Audio note: this article contains 54 uses of latex notation, so the narration may be difficult to follow. There's a link to the original text in the episode description.

0.1 tl;dr

This is an opinionated but hopefully beginner-friendly discussion of heritability: what is it, what do we know about it, and how we should think about it? I structure my discussion around five contexts in which people talk about the heritability of a trait or outcome:

(Section 1) The context of guessing someone's likely adult traits (disease risk, personality, etc.) based on their family history and childhood environment.
- …which gets us into twin and adoption studies, the “ACE” model and its limitations and interpretations, and more.
(Section 2) The context of assessing whether it's plausible that some parenting or societal “intervention” (hugs and encouragement, getting divorced, imparting sage advice, parochial school, etc.) will systematically change [...]

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Outline:

(00:17) 0.1 tl;dr

(02:57) 0.2 Introduction

(05:28) 0.3 What is heritability?

(08:47) 1. Maybe you care about heritability because: you're trying to guess someone's likely adult traits based on their family history, and childhood environment

(10:03) 1.1 The ACE model, and the classic twin study

(13:53) 1.2 What do these studies find?

(14:39) 1.3 What does C (shared environment) ≈ 0% mean?

(16:37) 1.4 What is E, really?

(18:34) 1.5 Twin study assumptions

(18:55) 1.5.1 The simplest twin study analysis assumes that mitochondrial DNA isn't important

(19:31) 1.5.2 The simplest twin study analysis assumes that assortative mating isn't important

(19:55) 1.5.3 The simplest twin study analysis assumes no interaction between genes and shared environment

(22:18) 1.5.4 The simplest twin study analysis involves the Equal Environment Assumption (EEA)

(27:14) 1.5.5 The simplest twin study analysis assumes there's no nonlinear gene × gene (epistatic) interactions

(31:28) 1.5.6 Summary: what do we make of these assumptions?

(33:02) 1.6 Side-note on comparing parents to children

(33:45) 2. Maybe you care about heritability because: you're trying to figure out whether some parenting or societal intervention will have a desired effect

(35:25) 2.1 Caveat 1: The rule-of-thumb only applies within the distribution of reasonably common middle-class child-rearing practices

(38:42) 2.2 Caveat 2: There are in fact some outcomes for which shared environment (C) explains a large part of the population variation.

(38:54) 2.2.1 Setting defaults

(40:12) 2.2.2 Seeding ideas and creating possibilities

(42:50) 2.2.3 Special case: birth order effects

(47:33) 2.2.4 Stuff that happens during childhood

(50:57) 2.3 Caveat 3: Adult decisions have lots of effect on kids, not all of which will show up in surveys of adult intelligence, personality, health, etc.

(53:04) 2.4 Caveat 4: Good effects are good, and bad effects are bad, even if they amount to a small fraction of the population variation

(55:47) 2.5 Implications

(55:51) 2.5.1 Children are active agents, not passive recipients of acculturation

(01:01:21) 2.5.2 Anecdotes and studies about childhood that don't control for genetics are garbage

(01:03:18) 3. Maybe you care about heritability because: you're trying to figure out whether you can change something about yourself through free will

(01:05:08) 4. Maybe you care about heritability because: you want to create or use polygenic scores (PGSs)

(01:07:10) 4.1 Single-Nucleotide Polymorphisms (SNPs)

(01:08:35) 4.2 Genome-Wide Association Studies (GWASs), Polygenic scores (PGSs), and the Missing Heritability Problem

(01:21:09) 4.3 Missing Heritability Problem: Three main categories of explanation

(01:21:17) 4.3.1 Possibility 1: Twin and adoption studies are methodologically flawed, indeed so wildly flawed that their results can be off by a factor of ≳10, or even entirely spurious

(01:22:27) 4.3.2 Possibility 2: GWAS technical limitations--rare variants, copy number variation, insufficient sample size, etc.

(01:24:18) 4.3.3 Possibility 3: Epistasis--a nonlinear map from genomes to outcomes

(01:31:49) 4.4 Missing Heritability Problem: My take

(01:31:55) 4.4.1 Analysis plan

(01:34:29) 4.4.2 Applying that analysis plan to different traits and outcomes

(01:41:19) 4.4.3 My rebuttal to some papers arguing against epistasis being a big factor in human outcomes

(01:44:30) 4.5 Implications for using polygenic scores (PGSs) to get certain outcomes

(01:47:32) 5. Maybe you care about heritability because: you hope to learn something about how schizophrenia, extroversion, and other human traits work, from the genes that cause them

(01:53:18) 6. Other things

(01:54:39) 7. Conclusion

The original text contained 10 footnotes which were omitted from this narration.

The original text contained 1 image which was described by AI.

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First published:
January 14th, 2025

Source:
https://www.lesswrong.com/posts/xXtDCeYLBR88QWebJ/heritability-five-battles

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“Heritability: Five Battles” by Steven Byrnes

LessWrong (30+ Karma)