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Topic of Book
Plomin argues that the latest genetic research shows that our DNA is by far the largest factor in determining our personality, behavior and life outcomes.
Many readers are very uncomfortable with the role that DNA plays in human behavior and variation in life outcomes. This is particularly true for those with strong ideological convictions.
Genetic research should force us all to reconceptualize many of our theories and belief systems. I believe that it is important to let these findings sink in and confront the fact that we each need to update our beliefs.
A good place to start is to not accept a non-genetic explanation until genetic factors have been controlled. With our current knowledge, this is very difficult, but as we learn more, this will get much easier. For now we must not quickly dismiss a genetic cause just because we are uncomfortable with its ramifications.
I highly recommend this book!
- Our genes are the most powerful determiners of our personality, behavior and life outcomes. They typically account for 50-70% of the variation. This is true even for complex behaviors such as social status, income and educational outcomes.
- All psychological traits show significant and substantial genetic influence.
- Heritability increases with age. In other words our genes predict outcomes more accurately for adults than young children.
- Increased equality of opportunity will not lead to equality of outcome. It will lead to a large influence of genes on outcomes.
- Many causes that were previously thought to be from the environment are now known to be caused to large extent by genes: parenting, peer groups, social support and even how much time children spend watching television
- Parenting account for close to 0% of the variation (although abusive parenting is known to cause problems).
- Other non-family environmental factors that explain the rest of the variation are more akin to random events in our lives. These causes are largely unknown, unsystematic, in the sense that they are mostly a matter of chance, and short in duration.
- Twin studies and adoption studies have long made it clear that genetics had a powerful impact, but until very recently we did not know which genes were responsible.
- Behavior and outcomes are rarely influenced by just one gene. It is usually caused by the subtle interactions among tens of thousands of genes
- Until very recently it was impossible to identify those genes because sequencing was expensive and databases were small.
- Starting in the late 2010s inexpensive SNP tests and very large databases have set off a tidal wave of definitive studies linking specific combinations of genes with disorders.
- These studies have enabled us to create “polygenic scores” that are very accurate in predicting the probability of personality, mental illness, intelligence and complex behaviors.
- Rather than looking for statistically significant effect for one genes (the old method), researchers now add genes to the polygenetic score if they increase reliability and validity of the composite score.
- These polygenic scores will enable us to deliver individualized treatment to people who are genetically predisposed to disorders long before they actually show symptoms.
- Polygenic scores are still a long way off from accounting for all the genetic variation derived from earlier twins and adoption studies, but they are getting better every year. This gap is the “missing heritability.”
Important Quotes from Book
What would you think if you heard about a new fortune- telling device that is touted to predict psychological traits like depression, schizophrenia and school achievement? What’s more, it can tell your fortune from the moment of your birth, it is completely reliable and unbiased – and it costs only £100.
This might sound like yet another pop- psychology claim about gimmicks that will change your life, but this one is in fact based on the best science of our times. The fortune teller is DNA. The ability to use DNA to understand who we are, and predict who we will become, has emerged only in the last three years, thanks to the rise of personal genomics. We will see how the DNA revolution has made DNA personal by giving us the power to predict our psychological strengths and weaknesses from birth. This is a game- changer that has far- reaching implications for psychology, for society and for each and every one of us.
Since the 1960s scientists conducting long- term studies on special relatives like twins and adoptees have built a mountain of evidence showing that genetics contributes importantly to psychological differences between us. The genetic contribution is not just statistically significant, it is massive. Genetics is the most important factor shaping who we are. It explains more of the psychological differences between us than everything else put together. For example, 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 – once we control for the impact of genetics. Genetics accounts for 50 per cent of psychological differences, not just for mental health and school achievement, but for all psychological traits, from personality to mental abilities. I am not aware of a single psychological trait that shows no genetic influence.
By studying genetically informative cases like twins and adoptees, behavioural geneticists discovered some of the biggest findings in psychology because, for the first time, nature and nurture could be disentangled. The implications of these findings are transformative for psychology and society and for the way you think about what makes you who you are.
For example, one remarkable discovery is that even most measures of the environment that are used in psychology – such as the quality of parenting, social support and life events – show significant genetic impact.
Genetic influence slips in because these are not pure measures of the environment ‘out there’ independent of us and our behaviour. We select, modify and even create our experiences in part on the basis of our genetic propensities. This means that correlations between such so- called ‘environmental’ measures and psychological traits cannot be assumed to be caused by the environment itself. In fact, genetics is responsible for half of these correlations. For example, what appears to be the environmental effect of parenting on children’s psychological development actually involves parents responding to their children’s genetic differences.
