Obesity Makes People Stupid...About Heritability

Heritability: I do not think it means what you think it means.

There's been a spate of posts about obesity, started by a post by Megan McArdle. In these posts, a high heritability for obesity is bandied about (0.9!!! ZOMG!! TEH GENEZ R MAKIN U FAT!). But this demonstrates a lack of understanding of what heritability estimates mean--and, more importantly, what they do not mean.

A couple of years ago, the Three-Toed Sloth wrote a wonderful post about heritability, and I'll quote liberally from it here (instead of rephrasing it inelegantly) and add some additional commentary. I should note that most people familiar with quantitative genetics will not see anything new here--as Three-Toed Sloth notes, this was covered over thirty years ago. Moving right along....

First, Herr Doktor Three-Toed Sloth gives a very succinct definition of what heritability does and does not tell us (italics original; boldface mine):

Heritability is a technical measure of how much of the variance in a quantitative trait (such as IQ) is associated with genetic differences, in a population with a certain distribution of genotypes and environments. Under some very strong simplifying assumptions, quantitative geneticists use it to calculate the changes to be expected from artificial or natural selection in a statistically steady environment. It says nothing about how much the over-all level of the trait is under genetic control, and it says nothing about how much the trait can change under environmental interventions.

One of the mistakes that is commonly made regarding heritability is that a high heritability (heritability ranges from zero to 1.0) means that most of the absolute value of a trait, such as BMI, is genetic. Not exactly (italics original; boldface mine):

Saying a trait is highly heritable is saying that, in a given distribution of genotypes and environments, most of the variance in that trait is associated with genetic differences. Maybe the most important point I'll make here is that this is not the same most of the value of the trait being genetically controlled. The textbook example is that (essentially) all of the variance in the number of eyes, hearts, hands, kidneys, heads, etc. people have is environmental. (There are very, very few mutations which alter how many eyes people have, because those are strongly selected against, but people do lose eyes to environmental causes, such as accident, disease, torture, etc.) The heritability of these numbers is about as close to zero as possible, but the genetic control of them is about as absolute as possible.

There's a flip side to this--heritability estimates are made within the context of the environments in which the estimates are made:

Similarly, heritability says nothing about malleability, about how much or how easily the trait changes in response to environmental manipulations: heritability is defined with respect to a given distribution of environments, and does not predict the response to environmental changes....

What heritability does predict is the response to selection, in a constant distribution of environments. This is why quantitative geneticists developed and retained the concept. If a population is subjected to directional selection on a trait, whether the selection is natural or artificial, and the trait follows the classical decomposition into additive, uncorrelated components, the degree to which the genetic component of the trait changes will depend on the intensity of selection, the variance in the trait, and its heritability. The response to selection, the phenotypic change in the next generation, will be large if the selection pressure, the trait's variance, and the trait's heritability are all high, assuming that the distribution of environments is held fixed and uncorrelated with genotype.

But the crux of the matter is can not environmental changes alter phenotypes, even if those phenotypes are highly heritable? Well, yes:

It's banging on an often-sounded drum, but it's worth doing because it makes the point clearly: height is heritable, and estimates for the population of developed countries put the heritability around 0.8. Moreover, tall people tend to be at something of a reproductive advantage. Applying the standard formulas for response to selection, we straightforwardly predict that average height should increase. If we select a population without a lot of immigration or emigration to mess this up, say 20th century Norway, we find that that's true: the average height of Norwegian men increased by about 10 centimeters over the century. But that's much more than selection can account for. Doing things by discrete generations, rather than in continuous time, height grew by 2.5 centimeters per generation. (The conclusion is not substantially altered by going to continuous time.) If the heritability of height is 0.8, for this change to be due entirely to selection, the average Norwegian parent must have been 3 centimeters taller than the average Norwegian [today]. This, needless to say, was not how it happened; the change was almost entirely environmental. The moral is that highly heritable traits with an indubitable genetic basis can be highly responsive to changes in environment (such as nutrition, disease, environmental influences on hormone levels, etc.).

Conversely, the very low heritability of eye number does not tell us that it is easy to increase how many eyes someone has by exercise, education and training, manipulating diet, manipulating ambient light, trepanation, etc.

