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Personality and Individual Dierences 200 (2023) 111896
0191-8869/© 2022 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Heritability and Etiology: Heritability estimates can provide causally
relevant information
Jonathan Egeland
University of Agder, Norway
ARTICLE INFO
Keywords:
Heritability
Causation
Locality
Twin study
GWAS
ABSTRACT
Can heritability estimates provide causal information? This paper argues for an afrmative answer: since a non-
nil heritability estimate satises certain characteristic properties of causation (i.e., association, manipulability,
and counterfactual dependence), it increases the probability that the relation between genotypic variance and
phenotypic variance is (at least partly) causal. Contrary to earlier proposals in the literature, the argument does
not assume the correctness of any particular conception of the nature of causation, rather focusing on properties
that are characteristic of causal relationships. The argument is defended against Lewontin's (1974) locality ob-
jection and Kaplan and Turkheimer's (2021) recent critique of Genome-Wide Association Studies (GWAS).
1. Introduction
The discipline of behavioral genetics aims to investigate and un-
derstand the relative inuences of genes and environment on behavioral
traits. With respect to such investigations, there are two fundamental
questions that must be kept separate:
1. What proportion of an individual's phenotype P are genes responsible
for, and what proportion is environment responsible for?
2. What proportion of the variance of phenotype P is genetic variance
responsible for, and what proportion is environmental variance
responsible for?
1
There is general agreement among researchers working on concep-
tual or methodological issues in behavioral genetics that the rst ques-
tion cannot be answered (e.g., Dowens & Lucas, 2020; Grifths et al.,
2005; Lewontin, 1974; Pearson, 2007; Sober, 2001). For example, if a
person has a bodyweight of 70 kg, it does not make any sense to say that
either genes or environment is responsible for a certain proportion of the
person's bodyweight (such as that 50 kg are due to genes and 20 due to
environment). Since both genes and environment necessarily contribute
in intricate interaction with each other to the development of the
phenotype in question, it is impossible to partition the phenotype into
portions that are due only to genes or environment respectively. Genes
and environment, nature and nurture, function as interwoven strands of
thread in the un-untieable Gordian knot that is the development of any
phenotypic trait (cf. Bateson, 2001, p. 565; Nufeld, 2002, p. 40).
On the other hand, when it comes to the second question opinions
differ as to whether it can be answered using current statistical methods
and technologies. More specically, the disagreement has to with how
heritability measures should be interpreted, and whether they can pro-
vide any information about the causal effects of genetic variance on
phenotypic variance. This paper develops an argument to the effect that
heritability measures indeed can provide causally relevant information
about the sources of trait variance and, moreover, it shows that the
argument is not vulnerable to either Lewontin's (1974) locality objection
or Kaplan and Turkheimer's (2021) recent critique of Genome-Wide
Association Studies (GWAS).
The paper is structured as follows. Section 2 lays the groundwork by
explaining the basics of heritability estimation and why there is so much
disagreement about how such estimates should be interpreted. Section 3
develops the argument that heritability measures can provide causally
relevant information. Section 4 shows that although heritability mea-
sures are contextual and local, it does not follow that they are scientif-
ically useless and without causal relevance. Section 5 shows that Kaplan
and Turkheimer's recent critique of GWAS is unsound. Section 6 con-
cludes and offers some reections on the threats posed by gene-
environment (G-E) interaction, G-E covariation, and indirect genetic
effects (IGEs).
E-mail address: jonathan.egeland@uia.no.
1
The rst question is about the phenotype of a particular individual, whereas the second is about the occurrence of certain phenotype in a particular population (since variance is a population statistic). Cf. Tal's
(2009) discussion of the rst question and of an alternative version of the second question that also is focused on the phenotype of a particular individual.
Contents lists available at ScienceDirect
Personality and Individual Differences
journal homepage: www.elsevier.com/locate/paid
https://doi.org/10.1016/j.paid.2022.111896
Received 21 July 2022; Received in revised form 30 August 2022; Accepted 1 September 2022
Personality and Individual Dierences 200 (2023) 111896
2
2. Heritability: analyzing trait variance
Heritability estimates are often calculated using a statistical method
known as the analysis of variance (ANOVA). ANOVA is based on a linear
model, which in the case of heritability estimation assumes that the
variance of a phenotypic trait V
P
is a linear function of genotypic vari-
ance V
G
and environmental variance V
E
(given that there is no G-E
interaction or covariation):
VP=VG+VE
And the most common way of dening a phenotypic trait's heritability
(H
2
) is as the ratio of V
G
to V
P
(Plomin, DeFries, McClearn, & McGufn,
2008)
2
:
H2=VG
VP
H
2
is a statistical measure of broad sense heritability, which is the
estimated proportion of phenotypic variance that is due to genetic
variance. However, there is also another heritability measure known as
narrow sense heritability (h
2
), which is the estimated proportion of
phenotypic variance that is due to additive genetic variance. Additive
genetic variance is simply the proportion of phenotypic variance that is
due to the additive effects of genes. The other sources of genetic variance
are dominance variance, epistatic variance, and variance due to assor-
tative mating. For the purposes of this paper, we will focus on herita-
bility in the broad sense.
