Twin Studies

Twin studies and adoption studies have recently come in for criticism as being far less useful than we have though. Scott Alexander has gotten involved with the discussion with Sasha Gusev and others, and seems to be partly convinced by this.  He continues to see value in the hereditarian side and is his usual scrupulous self in saying "here are the arguments.  Here are the objections. Here are the objections to the objections. Here is another possible explanation. I think you have to be a professional, or at least much smarter than I am, to get the arguments here. I have an added bias because Gusev quickly becomes dismissive and insulting with people who don't agree with him. That puts me off, but that often happens in the history of science.  I quote the ACX paragraphs here with links included in case you want to get into this.

Lots more good discussion about missing heritability. Sasha Gusev argues that twin studies might be a poor guide to anything else if there are many gene-gene interactions. That is, if we take the difference between identical twins (who share 100% of their genes and therefore 100% of their interactions) and fraternal twins (who share 50% of their genes and therefore fewer than 50% of their interactions), and incorrectly extrapolate it to other differences using a model that assumes there are no interactions, we will overestimate the size of (non-interaction) genetic effects. Most studies find that there are few gene x gene interactions, but commenters convinced me last time that this might be an artifact of the studies being bad.

And Unboxing Politics argues (against me in particular) that although it superficially looks like adoption and twin studies sort of agree, when you adjust out their known biases, it moves twin studies further up and adoption studies further down, such that now they disagree again (the objection I would have made is their Objection 2, which I think they at least somewhat refute). This is a good argument; without spending several hours checking all of their claims, my only weak partial objection is that I don’t think assortative mating can play quite the role they expect, because there seem to be the same twin/RDR differences even on traits where believing in assortative mating is absurd (like kidney function). But if you replaced it with Sasha’s argument above, you might have a pretty good case!

On the pro-hereditarian side, East Hunter takes aim at gene x environment correlations, comes down somewhere in the middle, and Sebastian Jensen continues banging the drum of how most objections to twin studies don’t work. I think these are good attempts to buttress existing research but don’t fundamentally change anything or respond to the novel arguments above.

And Emil Kirkegaard points out that the observed SNP heritability of facial features is only 23%. He argues that since it seems like facial features are extremely heritable, this reinforces the argument that SNP heritability numbers are too low (and therefore twin study numbers are more likely defensible). But should we be sure that facial features are more than 23% heritable? His argument is that identical twins have identical faces, but this might be vulnerable to Gusev’s point about interactions. Maybe a better argument would be that it seems very hard for shared environment to affect facial features (with a few exceptions like fetal alcohol syndrome), and facial features seem more than 23% heritable just by normal “he looks like his brother” common-sense observation?

One interesting potential consequence of this research: if we ever fully understand how genes affect faces, then embryo selection companies could show people what each of their potential future kids might look like. I suggest they not do this: it might spook me into becoming pro-life.