One tangentially related point is that I’m very skeptical of claims that environmental influences on g “wash away” in the long run mainly because I have the pessimistic view that almost all modern environments are terrible for humans, especially for (but not limited to!) those born with high innate ability. I mean this in a variety of different senses: that the modern world and modern education verge on being traumatically difficult to live in for the majority of people, that approximately ~99.999% of schools have embarrassingly poor pedagogical practices, that the vast and overwhelming majority of parents have no idea how to raise children (let alone inculcate intellectual tendencies in them), and so on and so forth … If the cultural standard of parenting and education is the equivalent of ineptly trying to put out a bonfire by blowing on it, then of course no effect will be found! (For an example of what I mean, just consider the embarrassingly small number of math textbooks that one can actually consider to be “written well” in an expository sense…)
All considered, I’m increasingly loath to make any confident extrapolations from studies of heritability, interactions, environmental effects, …, outside of the specific context in which they were performed.
(I know that one can in principle [10] use regularized regression to account for nonlinearity nearly optimally given certain reasonably plausible assumptions, although on priors alone I’m skeptical of practical efficacy until the method is further validated with biological data.)
The evolutionary argument for genetic additivity
I remember being struck by the argument
However, quantitative differences between individuals within a species may be largely due to independent linear effects of specific genetic variants. As noted, linear effects are the most readily evolvable in response to selection, whereas nonlinear gadgets are more likely to be fragile to small changes. (Evolutionary adaptations requiring significant changes to nonlinear gadgets are improbable and therefore require exponentially more time than simple adjustment of frequencies of alleles of linear effect.)
which seemed very elegant to me.
Upon further reflection, it seems that this might prima facie be in tension with the model of the genetics of intelligence where the minor alleles of the causal variants reduce intelligence. If we assume that natural selection has already been going on for so long that the major alleles are predominantly those which increase intelligence, that implies the existence of a long period of time over which one could imagine accumulation of random, fitness-neutral genetic drift, which plausibly results in epistatic and nonlinear mechanisms. It’s also not entirely clear that selective pressures are always biased toward the evolution of independent effects in a modular (rather than hierarchical or centrally controlled) framework. I quote from The frailty of adaptive hypotheses for the origins of organismal complexity [11]:
The hypothesis that expansions in the complexity of genomic architecture are largely driven by nonadaptive evolutionary forces is capable of explaining a wide range of previously disconnected observations (13,40) (Table 2). … The emergence of modular gene structure by the nonadaptive processes of duplication, degenerative mutation, and genetic drift is fully compatible with the known magnitudes of these forces in multicellular species (13). … [B]ecause of the mutational cost of allelic complexity, the likelihood of completion of semineutral modularization processes becomes negligible once 1/N_g becomes smaller than the excess mutational burden (66). Thus, contrary to popular belief, natural selection may not only be an insufficient mechanism for the origin of genetic modularity, but population-genetic environments that maximize the efficiency of natural selection may actually promote the opposite situation, alleles under unified transcriptional control.
See also An Evolutionary Perspective on Epistasis and the Missing Heritability [12]:
The relative importance between additive and non-additive genetic variance has been widely argued in quantitative genetics. By approaching this question from an evolutionary perspective we show that, while additive variance can be maintained under selection at a low level for some patterns of epistasis, the majority of the genetic variance that will persist is actually non-additive. … We demonstrate that the perception of independent additive effects comprising the majority of the genetic architecture of complex traits is biased upwards and that the search for causal variants in complex traits under selection is potentially underpowered by parameterising for additive effects alone.
