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One way psychologists estimate IQ heritability (the percentage of variation in IQ linked to variation in DNA) is by correlating the IQs of monozygotic (MZ) twins raised apart. The higher the correlation, the more genetic IQ is thought to be.

However skeptics argue that because MZ twins raised apart still shared the same womb, and still grow up in the same country and sometimes the same town, the high correlation doesn’t prove the genetic effects are independent of environment (maybe the same genotype that increases IQ in the U.S. would decrease it Japan, but we’ll never know if virtually all the twins raised “apart” are still raised in the same country).

As commenter “Mugabe” suggested, the ideal study would have genetic clones separated at conception and gestated and raised by random women all over the developed World, but such a study would be unethical. And even if such a study were possible, and even if it showed strong independent genetic effects, the nature of these effects would remain mysterious. Does DNA directly cause IQ (i.e. coding for bigger and more efficient brains), or does it do so indirectly (i.e. causing us to stay in school longer, where we learn how to think). The problem with even the best designed study of MZ twins separated into random environments is that only the starting environment is random. As we grow old, we select environments that fit our DNA, and although the effects of such environments are counted as genetic effects (since our genes made us choose those environments) they are actually gene-environment feedback loops.

But what if it were possible to clone just our brains, and these cloned brains were reared in environments completely alien to anything we have experienced. You grew up in a nice middle class family, and your cloned brain grows up in a petri dish, where its environment was 100% controlled with no gene-environment feedback loop.

image found here

Then we could be sure that any cognitive correlation between us and our cloned brains was not only an independent genetic effect, but a direct one to boot.

It sounds like science fiction, but something similar is actually happening in the lab of Alysson Muotri, a biologist at the University of California, San Diego. Muotri takes skin cells from volunteers, turns them into stem cells, and then makes them grow into tiny pinhead sized balls of brain tissue called organoids.

Of course these organoids are way too tiny to be considered cloned brains, but they are complex enough to make brain waves. And Muotri has already found that cognitively impaired populations have cells that produce underdeveloped brain organoids in the petri dish. For example brain organoids derived from autistic people had about a 50% reduction in synaptogenesis.

Muotri also decided to study Neanderthal brain organoids. Since it’s not possible to get cells from Neanderthals, he edited modern human DNA. Of the 20,000 protein coding genes, only 61 differ between us and them, and of these, only four are highly expressed in the brain so by editing just these four genes, he was able to produce Neanderthalized organoids, or Neanderoids as he calls them. Modern humans had far more spherical skulls than Neanderthals so it’s interesting that our brain organoids are spherical, while theirs look like popcorn.


Muotri notes that like the autistic brain organoids, the Neanderoids have a 50% reduction in synaptogenesis. Neanderoids also show 65% to 75% reductions in firing rate and activity level per neuron per minute. Muotri thinks this may help explain why it took them several hundred thousand years to progress from simple stone tools to, well, simple stone tools. By contrast, in just the last 50,000 years we jumped from simple stone tools to the internet, genetic engineering and traveling to the moon.

image from Muotri’s talk comparing our rate of cultural progress to Neanderthals’

So clearly brain organoids are very good at identifying cognitively impaired populations, but can they measure normal variation in human intelligence?

Muotri could greatly advance our understanding of behavioral genetics if he made brain organoids of a representative sample of Americans of known IQ scores, and then correlated the synaptogenesis, neuron activity level and firing rate of the organoids with the tested IQs of the people from whom they were derived. Perhaps a carefully weighted composite score of all three measures would give the best prediction of IQ, and perhaps such a formula could allow us to estimate how Neanderthal’s would score on IQ tests (if they were reared in our society).

If it’s too difficult to get a representative sample of Americans and test their IQs, he could simply have students at his university donate their cells, and then correlate their brain organoid scores with their SAT scores. Would there be statistically significant differences in the brain organoids of people who score a perfect 1600 on the SAT compared to those who score 1400 compared to those who score 1200 compared to those who score 1000?

Muotri is also trying to teach the brain organoids how to control a robotic body. The speed with which they learn might be considered a low level IQ test. So imagine taking a conventional intelligence test like the Wechsler Adult Intelligence Scale (WAIS) or the SAT, while your mini-brain, raised in a petri dish is taking its own IQ test (learning to control its robotic body). This could be the 21st century version of studies where identical twins raised apart have their IQs correlated. If your score on a conventional intelligence test predicts the speed with which your brain organoid learns to control its robotic body, then that proves IQ tests are measuring a genetic property of the brain that is completely independent from social class and culture because environment is perfectly controlled in the petri dish.

Perhaps in the future instead of universities testing candidates on the SAT, they’ll just test the student’s brain organoids instead to eliminate the cultural bias some think confounds the SAT. For there’s no culture in the petri dish (aside from bacteria culture :-)).

When a prosecutor suspects a murderer is faking his low score on the WAIS to avoid execution (because it’s illegal to execute people with IQs below 70 in some states) he could insist on testing the murderer’s brain organoid instead (since they can’t fake low scores-as far as we know).

On the other hand brain organoids might prove that normal variation in IQ is nowhere near as genetic or biological as its proponents think. I find it fascinating that just four brain genes separating modern humans from Neanderthals produced such dramatic differences in brain organoids. That implies each gene must have huge effects. That’s not at all consistent with research on normal IQ variation among modern humans, which estimates that some 10,000 genomic variants are involved, each one affecting IQ by only a fraction of a point. It’s also possible that brain organoids showcase too early a stage of brain development to correlate with the higher abstract abilities measured by IQ tests (for example infant development scales have weak correlations with adult IQ).

In the below video Muotri discusses his brain organoid research: