Turkheimer's Projects: Genetics and Human Agency | Statistical and Structural Replication

Posted at 12:13h in Uncategorized by Eric Turkheimer 0 Comments

In the first paper I wrote about GWAS, in 2012, I reviewed a paper about GWAS of height by Weedon et al.  After correcting for population stratification, Weedon et al identified a handful of SNPs with genome-wide significant correlations with height (news at the time), and declared, “This means that the associations are likely to reflect true biological effects on height.” (p. 580) I noted that they never paused to say what they meant by a “true biological effect”, and suggested that they were confusing two things: statistical significance, which involves testing the null hypothesis that an association between SNP and phenotype was the result of sampling error, and hypotheses about ill-defined “true biological effects,” which, whatever they might be, couldn’t be confirmed by null hypothesis significance testing.  True biological effects are structural hypotheses about the nature of the relationship between the SNP and the phenotype; statistical significance may be a necessary condition for structural significance, but they are certainly not sufficient.

This distinction has never been fully resolved in the GWAS literature, and we worked through it again last week, after I gave a talk at APS about heritability. I went through my usual arguments about the limitations of the heritability concept, including the idea that the heritability of a trait (itself a problem, because traits don’t “have” fixed heritabilies) doesn’t tell us anything about the likelihood of success for gene-finding. Michel Nivard demurred.

Depending on your definition of gene finding success, higher h2 is on of the ingredients of higher GWAS power (GWAS hits prob not what you mean, as you like your genes to be specific and come with a plausible path to effecting the phenotype before you call them “genes for”?)

— Michel Nivard (@michelnivard) May 25, 2019

I responded with a question:

Question: When you look at GWAS of some trait over time, are there good, replicable SNPs you believe in? “Oh, there’s rs655432, that always shows up!” And what of @StevePittelli ‘s case that “replication” is outmoded, because every new study is just added to the meta-analysis?

— Eric Turkheimer (@ent3c) May 26, 2019

At that point an argument about replication broke out. Michel pointed out that a recent study of depression had replicated most of the genome-wide significant hits from an earlier study. Steve Pittelli objected, saying that they weren’t really replications, because they had been tested at a much less stringent probability level than the original. Steve was then taken to task for not understanding the math behind Type I error-correction in replication. Tempers flared.

The two sides were talking past each other because one side was talking about statistical replication, and the other about structural replication. For statistical replication, it makes sense to conduct discovery at genome-wide significance, and then when the field of possible hypotheses has been reduced, to conduct replication analyses at ordinary significance levels. This is fine, but it must be borne in mind that only a very limited hypothesis is being tested, whether the association begtween the SNP and the phenotype is different than zero for reasons other than sampling error. Although I couldn’t find the information in Howard et al., presumably the effect sizes of the “replicated” SNPs were considerably smaller than they were in the original GWAS, because of the winner’s curse, but that doesn’t matter if all you care about is statistical replication.

Pittelli wants more (https://tinyurl.com/y3kn8p3n) and although I agree with him that there are more important hypotheses to think about, I part ways about how the case should be argued. Steve is always arguing that SNP associations are “false positives,” by which I think he means that the observed SNP correlations don’t really exist, that they would go away if the GWAS people didn’t spin them into existence. I think more or less the exact opposite. Based on a combination of the First and Fourth laws of behavior genetics, the Meehlian crud factor, and modern thinking about omnigenics, I am quite happy with the possibility that all SNPs have statistically significant associations with behavioral phenotypes, if you make the discovery sample big enough. It’s a lot like the old discussions about twin studies, where opponents of hereditarian conclusions about them thought they had to deny the results of the twin studies themselves, the very idea that rMZ > rDZ, based on the equal environments assumption and the like. That was a losing battle: rMZ is greater than rDZ, for reasons that can be very broadly characterized as genetic. The hill to die on involves the question of what heritability means, not about the heritability itself.

There is a lesson here for the GWAS community as well.  Simply establishing tiny, uninterpreted, but “significant” associations between SNPs and phenotypes will not lead anywhere interesting. Sometime in the future, some wag will declare the idea that all SNPs are associated with all complex outcomes, “The Fifteenth Law of Behavior Genetics.” The kind of GWAS that has recently evolved, the kind that leads to that foolishness about dog ownership, has an advantage that I long ago identified in twin studies: it can’t fail. If all you want to do is show that dog ownership is heritable, that there are SNPs with significant correlations that are statistically replicable, that it has genetic correlations with other phenotypes, then you are in luck, because it will work every time. Go ahead, do cat ownership, do golden retriever ownership, knock yourself out. But science that works every time is pointless science.

By way of contrast, consider FTO, a gene that was identified with GWAS, and which appears to play an important role in obesity. FTO and the SNPs associated with it are more than just statistically replicable. The FTO-related SNPs aren’t just p

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