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My Favourite Parts From: Robert Plomin's Blueprint
Welcome to the third edition of My Favourite Parts From. This time we’re looking at my highlights from Robert Plomin’s bestselling Blueprint: How DNA makes us who we are.
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.
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.
In contrast to this narrow but specific definition of ‘genetic’ as inherited DNA differences, environmental influence is defined very broadly to mean all influences that are not due to inherited DNA differences. This definition of environment is much broader than the typical environmental influences that are studied by psychologists such as family, neighbourhood, school, peer and work environments. As in the case of breast cancer, it even includes DNA differences that are not inherited. This broad definition of environment also includes prenatal influences, illnesses, and food and drink – everything and anything that is not caused by inherited DNA differences. In this sense, a better word for what geneticists mean when they refer to environment is ‘non-genetic’.
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 – indeed, they might be the same good traits that make people desirable as marriage partners in the first place.
It has long been known that the offspring of divorced parents are more likely to get divorced themselves. Possible environmental explanations leap to mind, for example, living through their parents’ divorce causes children to have relationship problems, or because they do not have good models for a stable relationship. However, a recent adoption study in Sweden showed that the link between divorce in parents and divorce in their children is forged genetically, not environmentally. For a sample of 20,000 adopted individuals, the likelihood of divorce was greater if their biological mother, who did not rear the individual, had later in life become divorced than if the adoptive parents who reared them had become divorced.
The heritability of divorce is about 40 per cent across studies.
heritability of school achievement is about 60 per cent across the school years, higher than the heritability of intelligence, which is about 40 per cent.
How can this be? One possible explanation is that universal education in the early school years reduces environmental disparities in skills like reading and maths which are targeted by tests of school achievement, and this leads to high heritability even in the first few years of school.
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, even though these abilities are thought to involve very different neurocognitive processes.
Education-related skills such as reading, mathematics and science show even higher genetic correlations: about 0.7.
The genetic correlation between reading familiar words and non-words is 0.9, making this one of the most powerful examples of generalist genes. That is, the same DNA differences are responsible for individual differences in fluency and phonetics, even though fluency and phonetics are thought to be completely different neurocognitive processes.
The average IQ score difference between siblings is thirteen IQ points, not that far off the average difference of seventeen IQ points for pairs of individuals selected at random from the population.
Grit is another personality trait that has been thought to be due to shared environmental influence, but it also shows the same results as other personality traits: moderate heritability and no shared environmental influence. Nurture does not teach children to be kind or gritty.
No identical twin differences have been shown to be stable over several years, which would be necessary if non-shared environment had enduring effects. This means that the non-shared environmental factors that make identical twins different are not stable. They are like random noise.
In 2010 Michael Gove, the recently appointed UK Secretary of State for Education, decided that UK schools should go back to teaching reading using phonics to sound out letters and words.
The Phonics Screening Check, mentioned earlier, involves reading aloud as quickly as possible a list of forty age-appropriate familiar words and non-words. For example, some easy words are ‘dog’, ‘big’ and ‘hot’ and more difficult words are ‘project’, ‘frequent’, ‘exercise’. Non-words are word-like combinations of letters the child has never seen before that are matched to the real words in difficulty level.
How well children performed on the Phonics Screening Check was assumed to be due to how well their teachers taught phonics.
we found that it was among the most highly heritable traits ever reported at this age, with heritabilities of about 70 per cent. This means that the test is not measuring how well children are taught reading. Instead, it is a sensitive measure of genetically driven aptitudes for learning to read.
Parents can make a difference to their child but, on average in the population, parenting differences don’t make a difference in children’s outcomes beyond the genes they share.
Over half of children’s psychological differences are caused by inherited DNA differences between them. The rest of the differences are largely due to chance experiences. These environmental factors are beyond our control as parents. As we saw in the previous chapter, we don’t even know what these factors are.
Anyone who has interviewed candidates for a job knows the complexity and capriciousness of selection. In the first place, you can only select from people who applied for the position. In addition, interviews are notoriously poor predictors of performance. These and many other unsystematic factors, including chance, contribute to individual differences in occupational status and income. These factors are not meritocratic, but they do not represent systematic bias.
…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.
Researchers are attempting to use CRISPR to treat several single-gene diseases in somatic cells that are not passed on, including muscular dystrophy, cystic fibrosis and certain blood diseases. The problem is that, unlike changing the DNA in an embryo, which has just a few cells, or in sperm or an egg, which are single cells, DNA needs to be edited in many cells in blood or muscle or lung to bring about a therapeutic effect. In contrast, genetic influence on psychological traits is not a matter of a hard-wired single-gene mutation. Heritability is the result of thousands of genes of small effect. For this reason, gene-editing seems unlikely to be used to alter genes involved in psychological traits.
…there is always concern about the unintended consequences of altering highly polygenic and pleiotropic systems, with their evolved checks and balances.
Unlike any other predictors, polygenic scores are just as predictive from birth as from any other age because inherited DNA sequence does not change during life. In contrast, height at birth scarcely predicts adult height. The predictive power of polygenic scores is greater than any other predictors, even the height of the individuals’ parents. Another advantage of polygenic scores over family resemblance is that parental height provides only a family-wide prediction that is the same for any child born to those parents. In contrast, polygenic scores provide a prediction specific to each individual. In other words, my polygenic scores at birth would have predicted that I would be taller than expected on the basis of the average height of my parents.
Recently, a family study of more than a million psychiatric patients in Sweden provided evidence to support these anecdotes, finding that the non-diagnosed first-degree relatives of schizophrenics were more likely to be in creative professions, such as actors, musicians and writers. A good example of the future use of polygenic scores is a recent study that asked whether the polygenic score for schizophrenia could predict creativity in healthy people. In several diverse populations the researchers found that people with high polygenic scores for schizophrenia were more likely to be in creative professions.
The forthcoming educational attainment polygenic score based on a GWA sample of a million predicts more than 10 per cent of the variance in intelligence. It will be difficult for GWA studies of intelligence itself to reach similar sample sizes because intelligence has to be tested, whereas years of education can be assessed with a single self-reported item. Until much larger GWA studies of intelligence are conducted, this polygenic score will continue to be the best predictor of intelligence.”
R. Population vs individual PGS for education attainment:
…the correlation between a polygenic score and a psychological trait does not tell us about the brain, behavioural or environmental pathways by which the polygenic score affects the trait.
It is remarkable that polygenic scores can predict psychological traits without knowing anything about these intervening processes.
The best predictor of whether children go to university is whether their parents went to university, a link which is widely assumed to be environmental in origin and which is thus thought to be a sign of immobility and lack of equality. In other words, university-educated parents are thought to pass on environmental privilege to their children, creating inequality in educational opportunity and stifling intergenerational educational mobility. In comparisons between countries, the strength of this link between parent and offspring attainment is used as an index of educational inequality and the lack of social mobility.
However, what we are talking about here is parent–offspring resemblance for educational attainment. I hope that by now you find it odd that people have assumed that parent–offspring resemblance is caused environmentally and that possible genetic influence has not been considered. Using the TEDS dataset, we found that DNA differences underlie this parent–offspring resemblance. That is, educational attainment polygenic scores of children were highest when both parents and their children went to university and lowest when neither parents nor their children went to university.
When infants are two years old, intelligence tests predict less than 5 per cent of the variance of scores when the individuals are eighteen years old. In contrast, polygenic scores can predict just as much variance in adult intelligence as they can, not just at two years of age, but even at birth.