What does "heritable" mean? (You won't believe it when I tell you.)

What does "heritable" mean, in the context of genetics? If you've casually followed genetics stories over the last few years, you've surely seen the term, and probably assumed that it refers to the extent to which a particular trait is genetically inherited.

For instance, here are a few recent uses in the New York Times:

"Studies of twins show that homosexuality, especially among men, is quite heritable, meaning there is a genetic component to it." (4/10/07)

"Since personality is heritable, this would be a mechanism for Yanomamo nature to evolve and become fiercer than usual." (3/12/06)

"A genetic analysis using tissue samples from 185 dolphins, 13 of them spongers, showed that it was highly unlikely that sponging was a heritable trait." (6/7/05)

So why does noted biologist and geneticist Tim Tully tell author Matt Ridley: "I can prove in a court of law that heritability has nothing to do with biology"?

And how is it that the heritability of the number-of-fingers on the human hand is close to zero?

And how can height be 90% heritable and yet subject to extraordinary influence from nutrition?

And how can I.Q. be 50-60% heritable in some well-designed studies and almost 0% in other, equally valid studies?

Welcome to the truly bizarre world of heritability and its misuses. If you're a non-scientist like me, you will be stunned to learn the word's actual definition, and its implications. In fact, a close look at how the term is misused by journalists and scientists alike goes a long way to helping us understand why the public still has such a profound misunderstanding of genetics and human development.

One thing is clear enough, to me anyway: the terms "heritable" and "heritability" should never, ever be used in a general interest publication without extensive context.

So -- here now the definition, from Merriam-Webster:

***
heritability: the proportion of observed variation in a particular trait (as height) that can be attributed to inherited genetic factors in contrast to environmental ones.
***

Let's break that down:

"the proportion"
-- a heritability estimate comes from a statistical analysis of a given population, not the results of a biological investigation (that, of course, doesn't mean it's invalid; but it does convey some important limitations); further, heritability can only be estimated, never actually known -- in fact, the "proportion" heritability seeks to estimate is ultimately unknowable, for reasons I will explain below;

"of observed variation"
-- heritability cannot actually look directly at what causes a trait (partly because every complex trait has multiple causes); it can only look at the amount of variation in that trait in a particular population. As Matt Ridley explains, heritability studies come from "measuring how similar identical twins are, how different fraternals are, and how both identicals and fraternals turn out if separately adopted into different families."

-- since the observed variation is going to differ from population to population, a heritability estimate is only relevant to the particular population studied. "It is strictly a property of a particular population," University of Arizona's Bruce Walsh explains. "Different populations, even if closely related, can have very different heritabilities."

    And, adds Matt Ridley, heritability is "meaningless for any individual person." This is critical to keep in mind.

"in a particular trait that can be attributed to inherited genetic factors in contrast to environmental ones."
-- What traits are we talking about? Not the basic Mendelian stuff like eye and skin color, obviously, but  the really complex stuff like sexuality, aspects of intelligence, personality factors, etc. How are those traits formed, to the best of our understanding? From an extremely complex combination of, and interaction between, genes and the environment. The actual equation used by behavioral geneticists is: V(p) = V(a) + V(d) + V(i) + V(e) + V(g X e) + COV(G,E). In layman's terms, there are four basic variables.

1. One or more separate genes, some of them interacting with one another (taking height, for example: hypothetically, you might inherit four different genes that help determine your height, two of whom are completely independent actors, and the other two which also have independent influence AND which will interact with one another to effectively produce a unique fifth genetic influence).

     plus

2. Environment (your in-utero and post-utero nutrition will have a direct and significant effect upon your adult height)

     plus

3. Gene-environment interactions (some of your nutritional experience will have a specific interaction with your particular genes -- with a *different* outcome than the interaction my genes might have with the exact same nutrition)

     plus

4. Gene-environment covariation (these are genetic influences on environment; for example, let's say from early on you are a particularly active baby, and as a result you get placed in a daycare group with other particularly active babies; that group happens to get a slightly different snack chosen by a different teacher, and that affects your nutrition, which effects your height. In effect, you have some non-height genes having had unexpected impact on your environment, which in turn is going to have an effect on your height. This indirect but very real phenomenon is called "covariation).

It actually gets more complex than that, but that's a fair summary. What this means, of course, is that for any such traits the nature vs. nurture paradigm simply DOES NOT EXIST. It's a false choice, like saying a meal either comes from the ingredients or the cook. Instead, the development process is continuously and inseparably nature-and-nurture.

If nature and nurture are inextricably intertwined, how can we determine what portion of a certain trait is due to genetics? We can't. We can only come up with crude averages from population studies. As a matter of biology, such a proportion does not exist. Which is why the Human Genome Project website calls heritability "a statistical construct," and warns: "heritability statements provide no basis for predictions about the expression of the trait in question in any given individual."

