This is an excerpt from what will eventually be my (next) book after Dr. Lisle and I finish ours.
The neo-Freudian argument that individuals are essentially “blank slates,” and that we learn who we are from our parents and childhood environments, is flat-out archaic and incorrect. That is not my opinion. That is the well-evidenced conclusion of six decades of longitudinal identical twin studies — and not a handful of twin pairs, or even a few thousand of them, but fifteen million pairs of twins.
Monozygotic (identical) twin studies are by far the most powerful evidence we have against the “blank slate” theory of human nature. Because identical twins share exactly the same genetic profile, any differences between them must be environmental — ie, the result of “nurture,” not “nature.” By investigating the personalities and life trajectories of identical twins who were separated at birth and raised in completely different home environments, we can very accurately tease out the true influence of parenting, education, family structure, and any number of other environmental factors that we typically think of as responsible for shaping our personalities and eventual success as adults. The “nurture” hypothesis — that we are blank slates — holds that if the twins experience very different childhood environments, each individual will turn out very differently to reflect the specifics of their environment. They absolutely do not. Instead, they turn out to have remarkably similar measurements of just about every trait we can figure out how to measure. Brain structure and IQ. Personality characteristics. Body size. School achievement. Propensity for drug use and addiction. PTSD and even exposure to trauma itself. Overall happiness and well-being. And approximately 17,000 other traits.
On average, the heritability of any given trait is about 50%, and the other 50% comes from what behavioral genetics researchers call the “non-shared environment.” But the non-shared environment is not what you think it is. People tend to hear: “it’s 50/50 nature/nurture, so that means my childhood plays a big role in how I turned out!” That’s not how it works. Things like whether your parents gave you Baby Einstein and listened to Mozart while you were in the womb, or if they spanked you, or got divorced, or your dad had an affair, or your mom was an alcoholic, or whether you listened to Rush Limbaugh or NPR matter almost not at all. Those things constitute the “shared environment” — the environmental features that are shared by all children in the family, and most estimates describe the shared environment influence as “very close to zero.”
Non-shared environment — the other 50% of “environmental” influence — describes, rather, everything that is unique to each child in a family, which most prominently includes how the child perceives their circumstances and how other people including the parents respond to the personality of the child. Sweet children receive more physical affection than more disagreeable children in the same family. Difficult children are more likely to struggle in school, even controlling for IQ. Siblings have completely different subjective experiences of family stress and parental divorce. The aspects of our personalities that are genetic essentially create the environment that, in turn, feeds back into our personality, which continues to structure our environment. Robert Plomin calls this phenomenon the “Nature of Nurture.” In essence, over time, we become who we are. And we become who we are not because of what happens to us, but in spite of what happens to us. In other words, virtually every trait increases in heritability over time, as lingering early life influence diminishes in relevance.
From these twin studies, we know some fundamental truths about human nature, and they are not always easy truths to swallow. Many are very much at odds with conventional “common sense” assumptions. To take one common and important example: it is certainly true that childhood adversity is indeed very strongly associated with poor life outcomes: obesity, drug use, and lower scholastic achievement, for example. But the fact that childhood adversity is associated with a given poor outcome does not mean that adversity caused the poor outcome. As we now know from the twin studies, it’s very clear that the correlation is not a causal one, but that a hidden variable is actually causing both childhood adversity and struggles later in life: namely, genetic factors.
Here is a simple example to show how this might work. Often, people who develop addiction as adults are quick to blame their upbringing for creating feelings of dislocation and disconnection that encouraged them to self-medicate with drugs or alcohol. This “psycho-spiritual disease model” of addiction is a very intuitive, humanizing, and compassionate approach to addiction, and it is widely believed to be true by leaders in mainstream pop psychology, medicine, and “spiritual” self help, including of course twelve-step programs.
Alcoholism, however, is extremely heritable — probably about 60% genes, with the remainder of the variance explained by the non-shared environment. How might the non-shared environment of someone genetically predisposed to alcoholism make developing it more likely? This is where the “nature of nurture” comes in.