From what has been said above, it may seem intuitive that non-nil
heritability measures provide information about genetic causation.
After all, discovering that a certain trait has a H
2
of, say, 0.5 (which is
not uncommon for behavioral traits, Plomin, DeFries, Knopik, & Nei-
derhiser, 2016; Polderman et al., 2015) means that genetic variation
(measured in phenotypic units) explains 50% of trait variation in the
population that is being studied. And it may not seem a stretch to think
that some of the genetic variation is causally responsible for some of the
phenotypic variation (e.g., Sesardic, 2005, p. 82ff.). However, there are
many objections to this idea. One is that the denition of H
2
given above
only says that there is an associative relation between the terms V
G
and
V
P
—and as we have all been taught in statistics 101, association does not
imply causation (Turkheimer, 2016). Another objection is that the
denition of V
P
given above is incomplete. More specically, it has been
noted that one cannot simply assume that there is not any G-E interac-
tion V
G×E
, or any G-E covariation 2COV(G, E).
3
The denition of
phenotypic variance should therefore be amended as follows
4
:
VP=VG+VE+VG×E+2COV(G,E)
And, moreover, many commentators have argued that covariation
between genotypes and environments constitutes a challenge to the
claim that heritability analyses of trait variance can provide causally
relevant information (Block, 1995, pp. 118–121; Block & Dworkin,
1976a, p. 480; Feldman & Lewontin, 1975, p. 1164; Jencks, 1980, pp.
726–730; Kaplan & Turkheimer, 2021, p. 61; Sober, 2001, pp. 72–75).
A famous thought experiment from Jencks et al. (1972) illustrates
why large G-E covariation renders any causal interpretation of herita-
bility unjustied. Imagine that there is a population in which children
with red hair experience systematic discrimination, and they are denied
access to education. In this population, red haired children will on
average perform worse on measures of intellectual aptitude and
achievement. Moreover, since there is large covariation between genes
associated with red hair and experiencing discrimination, the genetic
variants associated with red hair will be predictive of low scores on
measures of intellectual aptitude and achievement. However, according
to some of the aforementioned commentators, heritability measures of
aptitude and achievement cannot be trusted to be indicative of causal
relationships in this population, since on any intuitive understanding of
what it means for a gene to have a causal effect on a trait, one cannot say
that the genes associated with red hair are causally responsible for low
scores. To the contrary, it should be obvious that the etiological root of
the relatively low scores of red-haired children is the discrimination they
experience—or at least so the argument goes.
As things currently stand, there appears to be a lot of disagreement
concerning the interpretation of heritability measures and whether they
can provide causally relevant information. The next section will bracket
issues having to do with G-E covariation and develop an argument for
the claim that heritability measures are not causally irrelevant. After
that, the argument is defended against a couple of prominent objections
from the literature.
3. The argument for causal relevancy
It is not uncommon for both proponents and opponents of the claim
that heritability can be given a causal interpretation to rely on particular
views about the nature of causation (cf. Oftedal, 2005). For example,
Lewontin (1974) and several of his followers (e.g., Block, 1995, p. 24;
Block & Dworkin, 1976a, p. 482; Kaplan & Turkheimer, 2021; Keller,
2013) argue that ANOVA is not a useful method for discovering causal
relationships since ANOVA only provides associative information, and
knowledge about genetic causation requires knowledge of the exact
“process”, “function”, or “mechanism” by which genes produce their
phenotypic effects. Indeed, as Lewontin has put it, knowledge of genetic
causation requires that one can “provide a detailed molecular analysis of
the chain of causation between nucleotide substitution and cell devel-
opment and function” (Lewontin, 2006, p. 537).
Moreover, the same tendency is also found among those who support
causal interpretations of heritability. But whereas the opponents rely on
so-called process conceptions, the proponents tend to understand
causation in terms of probability or difference-making. Examples of this
can be found in recent work by Bourrat (2019, 2020); Lynch and Bourrat
(2017); and Tal (2009), where it is argued that heritability estimates can
be given a causal interpretation since they show that certain genes in-
crease the probability of having a phenotypic trait with a value that
deviates from the mean value of said trait.
