are all human traits heritable?

A new article by Polderman et al. in Nature Genetics, nicely summed up by Jeremy:

is a meta-analysis of essentially every twin-based study of heritability of any trait between 1958 and 2012. The top-line coverage, encouraged by the authors’ press release, is:

One of the great tussles of science – whether our health is governed by nature or nurture – has been settled, and it is effectively a draw.

This is based on the fact that, across 17,804 traits in 28 “general trait domains,” the overall mean heritability was 49%. That prompted me to write:

Read on for why I think that, what value there is in the below-the-fold part of the article, and why I think this kind of work is in desperate need of an injection of theory.

In the conclusion, the article claims: “Our results provide compelling evidence that all human traits are heritable: not one trait had a weighted heritability estimate of zero.” But there are obvious selection, measurability, and publication biases that go into the “all human traits” gloss. Traits for which heritability is not plausible to begin with; that are difficult to measure with precision; or that are very rare are unlikely to be in the population of traits investigated for heritability.

This is not just a methodological quibble; it’s an important theoretical point. A reasonable, naive reader could easily read “everything about people” in the “all human traits” phrase. But that’s not a defensible reading of the science here.

The problem is analogous to Gabriel Abend’s excellent critique of moral psychology. Abend argues that moral psychology investigates only “thin” moral concepts, and relies on an unspecified and largely implausible aggregation mechanism to scale up from “thin” to “thick” moral concepts. “At least some moral concepts and properties — thick ones — are ontologically dependent on institutional and cultural facts” (147). By analogy here, at least some traits — thick ones — are ontologically dependent on institutional and cultural facts. In other words, some outcomes of substantive interest are not just agglomerations of simpler traits, each of which displays a certain degree of heritability. (I have suggested [here and here] that we think about these as the result of arbitrary-length causal chains consisting of serial interactions between genetic [G] and environmental [E] effects.)

But wait, I hear you cry. Didn’t you say 17,804 traits? That’s an awful lot!

Well yes, it is an awful lot. But the 49-51 nature-nurture tie claim masks enormous heterogeneity among “general trait domains,” and that variation is the most valuable piece of the article IMHO. The trait domain with the highest overall heritability is ophthalmological, coming in at 71% heritability; the three lowest were environment, reproduction and social values, at 29%, 31%, and 31% respectively (you have to go to page 50 of the supplemental tables to get those numbers). Digging further, supplementary table 33 shows that that Environment category contains everything from “Diazepam Effects” to “Acquired Absence of Organs, Not Elsewhere Classified,” to “Maintaining a Job” and “Intimate Relationships.” “Social Values” includes just three areas: “Individual Attitudes of Strangers,” “Societal Attitudes,” and “Religion and Spirituality.” One of the social attitudes sources is this article, which measures conservatism and religious identity among adolescents; another was this one, with 30 attitudinal measures, ranging from doing crossword puzzles to sweets to the death penalty, being assertive, and riding roller coasters. (Attitude factors “Intellectual Pursuits,” “Treatment of Criminals,” and “Sweets and Games” showed no genetic variance at all, by the way [p. 853].) By contrast, the Ophthalmological category contains things like intraocular pressure, corneal hysteresis, and ocular pulse amplitude–I suspect far more straightforward to conceptualize and to measure.

As I said, I do think there’s significant value in this work. But I think it is in the variance, not the mean. What’s interesting here is the degree of difference in heritability among the different traits and domains. While some of this is probably measurement error, I suspect a lot of it has to do with inadequate theorizing. Specifically, I think there are three theoretical tasks that deserve integration here:

  1. Complex, hierarchically related traits. These are the “thick” traits referred to above: characteristics of people that cannot be decomposed into collections of isolated traits, but rather are the result of configurations where the structure of the configuration is relevant.
  2. Complex, sequential causes. These are the indeterminate-length GxE chains I mentioned above. To the extent that the effect of one cause is conditional upon the effect(s) of other cause(s), partitioning the variance between genetic and environmental causes is theoretically problematic (not just empirically).
  3. Theorizing the environment. Environmental contributions to outcomes are treated as residual; with few exceptions, they are not actually measured, which means they’re not actually theorized either. But environmental causes are likely even more complex and hierarchical than are genetic causes, since there are so many intersecting dimensions of the environments individuals experience.

Author: andrewperrin

University of North Carolina, Chapel Hill

8 thoughts on “are all human traits heritable?”