Consider two sisters. Their “shared” environment is the same, and it includes various factors that qualify as “adverse childhood events,” or ACEs: their father is an alcoholic (1), he rages when drinking (2), he can’t hold down a job so the children can’t go to the dentist or get new clothes when they need to (3), he is too occupied with his drinking and his legal troubles (4) to spend meaningful time with the kids (5), he is occasionally physically abusive toward their mother (6), and eventually their parents divorce (7).
An ACE score of 7 is in the top tenth percentile of the distribution across the population — only 1 out of 10 kids have it that bad (or worse.) We would expect these sisters to be much more likely to become alcoholics themselves because of their traumatic childhood, and we would be correct — but for the wrong reasons.
The sisters have slightly different personalities. They are both quite agreeable, introverted, and emotionally unstable, but Zoe has much higher conscientiousness than Chloe, and also slightly lower openness. Chloe is more likely to become an alcoholic — not because of her personality characteristics per se, but because of the experiences, circumstances, and relationships that her personality evokes and structures compared to her sister’s — the “environment” that she does not share with Zoe.
Both girls have a healthy fear of alcohol’s effects, having watched their father’s tragic decline and deterioration. With higher openness and lower conscientiousness, however, Chloe is more likely to be exposed to and experiment with it anyway, when she is given the opportunity. Zoe’s high conscientiousness plus her high neuroticism provoke a greater degree of anxiety around alcohol — she doesn’t want to “lose control,” so she never touches it.
Chloe also sees a college counselor to help work through her feelings about her parents’ divorce, and the counselor tells her that addiction is a disease of “disconnection” and isn’t genetic, which emboldens her even more. Soon, Chloe is binge drinking with her new boyfriend on weekends, initiating a cascade of biochemical changes in the brain that leads to increased depression (which she is also predisposed to) and the diminishment of executive function. She loses interest in her courses, which she was struggling in anyway because her lower conscientiousness wasn’t as concerned about grades. She drops out of college. She starts drinking daily with her (already significantly alcohol dependent) boyfriend, and slowly develops alcohol dependence herself.
Watching Chloe suffer so much in turn makes Zoe less likely to ever even try alcohol, and she never does. Zoe and her mother both attempt to make sense of Chloe’s tailspin and, understandably, blame the difficult circumstances of the siblings’ childhood. They agree, with the enthusiastic endorsement of their psychodynamic therapist, that because Chloe is a year younger, among other things she “didn’t have her dad around as much” and “took the divorce harder.”
This example, while simplified and stylized, is not entirely hypothetical. It’s drawn from my own experience, as the daughter of an alcoholic (though not abusive) father. My ACE score isn’t as high as Zoe and Chloe’s, but it’s relatively high — depending how you count it, somewhere in the top 30–40th percentile. I spent decades trying to explain my own alcohol dependence (and all of my other problems, for that matter) through the lens of childhood adversity, with strong encouragement from every therapist I ever met. But it was never my adversity to blame. It was my genes, and idiosyncratic, unpredictable chance events like alcoholic boyfriends and weekend binge drinking that were in turn made more likely by my (also genetic) personality traits, like my very high openness and agreeableness.
When you see through the lens of behavioral genetics, you realize how everything hangs together and how interdependent genes and environment actually are. If you are troubled by serious issues in your adult life — addiction, obesity, low achievement — it’s very likely that you had a lousier than typical childhood. But some of the same characteristics that are actually partly responsible for those outcomes — ie, low conscientiousness, high emotional instability, high disagreeableness, high openness — were inherited from your parents, who raised you in the fog of their very similar personality traits, which generated ACE-adjacent circumstances: substance abuse, divorce, neglect, legal troubles, and maltreatment. If you have PTSD, you are more likely to have been exposed to trauma in the first place.
We are fundamentally confusing causal pathways when we blame childhood adversity then for the state of anyone’s life, health, or happiness now. The real culprit is our genes, whether we are talking about obesity, substance abuse, trouble in relationships, dropping out of college, or even your lifetime income. It’s so intuitive to draw a clear line from our early childhood experiences to our later-in-life troubles (or triumphs), but we are barking up the wrong tree by doing so. The circumstances of our early lives do not make us who we are.
In other words, your past—good, bad, ugly, or in between — is not responsible for who you have become. Quite the contrary. We eventually become who we really are despite our greatest advantages and worst insults.