Book review: Eric Turkheimer's "Understanding the Nature-Nurture Debate"

From the book review:

One could imagine an alternative universe where the field takes twin studies "seriously but not literally” and follows a middle path: behavior is influenced by genetics to some extent, so observational studies need to incorporate genetically informed designs; but twin models are also influenced by environmental assumptions to some extent, so their estimates shouldn’t be taken at face value either. Instead, the idea of having a seemingly fool-proof natural experiment proved too attractive for caution. Twin heritability was enshrined as a fundamental biological parameter of deep value. A thousand twin studies bloomed, quantifying the heritability of every phenotype, behavior, or measurement one could think of. And at a certain point, the stamp collecting became and end unto itself[.]

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Twin studies are certainly not the only scientific gravy train, and Turkheimer has highlighted what may be a universal factor: a method that ostensibly answers an important scientific question but is, in fact, always guaranteed to come up in the investigator’s favor. Even better if running the method is resource constrained or otherwise expensive, so that the senior researchers who have “paid their dues” can be at the helm. And if you think you are hearing the echoes of some modern gravy trains — the countless single cell atlases, high-throughput screens, Mendelian Randomizations of dried fruit intake on time spent outdoors, etc. — well, yes, I hear those echoes too.

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Turkheimer’s answer is The Gloomy Prospect: a kaleidoscope of idiosyncratic gene-environment interactions and correlations that unfolds over the course of development as individuals encounter, select, match into, and reshape their environments and the environments of those around them. [...] This is where the field of behavioral genetics had landed at the end of the 20th century: both genes and environment matter and we have no idea how.

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Turkheimer reviews the early period of molecular behavioral genetics using Plomin’s hunt for IQ genes as a scaffold. In paper after paper, a molecular study is run, new IQ genes are identified, compelling stories are constructed. Then in the next study those genes fail to replicate. Rinse and repeat for ~15 years. Through this period Plomin is largely undeterred, ending each study with a promise that with just a few more samples the real IQ genes will be found. Here was The Gloomy Prospect in the “candidate gene era”. Every genetic association seems to tell a compelling story, and yet none of the stories ever replicate. But then, a breakthrough comes in Genome-Wide Association Studies: forget testing specific candidate genes in hundreds of samples, collect hundreds of thousands (and eventually millions) of samples and test every single variant, producing millions of highly sensitive correlations. Now the field was cooking with gas, and the “hits” started rolling in. But not just one or a handful of IQ genes; hundreds of them; then thousands; most not even in genes; and each “explaining” barely anything.

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Turkheimer’s prediction for the molecular era was largely correct. I’m not one of those people who shouts at the authors of GWAS papers that they only discover false positives — they clearly replicate. But, for many behavioral traits, they do so in a largely technical sense: the confounding in the UK is similar to the confounding in the US, so a false positive in one place will replicate in the other. Tweak the environment a little bit — for example, by restricting to individuals with high SES — and even the non-causal predictive accuracy can take a nose dive. Just as with twin studies, the correlations were cataloged and then the causes were left to worry about later. And here we are more than a decade later having learned nothing more about the biology of educational attainment other than that it has something to do with the brain, possibly, uh, neurons.