A second crucial discovery at the intersection of nature and nurture is the unexpected way in which the environment makes us who we are. Genetic research provides the best evidence we have for the importance of the environment because genetics accounts for only half of the psychological differences between us. For most of the twentieth century environmental factors were called nurture because the family was thought to be crucial in determining who we become. Genetic research showed that this is absolutely not true. In fact, the environment makes siblings reared in the same family as different as siblings reared in separate families. Family resemblance is due to our DNA rather than to our shared experiences like TLC, supportive parenting or a broken home. What makes us different environmentally are random experiences, not systematic forces like families.
The DNA differences inherited from our parents at the moment of conception are the consistent, lifelong source of psychological individuality, the blueprint that makes us who we are. A blueprint is a plan. It is obviously not the same as the finished three- dimensional structure – we don’t look like a double helix. DNA isn’t all that matters but it matters more than everything else put together in terms of the stable psychological traits that make us who we are.
These findings call for a radical rethink about parenting, education and the events that shape our lives.
We are the same as every other human being for more than 99 per cent of these 3 billion DNA steps, which is the blueprint for human nature. The less than 1 per cent of these DNA steps that differ between us is what makes us who we are as individuals – our mental illnesses, our personalities and our mental abilities. These inherited DNA differences are the blueprint for our individuality.
Recently, it has become possible to directly assess each of the millions of inherited DNA differences between us and to find out which of these are responsible for the ubiquitous genetic influence on psychological traits. One of the extraordinary discoveries was that we are not just looking for a few DNA differences with big effects but rather thousands of small differences whose weak effects can be aggregated to create powerful predictors of psychological traits. The best predictors we have so far are for schizophrenia and school achievement, but other DNA predictors of psychological traits are being reported every month.
These are unique in psychology because they do not change during our lives. This means that they can foretell our futures from birth. For example, in the case of mental illness, we no longer need to wait until people show brain or behavioural signs of the illness and then rely on asking them about their symptoms… In this way, DNA predictors open the door to prediction and, eventually, prevention of these problems before they create collateral damage that is difficult to repair.
I hope this book conveys the excitement I feel about this historic moment in psychology. The message from earlier research has begun to sink in, that DNA is the major systematic force, the blueprint, that makes us who we are. The implications for our lives – for parenting, education and society – are enormous. However, this only sets the stage for what will be the main event: the ability to predict our psychological problems and promise from DNA. This is the turning point when DNA changes psychology – scientifically and clinically – and the impact of psychology on our lives. Our future is DNA.
For most of the twentieth century it was assumed that psychological traits were caused by environmental factors. These environmental factors were called nurture because, from Freud onwards, their origins were thought to lie in the family environment. Because these traits run in families, it was reasonable to assume that the family environment is responsible for these traits.
But genetics also runs in families… . So the reason why psychological traits run in families could be nature (genetics) as well as nurture (environment).
Heritability is frequently misunderstood. For example, it is not a constant like the speed of light or gravity. It is a statistic that describes a particular population at a particular time with that population’s particular mix of genetic and environmental influences. A simpler way of expressing this is that it describes what is but does not predict what could be.
One of the biggest discrepancies between people’s ratings and research results is for school achievement, which is a focus of my research. The average rating in our survey was 29 per cent, but genetic research consistently shows that performance on tests of school achievement is 60 per cent heritable on average. That is, more than half of the differences between children in how well they do at school is due to inherited DNA differences.
Heritability is so ubiquitous that this has been called the first law of behavioural genetics: All psychological traits show significant andsubstantial genetic influence.
One way to disentangle nature and nurture is to find relatives who share nature but not nurture in order to test the power of genetics. Adoption is like a social experiment that does just this. We can see how similar children are to their biological, or ‘genetic’, parents when the children are adopted away at birth. These parents share nature but not nurture with their children. If nature is why weight runs in families, adopted children should resemble their genetic parents, not their adoptive parents.
Adoption studies also provide a direct test of nurture. If nurture is why weight runs in families, adopted children should resemble their adoptive parents, who are their ‘environmental’ parents. Just like parents who rear their genetic children, adoptive parents provide their children’s family environment, including the food they eat, and model healthy or unhealthy lifestyles.
If adoption is a social experiment separating the effects of nature and nurture, twins are a biological experiment. Where you can most clearly see heredity in action is identical twins. Identical twins come from the same fertilized egg, or zygote. This is why they have the same inherited DNA and why, in scientific terminology, they are called monozygotic twins (MZ). About one in 350 people is an identical twin, so chances are you personally know at least one pair of MZ twins.
