IQ differences of identical twins reared apart are influenced by education
50 comments
·November 25, 2025everdrive
An interesting tidbit in the nature vs. nurture debate is that nature and nurture interplay in ways you might not expect. For instance, height is approximately 90% heritable in the United States -- but this does not mean that in a vacuum height is mostly genetic. It means that in the United States nutrition has mostly been solved (and yes, even the "food insecure" in the US rarely lack for the actual calories which would impact their height -- food insecurity causes other problems) and therefore the only real differences that can remain are the genetic differences.
It might be useful to look at any twin study through this lens; if we know for sure the genes are the same and nature is off the table, how much variance remains?
the__alchemist
Things get even fuzzier when you throw in heritable epigenetics too. We have a balance of these factors at least:
- Genetics (DNA seq)
- Epigentics (Histone acetylation, base methylation etc)
- Brain wiring from experiences
- Chemical impact from experiences, e.g. nutrition, toxins, sunlight, muscle dev etc etc.everdrive
> - Brain wiring from experiences
> - Chemical impact from experiences, e.g. nutrition, toxins, sunlight, muscle dev etc etc.
Are these not all part of the nurture / environment bucket? Or are we drawing a hard boundary between nurture (eg, parenting) and environment? (eg, lead in the pipes)
the__alchemist
I'm splitting up both "nature" and "nurture" into slightly less-broad categories. In the case of what you highlighted, yes.
For example, epigentics is sort of both "nature" and "nurture", in that you can pick up these traits, and pass them on/get them passed on.
tptacek
Genes interact with the environment. There aren't hard boundaries the way you've phrased them.
xattt
Re: nutrition
I am curious about the influence on height by self-limited diets in children who are picky eaters. Is there some self-regulation process that decreases pickiness when nutritional status is at risk?
SilverElfin
That’s a good way to put it - eliminating other factors ends up deceptively making it look like genetics are the only difference. On the topic of IQ, the importance of those other factors is implied by things like the Flynn effect (https://en.wikipedia.org/wiki/Flynn_effect#Proposed_explanat...)? It’s also evident in how IQ varies by country. In developing countries, average IQ can measure low due to issues like malnutrition, access to education, etc. Those differences change as those countries develop or across different parts of those countries or when you track immigration populations from those countries.
Oddly, denial of these other variables has become core to scientific racism (https://en.wikipedia.org/wiki/Scientific_racism). For example, a Danish white supremacist named Emil Kirkegaard went so far as to create a fake journal (https://en.wikipedia.org/wiki/OpenPsych) to publish flagrantly incorrect papers on IQ that try to deny these factors and paint the IQ of non-white countries as low, with the only explanation being their “inferior” race/genetics. In fact, he just recently wrote a new “paper” (https://openpsych.net/paper/85/) that has been widely retweeted by supremacists on X.
tptacek
In all these discussions it's helpful to understand the definition of broad-sense heritability (the statistic almost always in play when we're discussing heritability) --- it's a correlation, not a demonstrated causation, and your environment (and gene/environment interactions) are inherited as well.
gishh
As an anecdote: late 30s male, mother was 5'6 father was 5'9, I'm 6'4.
I have three kids with two different women. All 3 are blonde-haired/blue-eyed. I have brown hair and green eyes, my ex has brown hair and blue eyes, and my spouse has blonde hair and blue eyes. Not as interesting I suppose. Yes, they're all my kids biologically. :)
churchill
[dead]
powerclue
IQ testing was recently found to be highly driven by response to difficult challenges, and could be influenced significantly by just tuning the rewards for participants doing well. Which suggests that measuring iq is a pretty fraught science, if you are trying to draw conclusions about heritable intelligence...
testtaker
which will bias all correlations towards zero, while height measurement is very accurate.
yet iq inheritability is only a bit lower, around 80%
tptacek
The most generous twin study numbers get you to 80%; modern study designs get you nowhere close to that. This is the "missing heritability" crisis.
seizethecheese
I would guess that people will do better on just about any test when given a reward for good performance.
timenotwasted
As a parent of identical twins watching them develop and grow is fascinating. I do wonder at times how much of it is due to going through every single life stage together but then again there are times where that bond seems to go beyond environment. There was a sobering but very interesting documentary on identical twins called Three Identical Strangers, if you are interested in this type of stuff it's a good watch.
