You Will Be Who You Are

Heritability for most traits increases rather than decreasing over time

J.S. Kasimir has recently made a foray into allegorical fiction, in which Death contemplates the human condition. The title of his story is Death and the Worm, and you can read it here. There’s a kind of rawness and accidental sincerity there typical of the work of younger writers, and one wonders how closely some of the interactions follow real events. In his vivid depiction of Death’s disappointment with humanity, Kasimir explores the idea that children psychologically start out where they start, and are slowly pushed to where they end up:

When an infant is born, its mind is at its purest state… Over several years, a child studies the world in an archaic manner, letting intuition and instinct guide it… However, knowledge is rather fragile in this state. A child needs guidance in order to know the Truth. Unfortunately, Worm, at this stage, people are most likely to lie to a child and train it on the art of hypocrisy and deceit. This muddles the goal for a child, whose sole duty is to understand the very cosmos it was born into. As it is raised in society, its mission is only further impeded. A child’s mind is drained of its wonder through dogma and sin. Society replaces purity with frivolity and ignorance. It enshrouds the soul with endless layers of ego until the child can no longer remember who they really are.

Though I could evaluate this piece primarily as a work of literature, I want instead to focus on its message, which I read as more important. The idea of the noble savage, of Adam and Eve living in innocent nakedness, of the truest self arising in childhood or even infancy, has a broad and intuitive appeal, but I don’t believe it anymore. I used to, but then I discovered psychology, psychology discovered the meta-analysis, and that as they say was that.

Here is what I mean:

A Review

So, speaking at the broadest level, most people everywhere are a lot the same. If extraterrestrials visited us from Alpha Centauri, they’d probably notice pretty quickly that humans generally have two eyes, two thumbs, and a habit of breathing air. But after they’d landed, had a few beers, and gotten over the outgroup homogeneity effect enough to consider the ways in which we are unique, they might notice people also have traits on which they differ, like hair color, or IQ, or opinions about whether caffeine infused underwear will destroy fat cells.

Now every person has a unique genetic makeup, which they share around 99% with everybody else. This may account for our broad agreement regarding the number of thumbs a person really ought to have, and personal preferences about breathing air.

Gen-Z Dating tip: Shortly after your date has begun, seek out areas of consensus in conversation to build rapport by broaching topics such as thumbs, or your preferred mode of respiration. More controversial subjects, such as whether water is wet, the number of genders in human society, or whether factory farmed meat is food, may end your date prematurely.

2024 is going to be a real fun year

But once you’ve really explored the number of thumbs people have, and the way we all breathe air, that 1% that differs starts to seem a lot more interesting—and it’s implicated in many human differences. Naturally we wonder about those differences, and scientists have spent a lot of time finding out how much of those differences in traits arise because of differences in our genes.

For example, consider the heritability of a trait like height. Some people are taller, and some are shorter, but why?

• In a society like the modern West, where the environment is standardized well enough that there's virtually no starvation, the heritability of height is very high, around 80%.

• In poor countries, where some people experience extremely different situations in terms of the availability of food, the heritability of height is closer to 50%.

• In a weird society of cloned hunter-gatherers, the heritability of height would be 0%. That's because any variation their heights would be caused by environmental differences—they all have the same genes, but some of them would just have better luck finding food and provisioning their (cloned) children than others.

In a world like that last one, where everyone is a clone, it makes perfect sense to think of differences in upbringing and experience adding up over time to produce unique individuals by the end of their lives. One person had her leg broken by a fall; another endured a winter without any food, and another was driven out to become a lonely hermit after an attack by a bear left him disfigured and unable to speak.

But we are not all clones.

So the World We Live in isn’t Like That at All

Instead, the world we live in is one where personality is moderately heritable—near 50%, depending on the trait in question.

Take conservatism as an example. One study found that the degree to which people agreed with things like "Bible Truth," "Conventional Clothes" and "Death Penalty" was 56% heritable.¹ This means 56% of the variation in scores on the study authors’ measure of conservatism was attributable to differences in people’s genetic makeup. This is just one study, but it's very typical of what we see in psychology.

