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[!summary]Many famous “twins reared apart” studies are flawed because those twins were not truly raised in different environments. This undermines claims that IQ is almost entirely genetic and shows twin studies can overestimate heritability. Genes matter, but environment and prenatal factors also play a large and complex role.
Highlights
id984731211
While heritability is an imperfect notion—with nasty caveats that are beyond the scope of this post—it has become a de facto standard and, for better or worse, the complex debate on cognitive inequality is often reframed as a one-dimensional debate on the heritability of IQ.
Heredabilidad es una técnica controversial, pero se acepta como estándar.
id984731653
Heritability is, by construction, a population-level aggregate. Before it can inform policy-making (or even personal decision-making), it must be interpreted at the level of individuals. This is where things get interesting and counterintuitive.
id984734347
By design, IQ is a statistical construct that captures some common denominator, the “g factor”, among a variety of cognitive tasks. It’s as if someone had pooled a large number of biomarkers to extract a general health score, the “h factor”. That doesn’t mean the construct is useless. The concept of good health is a meaningful one and simplistic proxies like “biological age” do make sense in particular contexts. IQ was designed as a cheap, scalable way of identifying students with learning disabilities and, for this specific use case, it is reasonably pragmatic. Do note that the “g factor” is an abstraction that is impossible to directly measure. All you can get is a proxy that has some degree of correlation (say, 80%) with the actual thing you care about.
id984763493
For all their performative battles, hereditarians and blank-slatists operate within the same deterministic frame (genetic determinism vs social determinism), denying any meaningful role for human agency and the messy, noisy process we call life.
id984767786
I agree that there is something irresistible in the idea of studying identical twins reared apart. It feels like a “natural experiment” that would magically tell nature and nurture apart. In fact, this appearance of simplicity, of being “cutting and definite”, is precisely what made Cremieux’s visual so compelling and viral. But even under unrealistically perfect conditions, with twins truly separated at birth and placed in truly random families—as we’ll see, that was far from being the case—the experiment would still be confounded by a substantial bias: twins share a womb for nine months.
id984938743
Regardless of their prevalence in the general population, these risks are compounded for twins reared apart, as 1/ twin pregnancies have a much higher chance of complications, and 2/ pregnancies resulting in adoptions have a much higher chance of being associated with inadequate prenatal care.
El ambiente perinatal tiene un efecto potente y documentado, y es una variable confundente en el caso de gemelos adoptados.
id984938843
On average, the twins in the Bouchard study were separated at 5.1 months. In the Shields study, 25 pairs were separated during their first year, but 6 were separated during the second year, 4 between ages two and four, and individual pairs were separated as late as seven, eight, and nine years of age. In other words, the phrase “reared apart” should be taken with a substantial grain of salt. It subliminally suggests “separated at birth”, while it effectively means “separated in early childhood or later.”
id984942833
For the experimental design to be “cutting and definitive”, as Bouchard dreamt it to be, the twins must be selected at random (bias #3), incubated in 100%-consistent artificial wombs (bias #1), and placed right after birth (bias #2) in truly random families—which never happens in real-life. This is bias #4. In our societies, adoptive families are the opposite of random. They tend to have specific family structures, live in specific neighborhoods, and share a specific set of values and cultural habits. The simply act of wanting to adopt a kid—and being entrusted with one—filters out the most dysfunctional environments. If you take two seeds from the same apple and plant them in separate pots, you might be able to derive meaningful insights on the relative importance of the environment. That is, unless you place them side-by-side on the same balcony.
id984958832
As I eventually found out, all this was well known to those with a strong interest in the topic. The study of “twins reared apart” is widely regarded as a scientific dead-end and behavioral geneticists have entirely moved on to new strategies.
Interesante! Hoy por hoy, ningún investigador serio considera que los estudios de gemelos son realmente útiles.
id984960699
Most people agree that twin studies overestimate heritability while genomic studies underestimate it, and the truth must lie somewhere in-between. People are working to resolve the gap with new techniques and meta-arguments. As far as I understand, the frontline seems to be stabilizing around the 30-50% range.
id984961473
As we learned in the schoolyard, nitpicking is for losers, and no one is going to listen to technical jargon in the face of glaring evidence. This explains why this piece is so long. I don’t think one can effectively argue against the Jim Twins narrative. To fully understand what was going on, I had to build my own counterstory, with secondary characters and subplots, that transformed the abstract concept of “biases” into concrete details that were memorable and real.
Esto es importante en relación a la justificación de la postura de escepticismo. Los relatos tienen mucho poder y activan todos nuestros sesgos que buscan hacer calzar la realidad con nuestros priors.