The most dramatic test of genetic influence is to study MZ twins separated by adoption early in life. They share nature completely but do not share nurture at all, so their similarity is a direct test of genetic influence.
MZ twins reared apart are of course extremely rare. Only a few hundred pairs have been studied worldwide. These cases have produced some amazing examples of similarity.
Like CAP, TEDS has shown that many traits (some of them in addition to those investigated in CAP) obey the first law of behavioural genetics. For example, in the cognitive domain, we found that how well children do at school in all subjects, from humanities to sciences, is substantially heritable. We also found that components of reading (for example, phonetics) and of language (for example, fluency) are highly heritable. For the first time, we showed that individual differences in learning a second language are highly heritable. We looked in depth at aspects of spatial ability, such as navigating from a map, with results again showing ubiquitous heritability.
The answer is an example of another of the ‘big findings’ of genetic research: heritability increases during development. Twins in TEDS are adolescents, but most other twin studies involve adults. In an analysis of forty- five twin studies, the heritability of weight increases from about 40 per cent in early childhood to about 60 per cent in adolescence to about 80 per cent in adulthood.
The point is that these very different designs – twin and adoption studies – converge on a simple but powerful conclusion: most of the differences between people in weight can be explained by inherited differences in DNA.
It cannot be overemphasized that genetic effects on psychological traits are not only statistically significant, they are massive in terms of how much variance they explain.
Few effect sizes in psychology are greater than 5 per cent.
This is why it is incredible to find that 50 per cent of the differences between people in psychological traits are caused by genetic differences between them.
One example is that general intelligence accounts for about 25 per cent of the variance in educational achievement. On this scale from small (1%) to medium (10%) to large (25%) effect sizes, heritability of 50 per cent is literally way off the scale. Inherited DNA differences are by far the most important systematic force in making us who we are.
The heritability of divorce is about 40 per cent across studies. This is a long way from 100 per cent, 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. Controlling for genetics is crucial.
Calling any measure ‘environmental’ does not make it a measure of the environment. Genetic studies of environmental measures have found significant heritability for most measures of the ‘environment’ – parenting, peer groups, social support and even how much time children spend watching television.
Since then, more than 150 papers have looked at environmental measures in genetically sensitive studies. They consistently find substantial genetic influence and the average heritability is still about 25 per cent.
Characteristics of adolescents’ peer groups are especially highly heritable, such as the peer group’s academic orientation or their delinquency. The reason for this high heritability may be that you can choose your friends but you cannot choose your family.
The only events free from genetic influence are those over which we have little control, such as the death or illness of relatives and friends. As expected, research finds little genetic influence for these uncontrollable events. Nonetheless, our reaction to such events – our psychological experience of the events – can be influenced by our genetic make- up.
Genetics typically accounts for about half of the correlation between environmental measures and psychological traits. This finding about the nature of nurture is one of the most unexpected and important examples of how DNA makes us who we are. Instead of assuming that correlations between the ‘environment’ and psychological traits are caused environmentally, it is safer to assume that half of the correlation is due to genetic differences between people.
One of the big findings from behavioural genetic research is counterintuitive: genetic influences become more important as we grow older. No psychological trait shows less genetic influence with age, but the domain where heritability increases most dramatically during development is cognitive ability.
The heritability of 50 per cent for intelligence is just the lifetime average across all studies. The impressive increase in heritability from 20 per cent in infancy to 40 per cent in childhood to 60 per cent in adulthood stands out from other traits that show little developmental change in heritability, most notably personality and school achievement.
The most plausible possibility is that slight nudges from genetics early in development are magnified as time goes by. That is, the same genetic factors snowball into larger and larger effects, a process that is known as genetic amplification.
Genetic research shows that the medical model is all wrong when it comes to psychological problems. What we call disorders are merely the extremes of the same genes that work throughout the normal distribution. That is, there are no genes ‘for’ any psychological disorder. Instead, we all have many of the DNA differences that are related to disorders. The salient question is how many of these we have. The genetic spectrum runs from a few to a lot, and the more we have, the more likely we are to have problems.
In other words, the genetic causes of what we call disorders are quantitatively, not qualitatively, different from the rest of the population. It’s a matter of more or less (quantitative), not either/or (qualitative).
When we talk about genetics, it is easy to slip into thinking about the gene for this and the gene for that. I call this the OGOD hypothesis, for ‘one gene, one disorder’, which is misleading. Our species has thousands of single- gene disorders, but they are rare. In contrast, common disorders, including all psychological disorders, are not caused by a single gene.
