99% of the human genome is the same in everyone. Less than 1% differs. The genes that differ make us who we are, says Robert Plomin, our mental illnesses, our personalities and our mental abilities. He is a psychologist, who works on how genes help us understand who we are as individuals and predict what we will become.
Most of psychology is based on our environment, and how that affects us. Nurture was thought to make us who we are. Since the 1960s scientists conducting long term studies on separated twins have shown genes contribute hugely to our psychological differences. The most important environmental factors, such as families and schools, account for less than 5% of our difference in mental health, once we control for the impact of genetics. Every psychological trait shows some genetic influence. Even our experience of our parents and our schools is influenced by our genetic personalities.
Nurture is the wrong word: siblings raised in the same family are as different as siblings raised in different families. Plomin uses the word Environmentalism for the idea in psychology that the environment influences us. There are not individual “genes for” psychological traits, rather the weak effects of thousands of small differences aggregate to create powerful predictors of traits. You are 50% different to each parent and sibling. One genetic difference can cause cystic fibrosis, if it occurs in the CFTR gene inherited from each parent, but schizophrenia does not work in that way. Polygenic testing correlates thousands of genes with behavioural differences. The larger the study group, the more accurate the predictions- and more and more people are having their genomes mapped.
Eye colour is 95% heritable, that is, based on the genes we inherit. Breast cancer only 10%. Schizophrenia is 50%, and autism 70%. Reading disabilities, school achievement, verbal ability and the ability to remember faces are all 60% heritable. General intelligence including reasoning is 50% heritable.
Heritability refers to a particular population at a particular time. For example, heritability of body weight is greater in wealthy countries where there is greater access to fast food and high energy snacks, than in poorer countries. There is a difference between group and individual differences: the average height of northern European males has increased by more than 6″ in the last two hundred years, which is clearly due to changes in environment, but the differences in height between individuals is down to genetics.
Average differences between groups could be entirely environmental, perhaps as a result of discrimination.
What would this mean for the good society? Not necessarily eugenics and preventing some people from breeding: remember that our parents’ genes are mixed, not a predictable half from each, and many genes have tiny influences. To me, as a socialist, it would mean that as success in life was to a great extent genetic, society should mitigate against the inequalities it causes, by progressive taxation and a good safety net. It would mean that punishment of crime should be about containment and reform rather than deterrence or retribution. The moral deservingness someone had of their position in society, exalted or miserable, would be less, so it would be moral to mitigate the differences.
It would mean trans women are just as much real women as women with female genes, gonads and genitals. I resisted transition, and the idea that I was transsexual, for many years, but insofar as I made a choice to transition it was a choice between being who I am, or pretending to be male, which was making me miserable.
It would be wrong to deny the truth based on what it implied for humanity. Left wing policy should address the scientific facts. Denial is not an option. However Eric Turkheimer accused Plomin of describing effects as genetic which could just as well have been environmental. This is a dispute over which I am unqualified to judge.
I considered the kindle sample of Plomin’s book Blueprint and this Guardian interview.