Summary
Blueprint: How DNA Makes Us Who We Are follows the story of the growing scientific evidence for the heritability of psychological traits. Dr. Plomin makes the strong claim that DNA is the most important factor in making us who we are. The book is broken up into two sections. The first section is devoted to defending that strong claim, and the second section in discussing the impact of the DNA revolution on health and society.
He begins by presenting research on twins and adoption studies. Since identical twins share more genes than fraternal twins, but both sets of twins have very similar shared environments, the difference between the similarity of identical twins vs. fraternal twins gives researchers some sense of how much a trait depends on genes. Similarly, in adoption studies, it is possible to separate some of the genetic and environmental factors in predicting traits because shared genes are no longer coupled to shared environment. Dr. Plomin surprises us all with the heritability of all sorts of psychological traits, which tend to hover around explaining 40-50% of the variance in a trait. Critical to his argument, is that these massive effects of heritability are far higher than any individual environmental factors.
Through interweaving methodological details and explanations of heritability, Dr. Plomin identifies some major paradigm shifting findings from heritability research in psychology, which comprise the themes of the chapters in the first part of the book.
First, by studying environmental measures in these kinds of genetically sensitive studies, we learn that many environmental effects in correlations between 'environmental' measures and psychological traits, are actually genetic effects, which reflects a principle that Plomin calls the nature of nurture. So, for example, socioeconomic status is typically considered an environmental variable when predicting something like career achievement, but socioeconomic status itself has significant heritability.
The second finding was about how heritability changes over the course of the lifespan. You might think that heritability for a trait would be highest in childhood, when differences in environmental exposures haven't set people on different paths. But actually, the opposite is true, especially for intelligence. Many traits become more heritable later in the lifespan.
The third finding is related to the concept of polygenicity, that many genes are involved in most individual traits. This has the effect of normally distributing the phenotype in a population. So, there aren't really binary mental conditions as implied by psychiatric diagnoses.
The fourth finding is related to the concept of pleiotropy, each gene impacts multiple, sometimes very unrelated, phenotypes. This research suggests that there are generalist genes, which seem particularly implicated in mental illness.
Finally, studying environmental effects while controlling for genetic effects showed that shared environment plays a very small role in predicting psychological traits, and environmental effects tend to be unstable and random over the lifespan.
An important thing to remember about heritability is that it is variable and dependent on a particular population, and he made the interesting point that if you observe higher heritability for a trait, which is reflective of a society with greater equality of opportunity and meritocracy.
The second part of the book focuses on genome-wide association studies, rather than twin and adoption studies, and what Dr. Plomin calls the DNA revolution, because with the ability to know inherited DNA differences in individuals at birth, it will become feasible to predict many of those heritable traits and outcomes. He starts off by giving us a primer on DNA and continues to discuss GWAS research. Probably the biggest lesson from genome-wide association studies is that each DNA difference predicts only a very, very tiny portion of the variance in a trait. Therefore, you need massive sample sizes to uncover a DNA difference that will reliably help predict a trait. You can combine these relevant differences together in a score called the polygenic score. Plomin makes some predictions about how these scores will and will not be used but thinks that they will be used in mostly positive ways to predict health risks and aptitude for certain skills.
Thought Questions
- Heritability can be a controversial issue. On one hand, thinking about things as very heritable can make you more compassionate toward people. Maybe a parent can be more accepting of a child who doesn't perform well in school or maybe a society can be more compassionate towards people who make a lot of poor life decisions. But on the other hand, the pro-hereditarian people don't have a great track record with perpetuating some pretty nasty things like eugenics movements, racism, and caste systems. I'm curious where you fall in how you feel about how this research was presented? Does it give you more of a hopeful compassionate feeling or an icky, elitist feeling?
- How does the book challenge or reinforce your understanding of the role of genetics in shaping who we are? Despite DNA accounting for around 50% of the variation of differences between psychological traits in the population, that still leaves 50% of the variation due to other factors. Dr. Plomin suggests that these other environmental factors are random and unsystematic. Do you agree with his interpretation?
- Let's talk about racism. This book is not the first book that painted a very pro-hereditarian interpretation of all sorts of phenotypes, including psychological traits. But many of those previous books were more explicitly focused on race. How do you feel about Dr. Plomin not addressing what I felt was the elephant in the room? If DNA differences due to race affect a trait, it seems like there is a huge confound in that those DNA differences may be related to systemic racism in society. For example, if someone showed me a polygenic score that was highly predictive of salary, but then the genes that were involved had to do with how much melanin someone produced in their skin, it seems silly to interpret that as DNA causing the low salary (in the same way as say DNA involved in serotonin regulation might cause one’s risk of depression). I would interpret it as indicative of racism.
If there are not major detectable DNA differences due to race, i.e. polygenic scores for traits like career success, salary, intelligence, etc. are not different across racial groups, I really think that Dr. Plomin should have made that clear given the history. In either case, he should have addressed it - either the risk of bias or the fact that there is no difference.
- How do we reconcile that heritability changes based on manipulating the environment with his gloomy outlook on environmental interventions?
- Where should society draw a line in the use of polygenic scores? What are the risks to inequality?
- (Stealing from Graeme here) In a situation where we have limited resources, should we distribute those resources to the most gifted or those who need the most help. For a real world example, should the very best high school teachers be assigned to teach the most gifted, curious students who love learning, or the kids who are struggling the most to learn and hate school, or should every teacher get a random selection and divide their time equally to all their students.