The Heritability of Everything

The gold standard in heritability estimates is the twin study, which involves looking at identical and fraternal twins, raised together or apart. This allows the cleanest test of decomposing the variance in observed traits into genetics, shared environment (factors equally affecting all children raised together), and non-shared environment (everything else, including random noise) contributions.

Generally it is assumed that the effect of parenting is equated with the shared environment, though there is clear evidence that parenting can differ substantially between siblings of the same parents and account for a significant fraction of non-shared environment, and the shared environment by definition also captures e.g. the neighborhood in which you grow up. Generally there are many caveats to apply to heritability estimates, particularly that they are only defined within a given population and may not apply as well in extreme cases, but nonetheless they are our best estimates as to the effects of genetics, and the effect is undeniably large.

An extremely ambitious meta-analysis of all twin studies was published in May 2015, reporting heritability estimates from 2,748 studies featuring over 2 million twin pairs, encompassing virtually every published study to date. The researchers have made a data visualization tool available if you wish to dig down into various aspects of the study, though it’s fairly opaque if you’re not familiar with the field’s jargon.

Across very broad domains of health outcomes, almost everything falls within the 40-60% heritability range, with cancer as a representative example being 46% heritable. Similarly, neurological variables show about 50% heritability (with little shared environment involvement), while cognitive and psychiatric outcomes are similarly heritable, but also have a nearly 20% shared environment component. Social values appeared to be 31% heritable, but shared environment played nearly as big a role at 27% explained. Similarly, social interactions were 32% heritable, with a somewhat smaller shared environment component of 18%.

Drilling down into more specific categories of interest, intellectual functions broadly were highly heritable at 67%, while more specific executive function metrics were 51% heritable with a high 24% shared environment contribution. Mood disorders were highly variable, from bipolar being 68% heritable to depressive episodes being 34% heritable. Height and weight showed 63% heritability, with relatively large 30% and 20% shared environment contributions respectively. The more specific values and social variables were mostly in line with the overall findings. Tendency towards religion and spirituality was 31% heritable with an even larger 35% shared environment component. Basic interpersonal interactions were similar, with 30% heritability but 36% determined by shared environment.

To summarize, basic variables in terms of intelligence, height, and weight are primarily determined by the genetic contributions. Most health and psychiatric outcomes fell somewhere in the middle, but still showed roughly half of variance explained by genetics. Variables relating to fundamental values (e.g. religion, politics) and social interactions (e.g. emotional intelligence, relationships) were by far the most malleable traits, with roughly equal contributions from genetics and shared environment.

Summary of How To Talk So Kids Will Listen & Listen So Kids Can Talk

How To Talk So Kids Will Listen & Listen So Kids Can Talk is a parenting / communication book written by Adele Faber and Elaine Mazlish. While this book is specifically intended for parents to have better relationships with their children, the vast majority of the advice contained within applies universally to all interactions, and I have written this summary specifically to abstract away from parent-child relationships. I consider the first chapter alone better at helping people internalize the principles behind nonviolent communication than Rosenberg’s entire book. HTTSKWL is currently by far my most highly recommended communications book, and because it is appealing to parents and children it is a remarkably easy read.

Note that unlike most books, this one contains a very high ratio of exercises and prompts and anecdotes relative to its advice. The authors recommend going through the book slowly, and doing all the exercises. This summary will only contain their explicit instructions – I highly recommend buying a copy of the book and completing it. The many specific example conversations will give a much better understanding of the principles I lay out here than I can convey in a summary.

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Parenting and the Non-Shared Environment

There is a major thread in the parenting literature that claims, in short, that parenting (short of outright abuse) has little to no effect on adult outcomes that we care about. Two exemplars in this category include The Nurture Assumption and Selfish Reasons to Have More Kids. This claim is largely based on twin and adoption studies, which allow us to attribute the observed variance in traits to genetic, shared environment, and non-shared environment factors. The general pattern is that lots of traits are about half genetic and half non-shared environment, with little contribution from shared environment. There is a major embedded assumption in this line of reasoning: that parenting effects are mainly in the shared environment. It turns out that this assumption is not a particularly good one.

A review article on research into the non-shared environment was released about two years ago, and it provides some fascinating data on this subject. I will quote some sections at length, but the entire paper is worth reading. Here is the first excerpt of interest:

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