Genetic enhancement of cognition

Genetic enhancement of cognition is the proposal to raise human cognitive ability by choosing or altering genetic variants, either by selecting among embryos or by editing them. Cognitive ability is substantially heritable, which is what makes the proposal seem tractable. It is also among the most polygenic traits ever mapped, which is what makes it intractable in practice. The distance between those two facts accounts for most of the disagreement about whether the idea is a near-term prospect or a category error.

§What the genetics shows

Twin, adoption and family studies place the heritability of adult cognitive test performance somewhere in the range of one half to four fifths, rising through childhood into adulthood. Heritability estimated from measured common variants is considerably lower, on the order of a fifth to a quarter, and the polygenic scores built from those variants explain less again.

The largest relevant studies target educational attainment, a proxy that is cheap to collect at biobank scale. A study of about three million individuals produced a score explaining something in the region of twelve to sixteen per cent of the variance in years of schooling in independent samples.¹ Direct genome-wide studies of measured intelligence are smaller and identify hundreds of loci.² In both cases the individual variants are minuscule: the largest common-variant effects correspond to a small fraction of a point on a standardised test.

Two asymmetries follow. First, there is no small set of high-effect "intelligence variants" to install. Second, the variants of large effect that do exist are almost uniformly harmful, because mutations that disrupt neurodevelopment produce intellectual disability. Nature offers a long list of ways to break cognition by changing one gene and essentially no examples of improving it that way.

Polygenicity is the whole problem

A trait built from ten thousand variants of a hundredth of a point each cannot be enhanced by a technique that acts on one locus at a time. Every proposed route has to solve the many-loci problem before anything else.

§Proposed routes

§Embryo selection

The nearest-term route is Polygenic embryo screening, ranking embryos by a cognitive or educational-attainment score. The expected gain is set by how much genetic variation exists among one couple's embryos, how accurate the score is between siblings, and how many embryos are available. Modelling by Karavani and colleagues put the expected shift at a couple of points at most when choosing the best of ten, with wide variance around that expectation, and less again for the number of embryos a real cycle produces.³

Two proposals attempt to relax the binding constraint. In vitro gametogenesis would generate large numbers of embryos from a couple's cells, raising selection intensity. Iterated embryo selection, described by Carl Shulman and Nick Bostrom, would run several rounds of gamete derivation and selection in vitro to compound gains across simulated generations without waiting for real ones.⁴ Neither has been demonstrated in humans, and the second requires deriving gametes from embryonic stem cells at a scale nobody has approached.

§Editing

Human germline editing could in principle install alleles that neither parent carries, escaping the sibling-variance limit entirely. Three obstacles stand in front of that, and only one of them concerns cutting DNA.

The first is that association studies do not hand over a list of variants to install. A genome-wide hit marks a stretch of chromosome correlated with the trait; the base actually responsible may be any of hundreds nearby. Statistical fine-mapping resolves a minority of loci to a plausible causal variant, and for a trait with thousands of contributing regions the causal set is mostly uncharacterised. Editing a marker rather than a cause changes nothing.

The second is scale. Effects measured in hundredths of a point mean that a shift worth having requires very many substitutions in one cell. The highest multiplex edit counts on record were achieved in cultured cell lines, which can be screened, expanded and thrown away; an embryo offers a single attempt, and confirming what happened to it means consuming cells it needs.

The third is damage. Every additional cut raises the probability of the large deletions and rearrangements documented in Off-target effects in genome editing, and a one-cell embryo has no spare copies. Base editing and Prime editing rewrite bases without severing both strands and are the only plausible chemistry for a high-multiplex attempt, but their behaviour across many loci at once in a human embryo has not been measured.

§Somatic intervention

Editing an adult brain is a different proposition. Most of the relevant biology acts during development, delivery to the central nervous system is the standing obstacle for AAV vectors, and Somatic gene therapy for a non-disease indication would face a regulatory bar no sponsor has tried to clear. Interventions aimed at adult cognition are, for now, pharmacological rather than genetic, and the evidence there is covered in Nootropics.

§Animal and biological evidence

The clearest single-gene demonstration remains a mouse. Overexpressing the NR2B subunit of the NMDA receptor in the forebrain produced animals that learned faster on several tasks.⁵ The result held up, but so did a subsequent finding that the same animals showed heightened sensitivity to inflammatory pain, an early and instructive illustration of pleiotropy.