So fine, this may not seem too big a deal; some of our unique personality comes from our genes, and some from our experiences and the way we are raised. Right?

Well, we can investigate what parts of the environment have an effect on the way people turn out. Specifically, you can compare the similarity between children reared together, versus those who are reared apart, to look at the effects of living in a specific home, under the same parents, and going to the same school. Researchers call this the “common” or “shared” environment.

Another way of investigating the shared environment is to compare monozygotic (identical) twins reared together versus dizygotic (fraternal) twins reared together. Remember everybody shares the 99% of genes common to humanity, but of those genes that do vary, dizygotic twins share about half, while monozygotic twins share nearly all of them.² So what happens when we look at human traits in this way?

The above table is adapted from a meta analysis of fifty years of twin studies investigating numerous physical and behavioral traits.³ Although there’s a lot more there, I’m just going to focus on personality for now. The correlation for monozygotic twin pairs’ personalities comes to 0.470, compared to 0.234 for dizygotic twins. Mathematically, this implies that the correlation for unrelated persons who shared the same home environment—even the same womb—would be -0.002. That’s the effect of upbringing on personality: zero.

This observed pattern of twin correlations is consistent with a simple and parsimonious underlying model of the absence of environmental effects shared by twin pairs and the presence of genetic effects that are entirely due to additive genetic variation... This remarkable fitting of the data with a simple mode of family resemblance is inconsistent with the hypothesis that a substantial part of variation in human traits is due to shared environmental variation or to substantial non-additive genetic variation.⁴

The study authors’ conclusion is not only based on personality, but is roughly applicable to the entire spectrum of traits under examination. The environment does matter, in terms of random events that affect us uniquely. But for most traits, and most people, how we are raised doesn’t make much difference.

Rising Heritability in the Long Run

This may not square well with intuitions and observations about children. We remember lessons our parents and teachers taught us; we remember things we learned from interacting with our friends on the playground. And anyone who’s had kids, or taught in a classroom, will know that children do respond to the environment; you can train kids to do all kinds of things.

What’s interesting is what happens after those children leave the neighborhood they were born in, leave the schools they grew up in, leave the peer groups that were foisted on them by circumstance, and start choosing environments of their own. As they age, they increasingly choose their own friends, they choose their own classes, and ultimately they choose their own homes in their own neighborhoods.

I realize “choice” may not be the best term, here. Someone with uncompetitive SAT scores isn’t going to end up at Harvard University even if that’s their first choice. What I mean to say is that wherever they end up, whatever environment they land in, is going to be influenced by their genes. What about the influence of the environment they were raised in? Won’t that significantly impact the jobs they can get, the schools they qualify for, and the social environments where they end up later on?

Well, probably not:

What you’re looking at right now is IQ—a chart from a meta-analysis on the way variation in measured intelligence shifts with age, reflecting an increase in the impact of genes, and a decrease in the impact of the way we are raised.⁵

And this is pretty important if, as Death implies in Kasimir’s story, the primary purpose of mankind is to understand the universe:

Discovering the Universe is imperative, my dear, for that is the very reason man was placed upon the Earth. Man was given a mind so sharp and calculating. What else is one to do with it? You are creatures of creation, able to understand the very stars themselves. When you acquire knowledge, you are the universe looking back in on itself. The more mankind understands about itself and the ether, the more the cosmos grows. Every creation, thought, and epiphany fills the Psyche of Humanity, allowing it to overflow with greatness for time eternal. When one man takes it upon himself to discover the Universe, he is not only tapping into this wellspring of glory and chaos but adds to it when he applies the knowledge he has acquired for the benefit of those living. In this way, he has achieved a great honor in the eyes of God.

I’m not going to directly take issue with the sentiment that what makes human existence worthwhile is our shared exploration of the beauty and complexity of creation. These kinds of opinions are common to intellectuals; Socrates insisted that the unexamined life wasn’t even worth living. I will point out, though, that if we’re going to take this kind of stance, then this tends to invalidate the lives of a substantial proportion of the population, and well, maybe we don’t really want to be doing that?