However, there are problems with both strategies, stemming from
the fact that they (in part) wed their claims about whether heritability
measures can be interpreted causally to particular conceptions about the
nature of causation. Two problems, more specically, are that both
kinds of conception face intuitive counterexamples, indicating that
causation cannot be analyzed in terms of “process” or “probability”
alone, and that there is not at the moment a consensus about what the
correct theory of causation looks like.
5
(For more on the different the-
ories of causation, as well as their strengths and weaknesses, see the
review by Schaffer, 2016.) Continuing, the argument of this paper will
not assume the correctness of any particular conception of the nature of
causation. Rather, it will focus on properties that are characteristic of
2
For more on the various ways of dening heritability, see Bourrat (2015); Dowens and Lucas
(2020); Falconer and Mackey (1996); Godfrey-Smith (2009); Hartl and Clark (1997); Jacquard (1983);
Tal (2009).
3
G-E interaction occurs when different genotypes don't respond to the environment in the same
manner, or when the environment has differential effects that contingently depend on individuals'
genotypes; G-E covariation occurs when certain environmental factors are associated with genetic
propensities in a population.
4
The formula is still somewhat simplied: V
E
is usually divided into between-family
variance and within-family variance, and the formula should also include an
error term representing variance due to measurement error. However, for
present purposes, it is only necessary to keep in mind the simplied version of
the formula.
5
Another problem that primarily threatens the views of the opponents, is that they seldom spell out
their assumptions about causation in any detail. This should be contrasted with the approach of (e.g.)
Bourrat (2019, 2020), who explicitly relies on Woodward's (2003) interventionist account of causation.
J. Egeland
Personality and Individual Dierences 200 (2023) 111896
3
causal relationships, without presupposing them to be either necessary
or sufcient.
Briey put, the argument is that since a non-nil heritability measure
tells us that the relationship between V
G
and V
P
satises certain char-
acteristic properties of causation, it increases the probability that the
relation is (at least partly) causal—whatever the nature of causation
really is. The properties that will be focused on are association,
manipulability, and counterfactual dependence.
6
Consider association
rst.
A couple of variables stand in an associative relation to each other
just in case certain values of one variable make certain values of the
other variable more likely. For example, since UV radiation exposure
increases the likelihood that one will develop skin cancer, UV radiation
exposure and skin cancer are associated with each other. Another
property that often is relied upon to get a grip on causation is manipu-
lability (Woodward, 2016). It is not uncommon to understand causation
in terms of the idea that the manipulation of a cause (and no other
variable)
7
regularly will result in the manipulation of an effect. The
manipulation need not be experimental or to have actually occurred;
rather, it is enough that it can happen in principle. A third characteristic
of causal relationships is counterfactual dependence, meaning that the
cause is counterfactually necessary for the effect (Menzies & Beebee,
2020). More specically, a variable Y counterfactually depends on a
variable X just in case there are alternative, possible values for Y and X,
such that if X were to have an alternative value, then so would Y.
Association, manipulability, and counterfactual dependence are
characteristic properties of causal relations and, moreover, that is pre-
cisely why we acquire causal knowledge by relying on information
concerning whether, and to what extent, certain relations of interest
satisfy these properties. In fact, the evidential support of causal claims in
the sciences is typically considered proportional to the degree to which
the data indicate that the aforementioned properties are satised.
Consider as an example the claim that smoking causes cancer. The
consensus view is that smoking indeed causes various types of cancer,
and that we know this to be true. But how did we acquire this knowl-
edge? We know it because a number of well-designed studies have been
published, where it is has been found, based on analyses of large data-
sets, that smoking and cancer are signicantly associated even after
controlling for possible confounders.
8
The associations are interpreted
as evidencing causal relationships since the statistical analyses are based
on large and representative samples from different populations living
under varying environmental conditions, and they remove the effects of
other relevant factors. This means that smoking is associated with can-
cer, and since changing the value of only the smoking variable would
lead to a change in the value of the cancer variable, the relationship also
has the properties of manipulability and counterfactual dependence.
Although we may not know everything about the mechanisms by which
the cancers are developed, or about what exactly the necessary and
sufcient conditions for causation are, learning that a relationship be-
tween two variables satises the three aforementioned properties does
increase the probability that it is causal—sometimes to such an extent
that we can know that the relationship is one of cause and effect.