HPsquared
The other side to this is non-identical twins, especially when still very young and have had basically the same experiences (doing everything together), they can be very different.
lern_too_spel
I don't know how many more nails in the coffin of heritable intelligence are needed before public policy capitalizes on the opportunity here. We have seen huge shifts in intelligence in places like Korea after armistice, in China after economic and cultural reforms, and in the U.S. after COVID and smartphones. We have seen that individual tutoring reliably creates extreme outliers from the examples of Avant-garde, Williams, Tao, Polgár, the Hungarian Martians, etc. Aside from disorders, any heritable differences are dwarfed by environmental effects, and some disorders, like OCD, only magnify environmental effects.
ortusdux
Is there a 'teaching the test' element to this? Does more exposure to education increase your ability to take a standardized test?
tptacek
We don't know.
powerclue
We do know that response to difficulty influences performance in iq testing: https://pmc.ncbi.nlm.nih.gov/articles/PMC10990577/
This study found that adding or removing rewards for performing well can pretty dramatically impact performance.
"it is unclear to what extent the positive manifold reported in intelligence research since Spearman (1904) might be explained not through a shared component of intellectual capacity, but through a shared component of effort or time investment in testing tasks."
So, yes, we don't know, but the ways we don't know should also include "we don't know if iq testing is even measuring intelligence rather than stick-to-it-ness".
tptacek
I mean I agree with that too! I would just bucket all this under "we don't know". :)
DrewRWx
More exposure to upper middle-class culture increases ones ability to take IQ tests.
burnt-resistor
Define "more exposure to education". ;)
Also, NCBLA standardized testing has demonstrably ruined education in America.
l2silver
I don't know much about IQ. In the most extreme case, of dissimilar education, the different was about 15 points. Is that a lot? What does that mean to laypeople?
wjb3
On IQ tests, 15 points is a meaningful difference (one standard deviation), or roughly the gap between solidly average and clearly above average. It doesn’t make anyone a genius or a write-off. Still, we’d expect the higher-scoring person to generally find new learning and problem-solving easier, on average, if everything else is equal.
torginus
I'm not even sure it's mathematically appropriate to talk about IQ differences as there's no proof IQ is a linear metric. IQ is defined to have a normal distribution, unlike most things which are measured to have a normal distribution.
With IQ tests, the only thing you know that if 2 people fill it out and one of them scores higher, they have a higher IQ. Based on this you can sort people into a list by increasing IQ, but the standard distribution is implied not discovered.
This is like having a double-sided scale and a bunch of weights - you can similarly compare the weights to each other, and sometimes the arm of the scale will lean to the left, sometimes to the right, by a little or lot - you can postulate that the weights are normally distributed are normally distributed, but they are absolutely not required to be (I can choose them any way I like), so your assumption would be wrong. We know this because we have a direct, not just a comparative measure for weight.
I could make up an imaginary 'weight point' scale based on these comparisons, and say weights A is 5 WP heavier than B and C is also 5 WP heavier than A.
But A might be 100g, B might be 1g, and C might be 1kg.
This is what I think of when I see studies clamining the difference between 2 groups was 5 IQ points.
jakobnissen
IQ scores are calibrated to be normally distributed with a standard deviation of 15. So 15 is one standard deviation. That's the difference between average, and being in the smartest 16% of the population. Or being in the smartest 16%, and being in the smartest 2% of the population.
nabla9
15 points is significant difference.
If someone is 15 points above average, they are in 84th percentile, or in top 16%.
DaveZale
Well a sure component of test scores reflects test taking skills. Years ago, I purchased a book of a series of IQ tests, and my numerical result increased with every test. Another component is confidence. And another is ability. It is said by some that among the first big users of IQ tests was the US Army.
bena
15 points is right around one standard deviation.
It's not nothing, but IQ is already a little squishy. No one's IQ is a single number. But the article also goes into problems with the study and other potential issues.
Basically, they're saying there is this pattern in the data as recorded, but there are multiple confounding factors and issues with collecting the data in the first place.
Aloisius
Isn't the whole point of IQ that it is a single number? Or I suppose potentially two numbers if the quotient was expressed as a fraction.
tptacek
This is a deeper question than it sounds. The "point" of a modern IQ test is to identify cognitive deficits to target interventions. It's abused widely among non-practitioners as a ranking of intelligence, which it is not.
bena
I mean, nothing in the human body can be truly represented by a single number.
Even height and weight change throughout the day. People are typically taller and lighter in the morning than in the evening. Weight especially is variable, it can fluctuate up to 5 to 6 pounds.
austin-cheney
Sooo... yeah, but its not what you think.
There are two different kinds of IQ tests: convergent and divergent. Convergent tests are more common and test either knowledge or pattern matching. These tests are called convergent because they are a center of truth and conformance to that truth is the measured performance criteria.
Divergent tests measure the individual's creativity and abstract reasoning. The source of truth is the quantity of diversity of results submitted by the participant.
