A father, a quiet room, a toddler down for a nap. He's lying there keeping an eye on the kid, and he turns over, and there it is. Both of them, father and son, sleeping with one arm raised in exactly the same position. Fifteen-month-old Ezra, one arm up. Dad, one arm up. Same angle, same everything.
Daniel didn't even know he did it. His wife is the one who pointed it out. And once you see it, you can't unsee it. So he wrote in with the question. Can mannerisms and quirks like this actually be inherited? Not just the big stuff, not eye color, but the weird personal stuff, the way you hold your arm when you sleep. He points out we're always hearing that a kid has his father's facial expressions, or some other distinctive little thing. And he's honest that nurture and imitation are just as compelling an explanation as genes. So which is it? Can mannerisms be inherited, or is this just a comforting pseudoscience myth that we tell ourselves about our families?
It's a great question because it sits right on the fault line. Most people assume there's an answer and the answer is genes. And the actual science is much stranger than that.
So today we're going to untangle the genes, the imitation, and the stories we tell ourselves about family resemblance. And I'll warn you now, the honest answer is not clean.
It never is. But here's the thing that makes this particular question interesting. Sleep is the one domain where the genetic case is strong. If Daniel had asked about, say, a shared sense of humor, or the way they both gesture when they talk, I'd be much more skeptical. Sleep architecture is a different beast.
So the instinct might be more right than he thinks, just not for the reason he thinks.
So there are three buckets here. Genes, which are real and measurable. Nurture and imitation, which are also real and measurable and probably bigger than people expect for mannerisms specifically. And then the third one, which nobody talks about, which is that humans are pattern-matching machines and a father watching his sleeping son is about as primed to see resemblance as a person can be.
That third one is the one that makes me uncomfortable, because it means the observation itself is suspect.
It doesn't mean it's wrong. It means it's not neutral. And that's true of basically every observation we make about the people we love.
Alright. Start with the genes. What do we actually know?
So the big recent one is a study out of Edinburgh and Tartu, led by René Mottus. They pulled over a thousand pairs of relatives from the Estonian Biobank, more than twenty-five hundred participants, and they looked at personality traits and life satisfaction. And the headline number is that heritability of personality traits comes out around forty percent when you use a second informant rating. Usually a partner. Versus about twenty-five percent in the typical self-report study.
That gap between forty and twenty-five is interesting on its own. Why does the number move?
Because self-report is noisy. If you ask me to rate my own personality, I'm bringing in how I want to see myself, how I think I'm supposed to be, the mood I'm in that day. If you ask my brother, or my partner, you get a cleaner signal of what I'm actually like to be around. So the informant rating is probably closer to the truth, and the truth is higher than we used to think.
Forty percent. So genes matter.
Genes matter a lot. But here's the part that stops people cold. The same study found that people are only slightly more likely to share personality traits with their parents than they are with a random stranger.
Say that again.
Only slightly more similar to their own parents than to a stranger off the street. And they found no evidence that shared family experience, including being adopted into a family, makes personalities more similar.
So the whole thing we do, where we look at a kid and say, oh, he's got his father's stubbornness, he's got his mother's temper. That's...
That's mostly a story we're telling. Mottus put it well. He said the genes a parent passes on are not sufficient to make most of their personality traits similar. And then the line that really lands. About two-thirds of the reasons people differ in personality have something to do with their genes, but that is not enough to make parents and children much more similar than strangers.
Two-thirds of the variance is genetic, and it still doesn't make you resemble your parents.
That's the counterintuitive part. It's because the genes that make you different from other people aren't the same genes that make you similar to your parents. You inherit half of each parent's genes, but the combination is new, and the traits that pop out of that combination are new too. You can be highly heritable as a trait and still not resemble your family, because heritability is a population statistic. It describes how much of the variation between people is explained by genes. It says nothing about whether you specifically look like your dad.
So the warm feeling of, he's just like me, is running on a statistic that doesn't actually support it.
For personality. For mannerisms. But sleep is where the story flips.
Go on.
There's a twin study from twenty-thirteen, published in the journal Sleep. Sletten and colleagues, out of Monash and QIMR. A hundred and thirty-two adolescent twins, twenty-five identical pairs and forty-one fraternal pairs, and they wore wrist actigraphy for two weeks at home. So this isn't a lab, this is real life.
