Summary
For this month, we read, Innate: How the Wiring of Our Brains Shapes Who We Are, which was written by Kevin Mitchell, who is an associate professor of genetics and neuroscience at Trinity College Dublin. This book continues to speak to the Nature vs. Nurture debate, but focuses on the unique perspective that who we are - our personality, intelligence, perception, tendencies, and mental health - is not shaped by our genes in a simple way, but rather by how those genes guide the development of our brain. So the core thesis is that more of our nature is "innate" than we think, but that randomness in how development occurs plays a surprisingly large role.
One of the most counterintuitive and powerful points that reflects this are studies done on identical twins with identical DNA and nearly identical environments and yet are meaningfully different in core traits. One of the striking examples to me was that of homosexuality. If you cloned someone who was gay 100 times, only about 50 of those clones would be gay. I think that a common way to think about this fact is that there must be important life experiences that causally determine this outcome - maybe one twin spent a lot more time looking at Superman's rippling abs in comic books as a kid or something - but Mitchell argues that this is much more likely to be caused by developmental randomness when the brain is wiring, that actually has very little to do with learning or life experiences.
Another important theme focused on misconceptions about heritability, which was also a major focus of the other book we read on this topic, Blueprint, by Robert Plomin. One is that what we call "nurture" is often shaped by nature - people gravitate towards environments that match their innate tendencies, and so the heritability of traits tends to increase with age, rather than decrease, which is many people's intuition. Also, heritability of traits tends to increase in populations that have more equality in opportunity. So, for example, the heritability of intelligence is higher in countries where everyone has similar access to education. In this book, the most interesting way that this manifested to me, was when Mitchell was discussing innate sex differences. Mitchell discusses the paradoxical finding that sex differences in psychological traits, including personality, are often larger in more gender-equal and resource-rich societies, which seems counterintuitive - many people assume that as cultures become more egalitarian and supportive, gender differences should diminish, but instead the opposite pattern emerges. Mitchell reasons that in more egalitarian societies, people have more freedom to choose their own careers, hobbies, lifestyles and emotional expressions that align with their intrinsic dispositions. These dispositions, which are shaped by innate wiring, are then amplified.
Overall for me, this book really changed the way that I looked at the nature vs. nurture question because it revealed a sort of "third variable" of developmental noise, which makes a lot of sense, given that so many of the genes related to complex traits are mainly involved in development, with each gene only explaining a very small percent of variability in a trait. In Blueprint, I remember the "missing variance" being presented sort of like an unexplained mystery. Like traits would be 50% heritable, but then the best polygenic score estimates would only explain 15% of the variability in a trait and scientists also couldn’t find any environmental factors, that when combined with each other, explained anything close to 50% of the variance. I was very satisfied with the idea that developmental randomness could account for a significant amount of that missing variance.
Another fascinating theme for me in the book was the idea of developmental robustness and its relationship to mutational load. On average, we each carry about 200 severe mutations that negatively impair the function of some gene. But since we evolved in the context of this problem of mutations, we also seem to have a high level of development robustness. Where there are multiple processes that can help compensate for the things that happen to not be working optimally, especially in guiding the initial development of our brains. Mitchell actually suggests that part of the reason that we might have a relatively high degree of mental illness is because our brains changed so rapidly so recently and might not adapted enough robustness yet. This way of conceptualizing genetic origins of mental illness makes a lot of predictions that seem to be true - many genes are involved in mental illness, the genes involved in mental illness significantly overlap between different mental illnesses even though the phenotypes are different, and also mental illness doesn't seem to be ultra-heritable - like if you have schizophrenia, there is only ~50% that your clone would also have schizophrenia.
The final chapters do the obligatory wrestling with the implications. If people truly differ in innate capacities and tendencies, what does that mean for education, equality, free will, and moral responsibility? Mitchell walks a fine line: recognizing biological realities without sliding into determinism or inequality. He strongly argues that acknowledging individual differences should promote compassion and support, not discrimination. A society that assumes everyone starts from the same baseline sets many people up for failure.
Discussion Questions:
- Were there any ideas that particularly stuck with you or ideas that you felt were incorrect or over-stated?
- Did you think of the idea of developmental stochasticity more as part of nature or nurture or something else altogether? How does this idea influence how we interpret twin studies or design experiments about heritability?
- Kind of a related question, but how do you think we can experimentally quantify the effects of developmental noise in human brain wiring?
- How should we interpret heritability in general? It seems like we should think of it as more probabilistic?
- What do you think the role and responsibilities of scientists are in doing this kind of work. I think that findings about biological differences often can be interpreted in ways that reinforce stigma and inequality. How should we address that?
- I was thinking about Mendelian randomization studies in light of some of these ideas. The basic idea of Mendelian randomization is that genetic variants are randomly assigned at conception, similar to how a treatment is randomly assigned in a randomized controlled trial. These variants are known to influence modifiable traits (such as drinking alcohol). So if a genetic variant is also associated with an increased risk of disease, it supports a causal link between the trait and the disease (e.g. drinking alcohol causes heart disease). But this seems very much in line with the "genes for X" model, instead of the "genes that play a role in altering brain development in conjunction with a bunch of other genes" model.
- If we were able to find and change all of our "disadvantageous" mutations ahead of time, should that be allowed? His model of intelligence was that intelligence seemed to be more related to a lack of an accumulation of deleterious mutations rather than special genes that conferred intelligence.