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.

Religious Fasting Traditions

If you’ve ever talked to me about nutrition, you would know that I’m a huge proponent of fasting. I knew that cultural and religious traditions throughout the world had incorporated some form of fasting practice into their doctrines, though I had not systematically tried to document it myself at the time. There may be a decent review article on this somewhere, but I decided to sit down and do a little research myself last year, and these are my notes from that exercise.

tl;dr: there are lots of different kinds of fasting. Some commonalities include the use of sunrise/sunset, fasting as a spiritual practice (opposed to mere starvation) including prayer and charity, proscribed feasting days/periods, and not drinking water either. Longer-term fasts usually restrict eating during daylight hours and/or restricted types of food during the night, and these appear to be the most common. Fasts of 1-2 days can involve complete cessation of both food and water. The strictest fast is 2 meals over 5 days. Fasting is at most 250 days/year, and 1-2 times per week or month is very common. Exceptions for the young, the old, the sick, pregnant and breastfeeding mothers, laborers, and travelers are relatively common. Not every religion has a fasting tradition.

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Preventing Peanut Allergies

My default position when I started looking into peanut allergies was that exposure to potential allergens (particularly the ones outside our evolutionary heritage) would sensitize children to that allergen in the future. Eliezer once described changing your mind as the penultimate technique of rationality, and I always enjoy the opportunity to stretch that particular muscle. But I’m getting ahead of myself here…

Peanut allergies are one of the most common, and yet also most severe, food allergies. It tends to begin very early (unlike seafood allergies), and persist throughout life (unlike milk and soy allergies). The incidence is hard to measure, but potentially up to a few percent of the population will suffer from them over time. This may not seem large in an absolute sense, but it is one of the more common diseases, and is potentially fatal, requiring extensive lifestyle modification to avoid exposure. Allergies themselves, in one form or another, are much more common and often annoying, so if any underlying mechanism can be revealed and averted, so much the better.

I am involved with a small rationality and parenting mailing list, and in one thread I casually mentioned that we planned to exclusively breastfeed through 6 months, in part because of allergies. The benefits of (in some cases exclusive) breastfeeding are well established, and the WHO recommends exclusive breastfeeding up through 6 months. Furthermore hunter-gatherers don’t seem to supplement food until at least 6 months of age. That part is not so controversial. But I offhandedly said I’d heard the advice to avoid allergenic foods until two years of age. That prompted a response. Particularly a link to this paper.

To summarize the methodology, the researchers sent out surveys to a few thousand Jewish families in Israel and the UK, asking them about their weaning behavior and incidence of various allergies and other atopic disease. They had some very stark findings.

[Update: the randomized controlled trials I mentioned were coming have finally started to arrive – and the LEAP study from the UK finds a dramatic 81% decrease in peanut allergies from early exposure.]

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