A second line concerns klotho, a protein better known in aging research. Human carriers of one copy of the KL-VS variant show slightly better performance on some cognitive measures, and administering klotho to aged rhesus monkeys improved memory performance in a controlled study.⁶ This is a genuine cross-species signal, and it is also narrow: a single, modest, hormone-mediated effect, not a general method for moving a polygenic trait.

Comparative and evolutionary arguments are sometimes offered as evidence that large gains are available, on the grounds that human cognition changed rapidly in evolutionary time. That change involved coordinated modification of developmental programmes over hundreds of thousands of years, not the substitution of a few alleles.

§Why this is harder than it sounds

Within-family attenuation. A score's advertised accuracy is measured across unrelated strangers, and a substantial part of what it measures is not the child's own DNA. Some of it is the parents' behaviour, which correlates with their genotype and shapes the household. Some of it is ancestry and mating pattern, which align genotype with social position. Sibling comparisons strip those out, because siblings share parents, household and ancestry; what survives is the direct effect, and only the direct effect is available to an embryo choice. For educational attainment the surviving fraction is well below the headline figure, which matters because educational attainment is where nearly all the discovery power sits.

Pleiotropy. Genome-wide genetic correlations link educational attainment to a range of psychiatric outcomes, positively for some conditions and negatively for others.⁷ Selecting or editing on a cognitive score therefore moves risk for other traits in ways that are only partly characterised. The direction of some of these correlations is itself contested, since they are estimated from the same confounded population data.

Gene–environment interplay. Population mean scores on cognitive tests rose substantially across the twentieth century in many countries, a change no genetic mechanism explains. Whatever produced that shift was environmental, and it was larger than anything genetic selection currently offers.

Measurement. Test performance, educational attainment and cognition in any richer sense are not the same variable. Optimising the measurable proxy is not obviously optimising the thing of interest, a problem that recurs throughout Human enhancement.

Contested claims

Claims that cognitive enhancement by embryo selection is now practical rest on population-level score accuracy applied to a within-family decision. Mainstream statistical genetics regards that substitution as invalid, and no company offering the service has published outcome data on selected children.

§Ethics and politics

The arguments split along familiar lines. Procreative beneficence supplies the case that parents have reason to select for cognitive advantage as for any other expected benefit. Against it, Disability rights and enhancement raises the expressivist objection, and Access and inequality raises the prospect that a cognitive advantage available only to the wealthy compounds existing stratification faster than any other enhancement would. Because cognitive advantage is substantially positional, the dynamics analysed in Enhancement arms race apply with particular force: if the gain is competitive rather than absolute, universal adoption leaves everyone where they started while consuming real resources.

There is also a research-conduct problem. Cognitive genomics has a history entangled with eugenics, and much of the field's caution about enhancement framing reflects that history rather than the statistics alone. Several of the companies marketing cognitive scores for embryos have been criticised by the same researchers whose summary statistics they use.

§Outlook

The most likely near-term development is not a working enhancement but better evidence about how little the current tools deliver. Within-family genome-wide studies at biobank scale are the technically decisive experiment: they estimate direct genetic effects, and the size of those estimates sets a ceiling on what any selection or editing scheme could achieve. If within-family effects on cognition prove as attenuated as they are for educational attainment, the enhancement case rests on multiplex editing, which is not close.

The harder question is what a positive result would mean. A validated within-family score capable of shifting cognition by a few points would sit alongside interventions in nutrition, schooling and air quality that are known to move the same outcome by comparable amounts at a fraction of the cost, and would still be argued about far more.

Two things would change that calculation. The first is a mechanistic result rather than a statistical one: a variant whose effect on neural development is understood well enough to be engineered rather than merely selected. Nothing on the current maps looks like that, and the pleiotropy problem suggests few candidates will. The second is a shift in what is being enhanced. Most of the argument assumes general cognitive ability as measured by existing tests, which is the trait with the largest and least tractable polygenic architecture. Narrower targets — working-memory capacity, resistance to age-related cognitive decline, the specific deficits of a diagnosed condition — are smaller problems and land closer to therapy than to enhancement.

That last point is where the debate is most likely to actually go. The line between preventing decline and raising a baseline is not sharp, and interventions developed for neurodegeneration will be the first to test how societies police it.

§See also

• Polygenic embryo screening
• Designer babies
• Human germline editing
• In vitro gametogenesis
• Nootropics
• Intelligence amplification
• Bioethics of enhancement
• Enhancement arms race

§References