible with the environment – it is inherently dynamic and hard to measure systematically. As such, environmental risk scores are less quantifiable.
Turkheimer also disagrees with Plomin that the important environmental events which make a difference are chance or random, saying: 'What we really mean is that people react to environmental events in different ways that are very hard to predict […] it’s not as though parental divorce has no effects on children, it’s that parental divorce has effects on children that are very hard to predict. Just exactly, again, like the individual gene has effects on kids that are very hard to predict.'
Turkheimer’s view is that the mechanisms by which the genotype becomes the phenotype can’t be disregarded. 'We have no idea how those polygenic scores work. And in fact we do know that they work in part via routes that are mediated through your social environment rather than through your own body. It’s very easy to have intuition in your head when you hear about these risk scores that these are summarising genes that are telling you about how neurons are developing in kids’ brains and leading them to be more efficient thinkers.'
But if psychologists plan to use this information when working with individuals, Turkheimer believes we need to know how they work. 'Not knowing how they work and that some of the mechanisms are not through the body and the brain is a problem. So let’s say some of the polygenic risk score for educational attainment is DNA which is correlated with how dark your skin is. And people with dark skin get discriminated against, and that winds up correlated with their educational attainment. So it’s perfectly valid as a predictor but are you going to feel good about assigning kids to classes on this basis via mechanisms that are correlated with their skin colour?'
It’s a compelling argument. If our genes act on our environment, then many of the pathways for gene expression are going to be environmental. And if we don’t understand those pathways, how can we intervene?
For applied psychologists, to think of polygenic scores as causal (as Plomin says they are) without understanding the pathway doesn’t really make sense, because the pathways are what we work with. We take life histories, we ask about thoughts and feelings in response to circumstances and we develop hypotheses and interventions based on that individual’s (idiosyncratic and non-systematic) perspective. Turkheimer describes this process: 'What do you do with your therapist? You sit there and try and reconstruct your nonshared environment. You try to reconstruct your individual history with all the what [Plomin] would call random events and what I might call the non-systematic effects of non-random events and try and understand how all those things produce a narrative which led you to be who you are.'
He concludes, 'If I were to say, I want to understand my marriage of 35 years and all I’m allowed to use is main effects, correlations with questionnaire responses, I couldn’t do anything. So to come to any understanding of that you have to do things that are borderline non-scientific, and that’s the way the non-shared environment works.'
And here could be the fundamental challenge of Plomin’s book for psychologists. Just how do we understand who we are, and can that ever be separated from our personal experiences? When a psychologist meets an individual, standardised measures are only a small part of the picture. It’s hard to see how a polygenic score would significantly add to the depth of information we get when assessing and taking a history.
The problem for many of us is that it seems like we’ve been here before. The nature-nurture debate goes around, and each time the answer is, it’s a bit of both. Polygenic scores appear to be new and exciting, and promise so much – but their predictive power is currently low and we don’t know if that will change. As study after study finds that yet another behavioural trait is heritable, psychologists might be forgiven for saying 'so what? What difference does it make that everything is heritable, when the environment is the only thing we can change?'
There are practical implications of gene-environment interactions, which are sometimes overlooked. The same environment is experienced differently by different people, and thus our attempts to change things through changing the environment will be affected by the genes of the individuals involved. This doesn’t however, mean that we can’t deliberately change the environment, and that the experience of the other person won’t also change – it just means that we cannot usually predict how that will affect them. So in this way, parenting could make a difference, but we can’t predict exactly what difference it will make.
Geneticists and psychologists have at different times been accused of determinism about human development, and all deny the charges. Perhaps the word Blueprint is in itself misleading, as it implies (at least in the popular imagination) that we are planned out in advance. The truth is that our genome affects how we experience and interact with our environment on every level – but that without a complex environment, our genes have reduced potential to influence and shape our world. Thus our genes and environment interact in myriad ways, and through building an awareness of how these interactions work we may come to understand ourselves just slightly better.
- Dr Naomi Fisher is Chartered Psychologist and Clinical Psychologist registered with the Health and Care Professions Council. She is also an EMDR Consultant (EMDR-Europe). [email protected]
Plomin, R. (2018). Blueprint: How DNA Makes Us Who We Are. Allen Lane.
Maddox, L. (2018). Blueprint: How Our Childhood Makes Us Who We Are. Little Brown.
Turkheimer, E. (2000). The Three Laws of Behavior Genetics and what they mean. Current Directions in Psychological Science, Vol 09, issue 05. http://www.people.virginia.edu/~ent3c/papers2/three_laws.pdf
Turkheimer, E. Gloomy Prospect Blog: https://www.geneticshumanagency.org/gha/category/eric-turkheimer-gloomy-prospect-blog/
Scarr, S., & McCartney, K. (1983). How People Make Their Own Environments: A Theory of Genotype – Environment Effects. Child Development 54(2), 424-435.
Freese, J. & Jao, Y-H. (2017). Shared Environment Estimates for Educational Attainment: A Puzzle and Possible Solutions. Journal of Personality, 85, 79-89.
Plomin, R. (2018). In the Nature–Nurture War, Nature Wins. Scientific American.
Kaufman, S.B. (2018). In the Nature-Nurture War, Robert Plomin is fighting a losing battle. Scientific American.
BPS Members can discuss this article
Already a member? Or Create an account
Not a member? Find out about becoming a member or subscriber