What I am directly taking issue with is the idea that everybody starts out like a scientist or a philosopher and is warped by their environment into a drifting, incurious drone. The finding that the shared environment’s effect on IQ drops with age is a direct disconfirmation of this idea.

I’ve given a graphic from a recent study, but this finding has actually been known for decades; IQ’s rising heritability with age was the first surprise I experienced myself as I started looking into the subject of nature and nurture. As a child, I’d been explicitly told by my parents that early IQ tests were the most accurate assessments of a person’s ability to learn. If the point of IQ was to determine your ability to learn (specifically, in an educational environment most of us know and dread as school), it seemed pretty obvious that a dimwit could just be taught to answer questions and show an elevated score.

But the point of IQ isn’t (or, well, isn’t merely) to measure learning ability, let alone what you have learned. And a person’s practice with the kind of abilities tested by IQ is going to differ according to how much they feel like paying attention and engaging with cognitive tasks they experience in life. Dimwits are seriously going to get tired of concentrating hard on difficult stuff all the time, don’t you think? They aren’t going to be picking up new vocabulary from challenging foreign films or familiarizing themselves with logical principles as they explore philosophical ideas with their more educated friends in their spare time. They aren’t going to be honing their reason to a fine point in preparation for a penetrative exploration of the secrets of the universe. They’ll be watching clips of people squishing playdough through their fingers, and body filtered singers lip synching to pop music.

It’s The Same For Other Traits

OK, so the importance of upbringing diminishes with age, and the importance of genes increases with age when we look at IQ. But Kasimir’s post has Death concerned with more than mere intelligence—Death is scornful of those who let themselves drift without asking questions, with those people whose opinions were simply programmed:

The worst of all were those who were neither Builders nor Destroyers. These humans simply stumbled through life without purpose, following the orders of Builders and Destroyers with no questions asked. Their minds were near-empty voids, able to be swayed by almost anyone… Death’s presence hardly stirred any real motivation in them. He called them Worms. And they were the most pitiful humans of all.

Intelligence is obviously related to whether or not a person qualifies as one of Death’s Worms. But what about personality, and especially, what about values? Do our personalities and opinions show the same trajectory as intelligence?

I know of only one meta-analysis to have investigated the way heritability changes over the lifespan for traits other than IQ. But it’s one of my recent favorites.⁶ That study examined eight traits:

• Alcohol consumption (how much, and how often),

• Nicotine (i.e. smoking),

• ADHD,

• Externalizing behavior (aggression, conduct disorder, oppositional defiant disorder, and similar behavioral problems),

• Social attitudes (conservatism and religiousness),

• IQ,

• Anxiety, and

• Symptoms of Depression.

Here are the results:

When I look at this now, it’s the rising heritability of social attitudes that I find most striking. Political and religious values are obviously socially influenced; I think most people nowadays are aware of the way people can be drawn into online echo chambers and end up with an unrecognizable suite of opinions about election fraud, vaccines, or caffeine infused underwear. The social environment obviously exerts an influence.

But I have no way to account for the rising heritability of social attitudes except by saying that people must be sorting themselves into those influential environments and social groups that are most congenial to them.

A Natural Experiment: Church vs The Human Spirit

I’ve spent a lot of time discussing the scientific findings. The results may not be totally consistent for every trait imaginable—if you check the charts, we don’t have evidence for rising heritability in ADHD; heritability just starts high and stays there for ADHD. But the overall message is very clear that in most ways, we will be who we are.

But now that you’ve seen the science, you might be interested in how my own childhood was a natural experiment in social conditioning.

My parents lived on opposite sides of the political map, and they divorced when I was in preschool. Early on in my upbringing, my liberal parent backed off, giving the reigns to my conservative parent and leaving them in charge of my ideological education. Every week, I attended Sunday school, then the main service, and youth group on top of that—three doses of religion every week, representing more hours in church (and studying the Bible and praying at home) than I care to count.