A shift in vocabulary is needed so that we talk about ‘dimensions’ rather than ‘disorders’.
The normal- is- abnormal view is much more radical. We are not just conceding a bit of grey space in between normal behaviour and diagnosed disorders like schizophrenia and autism, setting up yet another diagnostic category called ‘spectrum disorder’. We are saying that the distinction between normal and abnormal is artificial. The abnormal is normal.
Studies of genetic causes chart a map of disorders that is almost unrecognizable from our current symptom- based diagnoses of disorders. That is, instead of finding distinct genetic influences that correspond to diagnoses, genetic effects are splashed out across many disorders. Genetic effects tend to be general rather than specific, which is why I call this topic generalist genes.
In other words, genetic risks are general across disorders; environmental risks are specific to a disorder.
Generalist genes are not limited to the domain of psychopathology. Most genetic effects are also general across cognitive abilities. For example, cognitive abilities such as vocabulary, spatial ability and abstract reasoning yield genetic correlations greater than 0.5.
This is why intelligence, more precisely called general cognitive ability, is such a powerful construct. It captures what is in common among diverse cognitive abilities. This makes intelligence a good target for hunting generalist genes.
Education- related skills such as reading, mathematics and science show even higher genetic correlations: about 0.7.
It also makes sense to assume that the brain evolved as a general tool for solving problems… Our ancestors’ survival depended on how well their brainpower translated into behaviour. Individuals who were better able to solve problems, including life- and- death split- second decisions, were more likely to survive and reproduce. Individual differences in problem-solving scooped up whatever advantages they had wherever they occurred in the brain.
What this research implies is that, far from the family being a monolithic determinant of who we are, environmental influences shared by family members do not make a difference. This is an astonishing conclusion because these are just the environmental influences that psychologists consider when they talk about nurture.
Heritabilities are typically 50 per cent, which completely explains the similarity of relatives. The environment accounts for the other 50 per cent, but there is no evidence for shared environmental effects of growing up in the same family.
Intelligence appears to be a major exception to the rule that environmental factors that affect psychological traits are non- shared. The correlation for half a dozen older studies of adoptive siblings was 0.25, suggesting that a quarter of the variance in intelligence can be explained by shared environment.
School achievement is another apparent exception to the rule. Tests of school achievement in all subjects from science to the humanities typically estimate that 20 per cent of the variance in performance can be explained by shared environment.
Genetic research shows that non-shared environmental influences are not only unsystematic, in the sense that they are mostly a matter of chance, they are also largely unstable, that is, inconsistent across time.
After thirty years of searching unsuccessfully for systematic non- shared environmental influences, it’s time to accept the gloomy prospect. Non- shared environmental influences are unsystematic, idiosyncratic, serendipitous events without lasting effects. The systematic, stable and long- lasting source of who we are is DNA.
Education is the field that has been slowest to absorb the messages from genetic research. In other fields, especially psychology, we have come a long way from the environmentalism that assumes that we are what we learn.
The most important thing that parents give to their child is their genes. Many parents will find this hard to accept.
There is no evidence that these parenting practices make a difference in children’s development, after controlling for genetics.
Parents are the most important relationship in children’s lives. Still, it is important that parents get the message that children are not blobs of clay that can be moulded however they wish. Parents are not carpenters building a child by following a blueprint. They are not even much of a gardener, if that means nurturing and pruning a plant to achieve a certain result. 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. It is also important for parents to know that, beyond genetics, most of what happens to children involves random experiences over which parents have no control. The good news is that these don’t make much of a difference in the long run. The impact of these experiences is not stable across time.
In the tumult of daily life parents mostly respond to genetically driven differences in their children. This is the source of most correlations between parenting and children’s outcomes… If we go against the grain, we run the risk of damaging our relationship with our children.
Genetics provides an opportunity for thinking about parenting in a different way. Instead of trying to mould children in our image, we can help them find out what they like to do and what they do well. In other words, we can help them become who they are. Remember that your children are 50 per cent similar to you. In general, genetic similarity makes the parent– child relationship go smoothly…. It is also useful to keep in mind that our children are 50 per cent different from us and that siblings are 50 per cent different from each other. Each child is their own person genetically. We need to recognize and respect their genetic differences. Most importantly, parents are neither carpenters nor gardeners.
Parenting is not a means to an end. It is a relationship, one of the longest lasting in our lives.
The same principles apply to education. Schools matter in that they teach basic skills such as literacy and numeracy and they dispense fundamental information about history, science, maths and culture.
But our focus is on individual differences. Children differ a lot in how well they do at school. How much do differences in children’s school achievement depend on which school they go to? The answer is not much.