What did they find?
Additive genetic factors accounted for sixty-five percent of the variance in total sleep time. Eighty-three percent in sleep onset latency, which is how long it takes you to fall asleep. Fifty-seven percent in wake after sleep onset. Fifty-two percent in sleep efficiency.
Those are big numbers. Eighty-three percent for how long it takes you to fall asleep?
That's enormous. But here's the twist. Sleep timing, so what time you go to bed and what time you wake up, was dominated by shared environment. Sixty-seven percent and eighty-six percent.
So the clock is the parents' rules and the family schedule.
What time you go to bed is your household. How you sleep once you're in there is substantially your biology.
That's a clean split.
It's cleaner than most of this literature gets. And it extends further. There's a twenty-seventeen study in Scientific Reports, Lopez-Minguez and colleagues, Murcia Twin Register, fifty-three female twin pairs. Heritability of taking a siesta was sixty-five percent. Daytime sleep duration, sixty-one percent. The other sleep parameters ranged from thirty-six to sixty-nine percent.
So napping is heritable.
Napping is heritable. I want that on the record, because my brother has been claiming his afternoon naps are ancestral for years and I've been treating it as a bit.
It's not a bit. It's sixty-five percent.
It's sixty-five percent in a Spanish female twin cohort, so let's not over-generalize to sloths. But yes. The point stands.
What about the actual posture? The arm. Is there anything on that?
This is the closest we get, and it's indirect. There's a twenty-eighteen twin study in the Journal of Oral Rehabilitation. Kang and colleagues, seventy-two Korean twins. They looked at hyoid position and the inclination of the cervical column. That's head and neck posture during sleep.
And?
High heritability. They describe head and neck posture during sleep as being under strong genetic controls.
So the way your neck sits while you're unconscious is partly written in.
Partly written in. Which is the closest thing to direct evidence we have for Daniel's question. There's no study that looks at whether sleeping with one arm raised is heritable. Nobody has done that. But head and neck posture during sleep being under strong genetic control is at least in the neighborhood.
And there's more, isn't there. The brain activity itself.
There is. A twenty-twenty study in Cerebral Cortex, Markovic and colleagues, looking at adolescent twins. Sleep spindle activity had about a forty-eight percent genetic contribution. But overall sleep EEG coherence, so how synchronized the brain's electrical activity is across regions, was mostly driven by unique environment. Sixty-six percent.
So even within sleep, it splits. Some of it's genes, some of it's you specifically.
Some of it's you. Not your family, not your genes. Your own particular life.
So the ledger on the gene side. Personality, barely resembles your parents. Sleep architecture, highly heritable. Sleep timing, environmental. Posture during sleep, some genetic control. And the specific thing Daniel noticed, one arm up, no study at all.
That's the honest state of it. And I want to flag one more thing, because it matters for how much weight we put on any of this. The Edinburgh study is a preprint. Not yet peer-reviewed. Treat those numbers as provisional.
Noted. But genes are only half of this.
Less than half, for mannerisms.
So what's the other half?
Imitation. And this is where it gets mind-bending.
Go.
So the old story was that imitation is inborn. Babies come out wired to mimic. You've seen the demo, the newborn sticks out its tongue and the parent sticks out theirs and everyone goes, look, he's copying me. That demo has been the poster child for innate imitation for decades.
And it's wrong?
It's contested. There was a twenty-sixteen study in Current Biology, Oostenbroek and colleagues, a comprehensive longitudinal study that challenged the existence of neonatal imitation in humans. And then Redshaw in twenty-nineteen found no evidence for neonatal imitation in rhesus macaques either. So the classic demo is now under serious question.
If babies aren't born imitating, where does it come from?
They learn it. And the study that shows this is beautiful. Twenty twenty-three, Current Biology, Essler and colleagues at LMU Munich. A hundred and twenty-seven mother-infant dyads, followed longitudinally. And what they found is that maternal imitation at fourteen months positively predicted the infants' imitation abilities at eighteen months. And it was mediated by maternal sensitivity at six months.
Slow down. What does that mean in plain language?
It means the more a mother imitates her baby, the better that baby gets at imitating others, months later. The baby learns to imitate by being imitated.