But over time, as childhood gave way to adolescence, I found that I got along less and less well with the typical person I met at church. Even though I was a staunch believer, I always gravitated to the company of the long-haired and less dogmatic members. I didn’t have the language to describe it, but I noticed that the descriptions of Jesus Christ in the Bible were considerably more “rebellious” and “radical” (i.e. politically left) than the comfortable, conforming churchgoers who filled the pews.

As I grew up, my conservative parent always hinted that it was the malign influence of my liberal parent that was making me less conservative, but looking back, it’s pretty obvious that—while this influence was real—it was actually something that had happened years ago, at the point of conception. Just a few scant conversations with unbelievers and Christians of other denominations gave me clues about areas of the Bible to investigate. Curiosity about religious history and access to a local bookstore revealed the breadth of beliefs within Christianity, and the overlap with other religions and philosophies. And ultimately access to the Internet gave instantaneous disconfirmation of everything I had absorbed in the pews and holding hands with believers in prayer.

Despite its weaknesses and detractors, Christianity still remains compelling today for millions of Americans. They go to church, and the experience defines them. But for me, it didn’t matter that I spent so much time being told what to believe. And it didn’t matter that I’d been offered only the most scanty and suspicious threads to tug on in the other direction. I would have had to’ve been raised in complete ignorance, deeper even than that experienced by the typical Medieval peasant, for my genes not to have had an opportunity to express themselves as they have.

Whether the spiral leads inward, towards the perfection and unity of the divine, or outward into the universe, depends on who you are

And now that I’ve lived through this, it’s clear to me that what seems to most people like the powerful effects of social conditioning is really a combination of social conditioning and individual predisposition. You can play Bach to baby squirrels all day long, and it isn’t going to convince them to gather around while you play Tocatta and Fugue in D Minor out the window after they’re grown. You can raise your dog vegan from birth, but adopt her away, and she won’t be eating vegan for long. And you can tell your child over and over again how they need to love God and believe in the Bible, but if that child is the sort to ask, “Well, why? How do we know that? And why doesn’t everybody else believe?” then forget it.

The only way you can truly condition people to believe things is if they already resonate with those beliefs to begin with. They may look like Worms, those who swallow what they’re told without questioning. But as the seasons turn, the years drift by, and childhood fades away, the only reason they continue to believe the things they were told as children, is because they resonated with those beliefs from the very beginning.

You will be who you are.

If you liked this article, you might also try:

1. Bouchard Jr, T. J., Segal, N. L., Tellegen, A., McGue, M., Keyes, M., & Krueger, R. (2003). Evidence for the construct validity and heritability of the Wilson–Patterson conservatism scale: A reared-apart twins study of social attitudes. Personality and Individual Differences, 34(6), 959-969.

2. Some details and weird effects create minor exceptions, here—dizygotic twins are statistically likely to share half of their genes, but they could randomly turn out more alike or less alike. Or, if the parents are already similar or related, their dizygotic children could be slightly more alike than if they had mated randomly, like drunken strangers sharing a quickie in a Las Vegas hotel. And even monozygotic twins don’t usually have perfectly matching genomes due to mutations that occur in utero. In fact, these mutations entail that even you don’t technically have a single genome.

3. Polderman, T. J., Benyamin, B., De Leeuw, C. A., Sullivan, P. F., Van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature genetics, 47(7), 702-709.

4. Ibid.

5. Briley, D. A., & Tucker-Drob, E. M. (2013). Explaining the increasing heritability of cognitive ability across development: A meta-analysis of longitudinal twin and adoption studies. Psychological science, 24(9), 1704-1713.

6. Bergen, S. E., Gardner, C. O., & Kendler, K. S. (2007). Age-related changes in heritability of behavioral phenotypes over adolescence and young adulthood: a meta-analysis. Twin Research and Human Genetics, 10(3), 423-433.