  1. What irks me about behavioral stuff in genetic studies is not the eugenics implications, but that people don’t talk about the effects as maximum heritability, which is what I think they are. For example, this from the NYT the other day: http://www.nytimes.com/2015/05/24/opinion/sunday/infidelity-lurks-in-your-genes.html, says: “Forty percent of the variation in promiscuous behavior in women could be attributed to genes.” Shouldn’t they say that’s the maximum genetic effect? Because consider what else is heritable besides the behavioral impulses of the person whose genes you’re measuring: lots of things that affect someone’s attractiveness to other people, thus influencing the odds of promiscuous behavior through the behavior of others. If facial symmetry or breast size affects the odds of being approached in a bar, is that a promiscuity gene? (And if so, does it work in some bars more than others?) Similarly, if twins are likely to be correlated on an education measure like skipping grades, that will reflect not only their academic ability but also correlation in other things that affect how they’re viewed and treated by others — height, for example, or voice quality, or other personality traits that only affect educational attainment in some contexts. So the twin effect on skipping grades gives a maximum effect. (If you’ve heard me complain about this before, sorry. Thanks for listening. If I’m wrong, please advise.)

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    1. Phil,

      Your comment is basically a special case of the general philosophical question of where a mechanism ends and a control begins. I’m generally on #teammechanism and I get the impression you’re generally on #teamcontrol. In some sense, it’s a semantic issue: infidelity is heritable regardless of whether the mechanism is being lustful, or being attractive, or having low self-esteem, or having an arbitrary stigma that others take as indicative of sexual receptiveness, or having personality traits well-suited to occupations involving travel, or whatever. In another sense, what you’re implying is that when people hear that infidelity is heritable they imagine the first type of causal pathway and so the others don’t count as real heritability of infidelity. My opinion is that it’s worth clarifying that the heritability coefficient (which is basically just a correlation coefficient, not a beta, let alone a beta netting out indirect effects) can have many causal pathways, including indirect ones, but that doesn’t mean they don’t count.

      In other words,

      And, that’s if we generously assume that most of the personality heritability effect occurs through the causal pathway of physical appearance, a premise that is hard to reconcile with studies of unrelated look-alikes, who as a rule show very low quasi-heritability, whereas were it all indirect effects of appearance they should show high quasi-heritability.
      (https://lesacreduprintemps19.files.wordpress.com/2012/12/personality-look-alike-twins.pdf).

      Also, FWIW, I have an old post that meditates on how we interpret heritability if we assume things like cumulative advantage.
      https://codeandculture.wordpress.com/2010/10/20/or-you-could-just-do-regressions/

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  2. The abstract of an article explains its point: “For a majority (69%) of traits, the observed twin correlations are consistent with a simple and parsimonious model where twin resemblance is solely due to additive genetic variation. The data are inconsistent with substantial influences from shared environment or non-additive genetic variation. This study provides the most comprehensive analysis of the causes of individual differences in human traits thus far and will guide future gene-mapping efforts.”

    It is indeed the case that the extent of additivity of traits has implications for strategies of doing genetic research, and, specifically, for the scope and promise of certain statistical genetics techniques for which some of the paper’s authors have been important contributors. So, while press releases are what they are, that is the point of this paper. (Whether the twin design is strong enough to support the inference that they want to draw regarding additivity is debatable, but it’s a different debate than what sociologists will have.)

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    1. Jeremy, I think you missed part of the abstract: “…the relative importance and specific nature of the influences of genes and environment on human traits remain controversial. …. Estimates of heritability cluster strongly within functional domains, and across all traits the reported heritability is 49%.”

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      1. Whether one wishes to understand the point of the article is their choice; all I can do is explain it. The quoted statement “Estimates of heritability cluster strongly within functional domains” is a statement about covariance, and as such about variance, and then the sentence also provides the grand mean.

        If the worry is that there are statistical or behavioral geneticists out there who believe that the heritability of every measurable human trait is the same, or that it is useful about to talk as if the heritability of every measurable human trait is the same, one can breathe easy. There are no such people.

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      2. Jeremy, I’m sorry if I pissed you off. That was not my intention.

        As I wrote, I agree that the covariance is the interesting finding in this paper. However, I also think that the press release (presumably approved by the authors) is very misleading, and I think there is significant overreach from “a whole heap of individual traits” to “all human traits.” That doesn’t undermine the value of the covariance finding. And no, I don’t think there are statistical or behavioral geneticists who think all heritabilities are equal. I do think we should hold claims about causality based on genetics to similar standards of evidence and theory to claims about causality based on things other than genetics.

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      3. I agree fully that the press release doesn’t correctly characterize the genuine point of the study, and, more generally, that press releases can be so maddening in this respect I don’t know where to begin.

        (Apologies for being pissy; I will confess to a general pissiness coming out of every pore as we head into the crunch time of the quarter here. Summer soon!)

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  3. Thanks, that’s very helpful to me. (And great clip.) I think what you assume your readers understand (which I would like to) is what’s missing from things like that NYT article: the context affects the heritability. Infidelity is only heritable if the genes it’s riding on work in this social context. So I agree it’s semantic if you’re extrapolating only to the same social context. But in the genre of reporting of that article (and sorry to get away from Andy’s nice post here), “genetic” is taken to mean extra-social, as in, it would have had the same effect in caveman times or in North Korea, as in, the opposite of the “social construction” obsessives that appear as anti-science villains in this story.

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