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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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