Heritability is not a useless measure, by any means. It has wide applications in genetics and agriculture. But in the context of conveying scientific understanding to a general audience, it is inherently misleading.

Comments

[Jan Newton]

Hi David,

What a fascinating project. I wish you good luck with it and with your new book.

The kind of stuff you're posting here usually makes my eyes cross and puts me to sleep - but I have to say you've really caught my attention and have yet to put me to sleep. Maybe it's your ability to relate these concepts in "plain English" - that's not exactly put the right way, but I think you get what I mean. It is a gift you have, being able to render quite complicated concepts and research into understandable, interesting prose.

I wanted to just add my two cents worth - not apropros necessarily to this particular post but appropriate in general to your research theme. If I understand you correctly, then the Polgar sisters (Susan, Sophia and Judit) are the perfect test cases that prove your thesis. I just finished reading Cathy Forbes' 1992 book "The Polgar Sisters - Training or Genius?" and found it fascinating. Now I'm reading Susan Polgar's new book "Breaking Through" which basically covers the same time period (and more) that Forbes' book covered, but from Susan Polgar's own pespective. From what I've read so far, Susan P. has very little to say about the "experiment" that Laszlo Polgar undertook in training up three chess prodigies. But I would not expect her to - it's well known the Polgars did NOT look kindly on Forbes' book back in 1992, and have not forgiven Forbes to this day for, in their view, portraying Father Polgar as a sort of Dr. Frankenstein (not true) and the Polgar sisters as some sort of freaks (also not true).

Following the line of my thought about chess geniuses, Bobby Fischer is another example - although he was self-taught/self-motivated. If you haven't read Frank Brady's biography of Fischer in his early days (way prior to the infamous/famous World Chess Championship Match with Boris Spaasky), I highly recommend it. Although it was written in 1965 (and caused a permanent breach in their friendship), it contains the kind of insights and information that only a close friend could have. When he was about 6 years old, Fischer and his older sister Joan, taught themselves how to play chess from reading the instructions on the back of a box a chess set came in - Joan had bought the game to amuse themselves during the hours they were alone together while their single mother worked and left them alone. At about age 12 Fischer said "he just got good" - but that's not true. He had been intensively studying and playing chess for years before he "just got good." Practically from the moment he and Joan taught themselves the moves of the pieces, Bobby was "hooked" and studied everything about chess that he could get his hands on, included quite advanced theoretical books from the library. He taught himself Russian so that he could read the analysis in Russian chess magazines - the most advanced information available at the time, in those long-ago pre-internet days. The rest is, as we know, true history.

Josh Waitzkin is another child chess prodigy - made famous by the semi-fictional movie "Searching for Bobby Fischer" after Fred Waitzkin's 1984 book of the same title. I think Josh would certainly be someone worthwhile to study further - he seems to be an adept at just about anything he touches. Now, of course, he is no longer that cute little seven year old from 20 plus years ago, he's a grown and quite accomplished man. He did not pursue chess as a full-time living (he earned his IM title but did not pursue further) but he's been involved for a number of years in the "Chessmaster" software program. He's also still very cute :)

And there's Alexandra Kosteniuk. I've been a fan of hers since I started reporting news about women chessplayers back in 2001. She earned a WGM title when she was barely 14 and has since earned a male GM title, something only 10 or so women chessplayers have managed to do. Alexandra is currently rated number 5 on the Women's FIDE list with an ELO of 2515 but, with that ELO, she does not rank in the top 100 players (the cut-off there is an ELO of 2622 currently). I read her book "How I Became a Grandmaster at Age 14" a few years ago and so the details are blurry, but I do remember a very young Alexandra (4 years old?) saying something like this "Poppa, I'm tired, I don't want to play anymore" - but of course Poppa didn't listen to her, and made Alexandra continue to practice. In no way do I mean to depict A's father as a heartless monster, but he certainly was the driving force behind A's learning chess and was constantly there pushing first Alexandra and later, Oxana, the younger sister, to incessantly practice and play.

I've written about your research at our new Goddesschess blog (http://goddesschess.blogspot.com) I think, in these days and times, your work in this book will be vitally important and may, perhaps, inspire many public school systems to adopt the techniques that you will be writing about. You and the research mentioned here have inspired me to write an article about chessplayers (trained or genius?). I hope to have the article finished and published at Goddesschess soon - but you know how these things go - it always takes much longer than one thinks :)

I can't tell you how much I look forward to your upcoming book. If it's anything like your "The Immortal Game", which is simply fabulous (I've still got about a third of the book to go, sorry for taking so long reading it but you know about press of other business), I hope it's a run-away best seller.

All the best to you, David.

JanXena

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