One general message that should emerge from these discoveries is tolerance for others – and for ourselves. Rather than blaming other people and ourselves for being depressed, slow to learn or overweight, we should recognize and respect the huge impact of genetics on individual differences.
At first glance, genetics seems antithetical to equality of opportunity. But ‘equal’ does not mean identical. If everyone were identical, there would be no need to worry about equal rights or equal opportunity. The essence of democracy is that people are treated fairly despite their differences.
The most important point about equality of opportunity from a genetic perspective is that equality of opportunity does not translate to equality of outcome. If educational opportunities were the same for all children, would their outcomes be the same in terms of school achievement? The answer is clearly ‘no’ because even if environmental differences were eliminated genetic differences would remain. What follows from this point is one of the most extraordinary implications of genetics. Instead of genetics being antithetical to equal opportunity, heritability of outcomes can be seen as an index of equality of opportunity. Equal opportunity means that environmental advantages and disadvantages such as privilege and prejudice have little effect on outcomes. Individual differences in outcomes that remain after systematic environmental biases are diminished are to a greater extent due to genetic differences. In this way, greater educational equality of opportunity results in greater heritability of school achievement.
It would be a mistake to see gene– environment correlation as inequality, because it is, ultimately, based on genetics. For this reason, gene– environment correlation is difficult to disrupt. We can’t stop parents from providing correlated nature and nurture to their children unless we adopt children away at birth. We could outlaw selection in schools, but in the classroom it is impossible as well as undesirable for teachers to treat children the same, regardless of their genetic differences. Finally, trying to stop children from actively seeking experiences correlated with their genetic appetites and abilities is futile.
What this means is that high heritability of school achievement indicates that educational opportunities are substantially equal. Attempts to increase equality of opportunity should focus on reducing shared environmental influence, although shared environment at most accounts for 20 per cent of the variance in school achievement. Non- shared environmental influences are out of reach because they are unsystematic and we don’t know what they are. Correlations between opportunity and outcome are genetically driven. This is another way in which DNA makes us who we are.
Both occupational status and income are substantially heritable, about 40 per cent in more than a dozen twin studies in developed countries. This should not be surprising, because occupational status and income are related to educational attainment and intelligence, which are heritable traits.
At first thought, it might seem that, given free rein, genetics will limit social mobility and calcify society into genetic castes, as happened in India, where for thousands of years mating was limited to members of the same caste. I would argue that this is not a problem in modern societies for two reasons. The first is simple: a lot of the environmental variation between us is not systematic. Random effects will not create stable castes.
The second reason is that parents and offspring are only 50 per cent similar genetically. Their genetic similarity means that, on average, brighter parents have brighter children. But their 50 per cent genetic dissimilarity means that children of brighter parents will show a wide range of ability, including some children of lower- than average ability.
In addition, children of high- IQ parents will on average have lower IQ scores than their parents for the same reason that tall parents have taller- than- average children but those children are less tall than they are. For the same reason, most prodigies do not have prodigy parents. This is a statistical phenomenon, not a specific genetic process.
Because children are 50 per cent similar genetically to their parents, genetics predicts that the children’s average IQ will regress halfway from their parents’ IQ to the population average.
The flip side of this argument is that parents of average ability also have children with a wide range of ability, including children of high ability. Because there are many more parents of average ability than of high ability, this guarantees that most of the individuals of highest ability in the next generation will come from parents of average ability, not from the most able parents. As long as downward social mobility as well as upward social mobility occurs, we do not need to fear that genetics will lead to a rigid caste system.
Self- selection is an important factor to the extent that people are free to choose what they do to earn a living. Self- selection involves listening to genetic whispers, not just about intelligence but also about personality and interests. These options include choosing a job that just pays the bills rather than a high- income occupation that might come with a high- stress price tag, or an especially enjoyable vocation that might not pay the bills.
- “The 10,000 Year Explosion: How Civilization Accelerated Human Evolution” by Cochran and Harpending
- “Who We Are and How We Got Here” by David Reich
- “The Story of the Human Body” by Daniel E. Lieberman
- “A Troublesome Inheritance: Genes, Race and Human History” by Nicholas Wade
- “The Gap: Science of What Separates Us from Animals” by Thomas Suddendorf
- “Is There Anything Good About Men?” by Roy F. Baumeister
- “A Farewell to Alms: A Brief Economic History of the World” by Gregory Clark
- “The Son Also Rises: Surnames and the History of Social Mobility” by Gregory Clark
- “Meta-analysis of … twin studies” by many