So imitation isn't a thing the baby has. It's a thing the baby is taught, by being on the receiving end of it.
That's the finding. Infants learn to imitate by being imitated by their caregivers. It's a loop. And it means the direction of transmission isn't what we assume.
The direction. Say more about that, because I think I see where you're going and it's strange.
Here's the strange part. Daniel notices Ezra sleeping with one arm up, same as him. The natural assumption is dad does it, kid copied dad. But if imitation is learned through being imitated, then it's entirely possible that Ezra held his arm that way first, and Daniel, unconsciously, mirrored it back. And now they both do it.
So the mannerism flowed child to parent.
The mannerism may have flowed child to parent. And Daniel has no idea, because automatic mimicry is exactly that. Automatic. You don't notice yourself doing it.
That's unsettling. The thing he's pointing at as evidence of inheritance might be evidence of him copying his baby.
It might be. It might also be coincidence. It might be genes. That's the whole problem with this question. You can't pull the threads apart once they're woven.
And automatic mimicry isn't just posture. It's everything.
Everything. Most people naturally mimic the way the people close to them speak and move, without realizing it. Accent drift is the obvious one. You move to a new place, and after a few years you're saying things the way they say them. But it's also gesture, it's posture, it's the little vocal tics. And for identical twins in the same home, the effect of shared environment and a close bond is particularly intense.
Which is exactly the population where people point and say, look, it must be genetic.
It's the population where the confound is worst. Two people with identical genomes, living in the same house, imitating each other constantly, and we look at their shared mannerisms and conclude genes. It's the least clean natural experiment imaginable.
But imitation itself. Is that heritable? The capacity to imitate?
Slightly. There's a two thousand seven study in Child Development, five thousand two hundred and six UK two-year-old twin pairs. Individual differences in imitative ability broke down as thirty percent heritability, forty-two percent shared environment, twenty-eight percent unique environment.
So the ability to imitate is mostly environmental.
Mostly environmental. Which means the thing that produces shared mannerisms is itself mostly learned. It's a double layer of nurture.
Alright. Now do the twins reared apart thing. Because this is where the anecdotal evidence lives and I want to know how much of it is real.
So the big one is the Minnesota Study of Twins Reared Apart. Thomas Bouchard, ran from seventy-nine to ninety-nine. A hundred and thirty-seven pairs, eighty-one of them identical, separated at birth and raised in different families. And the finding was that identical twins reared apart were about as similar as identical twins reared together, on multiple personality and temperament measures. About seventy percent of IQ variance was associated with genetic variation.
That's a strong result.
It's a strong result. And then there are the stories that come out of it. The Jim twins. Jim Lewis and Jim Springer. Separated at four weeks, reunited at thirty-nine, in nineteen seventy-nine. Both had tension headaches. Both bit their nails. Both smoked Salem cigarettes. Both drove the same model of car. Both took their vacations at the same Florida beach.
That's the one everybody knows.
Everybody knows it. And then Nancy Segal, who's done decades of work on this, describes the McKusick twins. She writes that they were seated alike, feet slightly apart, hands folded the same way with the thumbs resting on top. And she calls these behaviors scientific data.
She's defending them as data.
And there's a nineteen sixty-two book by Shields on monozygotic twins brought up apart, where he notes that voice, gait, vivacity, caution, have all on more than one occasion been noted as being much alike.
So there's a real pattern here. But.
But. These are small numbers. They're anecdotes. They're highly vulnerable to confirmation bias and selective reporting. Nobody publishes the twins reared apart who turned out to have nothing in common. And the stories that do get published are the ones that make you go, wow. So the sample of stories you hear is not the sample of cases that exist.
Which is exactly the comforting pseudoscience Daniel is asking about. We collect the hits and forget the misses.
We collect the hits. And the hits are the ones that feel good. Which brings me to the cautionary tale.
Handedness.
Handedness. Everybody assumes handedness is genetic. It runs in families, it's obviously biological, there's got to be a handedness gene. There's a twenty-nineteen twin study in Wellcome Open Research, Bishop and Bates, a hundred and ninety-four twin pairs. Heritability of handedness came out around zero point two five on one measure, zero point one eight on another.
