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Why Breeding Smarter Humans Won’t Work: Basic Genetics 101

Biochemist Michael Denton explains that, in human genetics, everything is connected to everything else; geneticists call it pleiotropy

Recently, we looked at the question of whether human IQ could be artificially increased via genetic engineering. One proposal was to mass produce human embryos, implanting only the smart ones and discarding the rest. All other issues aside, it’s unclear how to determine which kids will turn out to be the smart ones.

Now biochemist Michael Denton, author of a number of books including the recent Miracle of Man (2022), writes to tell us that the idea won’t work due to fundamental genetics. Noting that theoretical physicist Stephen Hsu, who advanced the idea of discarding embryos above, is not a medical geneticist, he told Mind Matters News,

Its true there are many genes involved in brain development but most genes ( alleles) in all organisms including those in humans are what are termed Pleiotropic. What this means is that few genes ( hardly any ) only affect one organ or trait ( like intelligence). So pleiotropy poses a very difficult (probably ) insurmountable challenge to attempts to improve a particular trait like intelligence by selecting particular genes or alleles.

According to ScienceDirect, “Pleiotropy is the property of a single gene or protein to act in a multiplicity of ways.” – The Cytokine Handbook (Fourth Edition), 2003. Most discussions of pleiotropy focus on medical issues and this one provides a quick look at how one gene can have a not clearly foreseeable variety of effects throughout the body:

Dr. Denton talks about pleiotropic genes in Evolution: A Theory in Crisis (Adler & Adler, 2002):

The effects of genes on development are often surprisingly diverse. In the house mouse, nearly every coat-colour gene has some effect on body size. Out of seventeen xray-induced eye colour mutations in the fruit fly Drosophila melanogaster, fourteen affected the shape of the sex organs of the female, a characteristic that one would have thought was quite unrelated to eye colour. Almost every gene that has been studied in higher organisms has been found to affect more than one organ system, a multiple effect which is known as pleiotropy.


He adds, in his note to us today, “It may be possible to improve slightly intelligence by genetic engineering but the notion that we can create super intelligent humans is pure sci fi.”

Well, the idea is certainly sci fi, dating back to the late nineteenth century. But in 2014, we were informed at Business Insider that “Humans Genetically Engineered To Be Super Intelligent Could Have An IQ Of 1000.” Stephen Hsu is quoted again,

“The possibility of super-intelligence follows directly from the genetic basis of intelligence. Characteristics like height and cognitive ability are controlled by thousands of genes, each of small effect,” Hsu writes.

From previous studies of the genome, Hsu says we can estimates that there are probably about 10,000 of these gene variants associated with intelligence. If we can tweak each of these to their “smart” version, so that they contribute a tiny bit to higher intelligence, we could engineer human beings who “exhibit cognitive ability which is roughly 100 standard deviations above average. This corresponds to more than 1,000 IQ points.”

Ajai Raj , “Humans Genetically Engineered To Be Super Intelligent Could Have An IQ Of 1000” at Business Insider (October 22, 2014)

No mention of pleiotropy. And the decade Dr. Hsu thought it would take, back in 2014, is nearly up. At least we have a clear idea why it isn’t plausible.

Except maybe in the movies:

You may also wish to read:

Could we really increase human IQ via genetic engineering?
One suggested approach is to only implant “intelligent” human embryos and discard the rest, to avoid editing individual genes. It’s not clear what, explicitly, human intelligence is or even how it originates. Ethics aside, there’s no way to decide who to save and who to throw away.


Can science really engineer a bigger human brain? Computational neuroscientist Daniel Graham wonders why we would bother. There is no strict relationship between brain size and intellectual achievement. The human brain has actually been shrinking in the last 30,000 years, the same period that has also shown great intellectual achievements.

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Why Breeding Smarter Humans Won’t Work: Basic Genetics 101