The first rule of behavioural genetics is that every trait is heritable. What makes us different is only about 1 per cent of our entire genetic code. Ninety-nine per cent codes for things like brains, heads, proteins, ears and bipedalism (ability to walk upright): the things that make us human. The other single per cent contains those DNA differences that make us who we are. This single DNA percentage has the most far reaching impacts on the social, economic and psychological differences between us.
Blueprint, by psychologist and geneticist Robert Plomin, is the product of a thrilling lifelong career in genetic research. The magisterial evidence it outlines provides a window into our nature. Using social experiments like the Colorado Adoption Studies (CAS) and biological experiments like TEDS (Twin Early Development Studies), behavioural genetics enables us to quantify the contributions of nature and nurture to specific psychological traits. One way to do this is to study relatives who share the same genes (nature) but different environments (nurture). Adoption acts as a natural experiment that allows researchers to do just that. Empirical studies have consistently demonstrated that adoptive children show greater resemblance to their biological/genetic parents than to their adoptive/environmental parents, across almost all traits. Another way of studying heredity is to compare the behaviour of twins, who can either be identical (sharing exactly the same genes) or fraternal (sharing 50 per cent of their genes). Like adoption studies, twin studies support the first rule of behavioural genetics: on average, psychological traits are extremely heritable, at around 50 per cent.
The Genetic Revolution
Three other major findings laid out in the first half of the book concern degrees of heritability, non-shared environments and the nature of nurture. Genetics provides the majority of the systematic variation between us, while environmental effects are random, and our chosen environments show genetic influence. Heritability is a tricky concept. When people hear that weight is 70 per cent heritable, they often see an imaginary pie chart above a person’s head with 70 per cent representing genes and the other 30 per cent representing the environment. This is a mistake. First and foremost, heritability is a property of populations, not of individuals, and it explains the variance between people who deviate from the mean. If you come from a culture that arranges your marriage, your best guess of the weight or height of your potential bride would be the average height or weight of your local population. If we consider an entire population, however, all the differences from the average are what we mean by variance and explain where heritability applies. Therefore, 70 per cent of the differences in people’s weight throughout a population are due to inherited DNA differences. Heritability describes how much of the differences between us (variation from the average) can be explained by DNA differences (the 1 per cent of our DNA that varies). The other 99 per cent of our DNA code is what we call innate and does not vary except in rare cases. The 1 per cent of DNA that does vary is what contributes to making us who we are psychologically.
Another incredible finding is that the heritability of certain traits increases over time. The heritability of IQ and weight, for instance, is 40 per cent in early childhood, changes to 60 per cent in adulthood and rises to almost 80 per cent in later life. Life events and environments certainly leave a mark, but their influence is minimal by comparison with that of genetic effects. The evidence shows clear genetic trajectories for many traits. On average, 50 per cent of the differences between people can be accounted for by genes and the rest by environment—but not the type of environment one usually imagines. Almost half of what we ascribe to environment is actually genetics in disguise. This is what Plomin refers to as “the nature of nurture.” Furthermore, parents are often responding to genetic differences between their children. The precocious child gives the book reading mother a false sense of influence. If your child were taken away from you at birth, she would turn out to be almost exactly the same person, although raised under an entirely different roof. Incredible as it may seem, this is what the evidence tells us. This explains the most controversial statement in the book: “Parents matter but they don’t make a difference.” Of course, we are describing the normal range of developmental upbringing and this does not apply to abusive or neglectful environments, whose psychological effects are acute and clear.
The Index of Equality
Taking note of some common effect sizes is a good way to compare the influence of different factors. It is often assumed that children learn more in smaller classrooms or that the quality of a good school explains why children who attend that school do so well. However, the correlation between the number of pupils in a class and educational achievement is only significant in so far as it is based on a large sample size. The actual effect is only 1 per cent. The GCSE is the UK’s final exam for 15–16-year-olds, and studies show that only 4 per cent of the variation in these results is explained by the quality of the school. This number drops to just 1 per cent when you correct for socioeconomic status, which means that the impact of school quality on outcomes is almost negligible. Schools matter, but they don’t make a difference. Even though these schools have little effect on individual differences, parents will still spend a small fortune on better schools, because of the social networks and future job opportunities they offer. Parents that cannot afford such schools can take solace in the fact that highly selective schools, which unintentionally select for highly heritable traits, have very little impact on overall school achievement.
A medium effect size, explaining 10 per cent of the variation in educational achievement, can be attributed to parental educational attainment. If a child does well at school, 10 per cent of the difference in outcome is due to parental achievement. A large effect is one that explains 25 per cent of the variance. One example is general intelligence, which accounts for about a quarter of the variation in educational achievement. There are very few effects of this magnitude in psychology. Inherited DNA differences are by far the most significant systematic force in making us who we are. Once environmental differences have been dramatically reduced, the largest remaining differences are going to be genetic, as Plomin explains:
This 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. The higher the heritability of school achievement, the less the impact of environmental advantages and disadvantages. If nothing but environmental differences were important, heritability would be zero. The finding that heritability of school achievement is higher than that of most traits—about 60 per cent in western countries—suggests that there is substantial equality of opportunity.
Abnormal Is Normal
Another revolutionary finding is changing the face of mental health. When it comes to psychological disorders, such as reading disability, autism, depression and schizophrenia, it is quantity not quality that matters. In this way, they differ from the thousands of single-gene disorders, like Huntington’s and sickle cell anaemia, in which a single mutation is both necessary and sufficient. Necessary because you will only get the disease if you inherit the mutation. Sufficient insofar as if you do inherit the mutation, you will succumb to the disease (though the number of repetitions present may also be an important factor in the severity of the clinical outcome).
OGOD, as Plomin calls it—one gene one disorder—is how many people think about psychological traits: as necessary and sufficient. However, findings from genetic studies have falsified the OGOD hypothesis and we now know that psychological disorders are the result of potentially thousands of genes, which each have small effects. This is known as polygenic inheritance. This finding means that we all share the genes for these disorders, but the quantity inherited increases or decreases the probability of suffering from any particular disorder. For instance, the average person may have 500 of the 1000 depression-causing DNA differences and will therefore have an average risk of depression. People with fewer of these DNA differences will have a lower risk and people with more will have a higher risk. Another way to express this is that the abnormal is normal. These disorders are normally distributed across a bell-shaped curve throughout the population. Does this mean that we all have some degree of schizophrenia? Indeed, it does. Anyone that has meditated will have made this empirical discovery long before the genetic revolution came along.
There are huge implications here for psychiatry and for clinical psychology in general. Genetic research has demonstrated that the medical model is wrong and that what we call disorders are merely the extremes of the same genes that are at work throughout the normal distribution. This finding means that the idea of curing a disorder is absurd because there is no disorder in the first place, just dimensions along a scale. Sliding your finger along that scale and deciding on a location where a disorder begins is an arbitrary operation. The word spectrum is already being used for conditions like schizophrenia and autism and this is a move towards a more quantitative approach. It is a case of more or less for all of us.
The Nature of Potential
All these propensities are descriptions of what is, not what could be i.e. potentials. If you are high on the scale for poor attention, there is nothing to stop you from engaging in practices like mindfulness to make your attention deficit more manageable. Having a reading disorder does not mean that you are fated to a life without books. Alas, being situated low on the spectrum for a sense of humour does not mean that you will never attempt to be funny or use irony. The cosmic lottery decides who becomes a Buddha, J. K. Rowling or Ricky Gervais. Nevertheless, discovering our own unique worth and dignity is far more important than becoming what parents, society or even genetics have in mind. Propensities significantly influence our behaviour, especially when they interact with certain environments. Meanwhile, an array of potentials are waiting to be actualized. As Plomin writes in Blueprint:
Throughout our lives we are bombarded with inspirational aphorisms. The barrage also comes from pop-psychology books where the message is all you need to succeed is some panacea, such as the power of positive thinking, growth mindset, or 10,000 hours of practice. Anyone who is interested in these modern maxims should understand that, to the contrary, genetics is the main systematic force in life. That is not to say genes are destiny. It just seems more sensible, where possible to go with the genetic flow rather than swimming upstream. As W. C. Fields said, “If at first you don’t succeed try, try, try again. Then quit. No point being a damn fool about it.”
This is what we mean by actualizing our potentials: we should explore life, but accept our natural limitations. This is what it means to mature. We vary in terms of our genes and we vary in terms of our potentials. Our ability to explore those potentials is one of the most magnificent properties of humanity. While we do vary, there are at least two potentials that are universal. The first is the moral potential, the potential to feel empathy toward sentient beings. The second is the insight potential, or seeing inward, the potential to self-reflect and become aware of the subjective facts of experience—namely, those born of a false sense of self, which produces needless suffering. Both these potentials interact at various times throughout our lives and are universally shared. Plomin comments:
Equality of opportunity, income inequality and social mobility are some of the most critical issues in society today. They are hugely complicated topics that heavily depend on values. My objective has been to look at these issues through the single lens of genetics, to show how DNA makes us who we are. However, no specific policies necessarily follow from genetic findings because policies depend on values. My values, not my science, lead me away from meritocracy towards a just society.
Genetics accounts for most of the systematic variation between us, environmental effects are random and our chosen environments show genetic influence. Everything is heritable—and what is not caused by genes is caused by non-shared environment or environments that are often genetic effects in disguise. Parents matter, but they don’t make a difference. Children actively select, modify and create environments that correspond to their genetic propensities. Every psychological problem is a problem of genetic degree, not of kind, and we all show major polygenic influence (thousands of genes with small effects) along a normal distribution. We share some propensities and potentials, while many others are unique. In a life well lived and a society well formed, we know our propensities and can explore our potentials. We need not emphasise one at the expense of the other.