That's low.
And the heritability of functional language laterality, so which side of your brain handles language, was zero.
Zero.
Zero. So a trait that people are absolutely certain is inherited, that they'll argue about at family dinners, turns out to be only weakly heritable, and the related brain asymmetry isn't heritable at all in that sample.
So the folk intuition is just wrong. On handedness.
Which should make us humble about every other intuition we have about what's inherited.
Alright. So where does that leave Daniel's question?
It leaves us with a tangle. Genes are real, and for sleep specifically they're doing more work than most people would guess. The head and neck posture thing is under strong genetic control. But mannerisms specifically, the way you gesture, the way you hold yourself, the way you sleep with one arm up, that's under-studied as a phenotype. There's no study. And imitation is doing enormous work, and it runs in a direction nobody expects, and it's mostly learned, and it's invisible to the person doing it.
The third bucket. The pattern-matching.
Daniel is lying in a quiet room watching his son sleep. He is primed to see resemblance. That's not a flaw, it's just what humans do. We're pattern-matching machines, and the patterns we look for hardest are the ones in the faces we love. So the observation itself is filtered.
Which cuts both ways. It doesn't mean he's wrong. It means the thing he noticed might be real and might also be a story he's telling.
Both can be true. The resemblance can be real and the meaning he's attaching to it can be a story.
Here's what I keep circling back to. Daniel asks whether it's a comforting pseudoscience myth. And the thing is, the pseudoscience cuts both directions. It's genetic is a comforting story. My kid is like me, it's in his blood, it's the family line. But he learned it from me is also a comforting story. He watched me, he wanted to be like me, he picked it up because I'm his whole world.
The honest answer is less satisfying than either. It's an unresolvable tangle of genes, shared environment, and automatic mimicry, and you cannot pull the threads apart in a single case. You can only do it in populations, with statistics, and even then the numbers are provisional.
The answer to Daniel is, we don't know, and we can't know, and the not knowing is the actual state of the science.
That's the answer. And I'd add one more thing. The specific heritability of mannerisms, gestures, facial-expression style, is under-studied. I went looking and there's nothing. No PubMed results for gesture style heritability. Nothing for facial expression resemblance between parents and children as a distinct phenotype. It's a gap.
Daniel's question is, in a real sense, an open research question.
It's an open research question. Which is a rare thing to be able to say about something this ordinary.
I had a pair of identical twins once. Sleep study, late two thousands. Both slept with one arm tucked under the head, same side, same angle. The researchers argued about it for weeks. Genetic or learned. Couldn't settle it.
Weeks.
What did they land on?
Bit of both. But the one who'd been adopted out at birth, raised in a different family, he slept the same way too. Nobody could explain that one.
That's the separated twin still sleeping the same way.
Which cuts against pure imitation.
It does. Though they couldn't rule out that he'd seen a photo. Or heard stories. So it's not clean either.
Not clean. But it's the closest thing I've got. I've been sitting on it the whole episode.
It's a useful thing to have. Because it shows you the actual texture of the evidence. Two researchers, in a room, with real data, unable to agree. That's what this field looks like up close.
The twin who was raised apart still doing it. That's the detail that keeps you up at night. Because if it's not imitation, then what is it?
That's the question. And Hilbert's case doesn't answer it. It just shows you how hard the question is.
We're left with an open question. No study directly tests whether specific sleeping positions like a raised arm are heritable. The closest evidence is indirect, head and neck posture, general sleep architecture. And the heritability of mannerisms, gestures, and facial-expression style is under-studied as a distinct phenotype.
Under-studied. And as genetic research gets more nuanced, we may find that the comforting pseudoscience is partly true. Just not in the clean, satisfying way we want. Not in the way that lets you point at your kid and say, that's me, that's my blood, case closed.
The honest answer is the tangle. Genes, shared environment, automatic mimicry, and a father who loves his son enough to notice.
Which might be the point. Maybe it doesn't matter whether it's genes or imitation. The resemblance is real either way. And the love that makes us notice it is real too.
That's a good place to leave it. Thanks to Hilbert Flumingtop for producing. This has been My Weird Prompts. If you enjoyed this episode, please rate and review us on your podcast app. It helps other listeners find the show. We'll be back soon.