The implicit success criteria for convergent testing is reading comprehension. A person with dyslexia, for example, will perform worse on these tests irrespective of their learning speed, learned knowledge, intellectual curiosity, or creativity. This is a form of bias. Other forms of bias include memorization of terms, such as SAT preparation.
To further complicate things these measures typically only account for academic intelligence. Other forms of intelligence include social intelligence, spatial intelligence, creativity, conscientiousness, and so forth. In the concept of multi-dimensional intelligence, which is what is actually addressed in practice in the real world after high school, academic intelligence alone has very little benefit. Its like height in basketball where after 6.5ft all other factors become more important for all participants.
bell-cot
Outside of a fascination with Intellectual Supremacism, why are people so obsessed with the genetic basis of IQ scores?
Presumably one could do similar identical twin studies on half-marathon race times and SAT test scores. Does no one bother with those, because widespread awareness of half-marathon training regimens and SAT prep courses would spoil the (desired) illusion of some "innate superiority of blood" being measured?
null
notepad0x90
IQ tests are very deceptive and often misused.
I don't dispute the differences in intellectual capacity between people but IQ tests are like weight lifting contests between people who didn't train to lift weights.
I don't understand why the scientific community keeps using methods that are so flawed. Perhaps it's due to my own lack of information.
The only way I could see IQ tests being a valid measurement of intellectual capacity is if participants were brought to the same level of knowledge and skills, and then were made to train for the IQ test using the exact same means, and even then cultural and language barriers must be accounted for.
Even if these twins have an identical intellctual capacity and they both had the same exact education, and same exact grades, that still doesn't mean they applied and exercised their brains in the same way for the questions of the IQ tests.
IQ tests to measuring intellectual capacity (instead of knowledge level) are like polygraphs to mesasuring truthfulness in terms of accuracy.
For measuring strength, i stand by my earlier correlation with weight lifting. If two people can dead-lift 500lbs, I only know that both participants are have reached that level of strength. What I don't know is how much effort each participant put into getting to that level of strength, which would tell me their natural muscular capacity per effort. IQ tests deceptively seem like they tell you what someone's capacity for intellect is, but they only tell you where that person is right now. Maybe that person worked hard and the score is their max, maybe the person rarely applies their brain to demanding tasks and this is their mid-level capacity.
My point is,it isn't just education or just genetics, it is also personality, effort, motivation. For all I know,someone with double-digit IQ score can work out their brain for a year and hit genius level. Choice. Can a person choose to be a literal idiot and succeed? Certainly, people choose to be incurious and ignorant all the time. Women can body-build and be as strong as many regular men for example. But simply because of a hormone difference, they have to work out a lot more than men to reach the same level of muscular strength. And men who never work out can be as weak as women who never work out.
There is also the question of brain development. Maybe the effort you put has different effect depending on age. small efforts at a young age at applying your brain might have huge impacts, where as if you're a teenager or an adult, applying the same effort might yield less results.
I mean, personally, if I tired, if I ate too much, or too little,if didn't get enough sleep, if I am distracted by something, if I'm unprepared and thus second-guessing myself, these are some of the things that throw me off wildly when taking exams. I've seen huge differences by simply getting enough sleep and calories.
There is no way that asking the same set of questions to to large population (even with control populations in place) can account for all the presumptions of the test.
cm2012
It is very hard to increase your own IQ score meaningfully by prepping to the test. That is why its a good measure.
retskrad
Speaking of IQ, when I listen to interviews and podcasts of supposed high IQ people like Mark Zuckerberg and Elon Musk, I struggle to see signs of intelligence (not saying that they don’t have above average IQ). I find that these 2 have a hard time sifting through the noise (competing thoughts) in their head to get to want they want to say (the signal).
When I listen to Steve Jobs, I hear someone who has very strong ability to sift through noise. So Jobs couldn’t see engineering in a new way like Elon does but Elon couldn’t do what Jobs did either.
Regarding Zuckerberg, from what I’ve read he is the Bill Gates type where he has the traditional variant of high IQ, aka raw hardware/horsepower but lower on creativity/imagination side.
So intelligence seems to have different shapes and sizes.
There is something wrong with this article, possibly just copyediting mistakes but it makes me question the whole thing.
For example, check out this mess:
> “Unfortunately, there is one significant issue with the aforementioned data: schooling. Seeing as the majority of work to date includes only aggregate data, it is impossible to account. The first concerns small N: seeing as most publish studies only include a handful of TRA data, there is a lot of room for error and over.
Unfortunately, there is a largely unaccounted for confound in this aggregate data which may make generalized analysis questionable: schooling.”