So when McArdle claims:

Twin studies and adoptive studies show that the overwhelming determinant of your weight is not your willpower; it's your genes. The heritability of weight is between .75 and .85. The heritability of height is between .9 and .95. And the older you are, the more heritable weight is.

...that's irrelevant. Phenotypes can be very environmentally malleable even though they are very heritable (see the aforementioned Norwegians). That is, diet can make a difference (if not necessarily in adulthood).

I would add that an additional complication is that the irreversibility of a trait is often confused with its genetic basis. In other words, if childhood (or young adult) behaviors and environments 'lock in' (or often do so) obesity in adulthood, that does not imply that obesity is 'genetic', in the same way that food deprivation as a child which stunts growth is also not 'genetic.'

What upsets me the most about all of this is that we know obesity is correlated with adult-onset diabetes. More importantly, we know that adult-onset diabetes, while having a genetic component, can be controlled with diet, yet this kind of obesity genetics-as-destiny fatalism gives cover to a lot of people with unhealthy habits.

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What upsets me the most about all of this is that we know obesity is correlated with adult-onset diabetes. More importantly, we know that adult-onset diabetes, while having a genetic component, can be controlled with diet, yet this kind of obesity genetics-as-destiny fatalism gives cover to a lot of people with unhealthy habits.
Not to mention a system that enables and encourages such unhealthy eating habits.

By ponderingfool (not verified) on 04 Aug 2009 #permalink

What gets me is when people use the idea of genetic obesity to justify eating more food. "Hey, I NEED a second helping; I'm a big guy!"

They've, um, got it backwards...

Reread the post. I did not say that environment wasn't interacting with genes--indeed, that was the entire purpose of the height comparison. I was responding to people who claim that individual outcomes can't be rooted in genetics because after all, there were no fat people in Auschwitz, plus we're all getting fatter. I understand heritability quite well, thanks--or at least, I already knew everything you wrote before you wrote it, and nothing in my post contradicts it.

Genetics is very much a factor. I'm 59, I've never 'watched my diet' until the last few years because of cholesterol concerns.

I am thinner than most of my family, friends and co-workers yet eat more than many of them. I still own and can fit the same work uniform from a job I had at 21.

It certainly wasn't self control, or healthy eating.

Heritability is a technical measure of how much of the variance in a quantitative trait (such as IQ) is associated with genetic differences

Even that seems too strong a statement to make. Unless the definition of "heritability" has changed, it's a measure of the inter-generational variability of a feature -- which may be genetic, or not. You can try to control for non-genetic elements of heritability, but you can't ever eliminate them completely -- particularly in studies of humans.

Or to put it another way, heritability is the maximum genetic contribution, unless you have particularly favorable model where you can switch the parents who gestate and raise your subjects and therefore control every environmental detail. Obviously, you can't do that in a human BMI study.

And that's not even including the covariance between genetic and environmental conditions!

Jay: still caught up in old-fashioned Descartian dualism?

Something may be out of your control -- yet not be "genetic". It could be due to placental influences, or some pattern in your early childhood. Who knows without a lot of research? But the fact that your behavior is out of your control does not imply that it is genetic -- just that for some reason the architecture of some part of your anatomy (such as your brain) is locked by its history.

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I was with you all the way up until here:

"What upsets me...yet this kind of obesity genetics-as-destiny fatalism gives cover to a lot of people with unhealthy habits"

1. Just because it ain't genetic doesn't mean that it is within people's control in any reasonable sense
2. It is misguided to believe that if we don't give people "cover" they will start to adopt "healthy habits" and, implicitly, stop being obese.

There are many great benefits to exercise and healthy eating, but turning a fat person into a skinny one isn't one of those benefits. Encouraging people to feel that their weight is within their control tends to just set them up for unrealistic expectations, that in turn undermines their motivation to maintain healthy habits. The path to widespread healthy habits involves decoupling the issue from weight and encouraging people to accept their bodies as being worthy of care, in their current form.

Being "upset" that people aren't sufficiently ashamed of their weight because we allow them too much "cover" is your personal issue that maybe you should work out with yourself. I know you said that the cover was for unhealthy habits, but you are confounding unhealthy habits with being obese. This might be true for you personally, but that doesn't make it true universally.