The situation is in many ways analogous when it comes to herita-
bility. When H
2
>0, there is an association between V
G
and V
P
. More-
over, although no ideal intervention is performed in either classical twin
studies or GWAS, the relation between genetic variance and phenotypic
variance does appear to satisfy manipulability and counterfactual
dependence since V
P
is dened as a linear function of V
G
and V
E
—meaning that a change in the value of V
G
only, should induce a change
in the value of V
P
. It is of course always theoretically possible that some
confounding factor (such as population stratication or assortative
mating) is responsible for the change in trait variation, or that there is a
very large G-E interaction effect that obscures the relationship between
V
G
and V
P
(cf. Lewontin, 1974, p. 406), but the likelihood of this pos-
sibility is greatly reduced when the samples on which the estimates are
based are large, appropriate statistical techniques are used (for more on
this, see Young, Benonisdottir, Przeworski, & Kong, 2019), and there is
no evidence for large G-E interaction.
Now assuming that there is not any large G-E covariation or inter-
action,
9
the same reasoning can be used to illustrate that non-nil heri-
tability measures increase the probability that genotypic differences
cause phenotypic differences. Since heritability measures satisfy certain
characteristic properties of causation (i.e., association, manipulability,
and counterfactual dependence), and cases where those conditions are
satised constitute a proper subset of all possible cases that exist with
respect to the relationship between certain genes and traits—one that
includes all, or at least most, cases of genetic causation—knowledge that
a certain phenotypic trait has a non-zero heritability value increases the
probability that genes play a causal role in the development of indi-
vidual differences in said phenotype. The reasoning is illustrated in
Fig. 1.
Let's summarize. Since we do not have direct epistemic access to the
causal structures of the world, we have to make inferences about
causation based on whether, and to what extent, certain relations of
interest satisfy properties that are characteristic (and thereby indicative)
of causation. Three such properties are association, manipulability, and
counterfactual dependence. Moreover, as has been demonstrated above,
heritability measures typically do satisfy these properties, which means
that they can provide causally relevant information. However, it does
not follow that non-nil heritability estimates always justify causal in-
ferences. Whether they do will have to be judged on a case-by-case basis,
No causal
informaon
Causal
characteriscs
are sasfied
Genec
causaon
Fig. 1. The outer circle represents all possible ways in which genotypes and
phenotypes can be related. The middle circle represents all the ways in which
genotypes and phenotypes can be related when the properties of association,
manipulability, and counterfactual dependence are satised. The inner circle
represents all the ways in which variation in genotypes can cause variation
in phenotypes.
6
An important implication is that Turkheimer (2016) and others are mistaken when they claim that
heritability simply provides information about genotype-phenotype correlation.
7
A manipulation that only changes the value of one variable is sometimes called an ideal inter-
vention (Woodward, 2003).
8
For a few examples, see Boffetta (2008); Lee, Forey, and Coombs (2012); Pesch et al. (2012);
Sasco, Secretan, and Straif (2004).
9
Scenarios involving G-E covariation or interaction will be discussed in Section 6.
J. Egeland
Personality and Individual Dierences 200 (2023) 111896
4
and in the light of evidence pertaining to the degree to which there is G-E
covariation or interaction.
4. Lewontin's locality objection
Critics of the claim that heritability estimates can provide causal
information usually rely on arguments presented in Lewontin's seminal
(1974) article. One of Lewontin's most inuential arguments is the so-
called locality objection, which points out that H
2
is a population sta-
tistic and infers that it cannot be applied to any other population or any
other measurement condition. This is how he puts it:
That is, the linear model is a local analysis. It gives a result that de-
pends upon the actual distribution of genotypes and environments in
the particular population sampled. Therefore, the result of the
analysis has historical (i.e., spatiotemporal) limitation and is not in
general a statement about functional relations (Lewontin, 1974, p.
403).
Moreover, since Lewontin assumes that a causal analysis or explanation
requires knowledge of the exact function or mechanism by which a
cause produces its effects, as indicated by the last sentence above,
10
it
follows that ANOVA cannot provide any information about genetic
causation.
The locality objection has been tremendously inuential, and it has
been reiterated by a number of scientists and philosophers who agree
that because heritability is a population statistic it can be justiably
inferred that it cannot be applied to any other population or any other
measurement condition (e.g., Block & Dworkin, 1976b, pp. 486–487;
Daniels, Devlin, & Roeder, 1997, p. 54; Nelkin & Andrews, 1996, p. 13;
Rutter, 1997, p. 391; Rutter, 2002, p. 2; West-Eberhard, 2003, pp.
102–103), and who also think that ANOVA cannot provide causally
relevant information because it does not give any insight into the
function or mechanism by which genes produce their phenotypic effects
(e.g., Block, 1995, pp. 117–119; Block & Dworkin, 1976b, p. 482;
Kaplan & Turkheimer, 2021, p. 61ff.; Keller, 2013). However, there are
ve reasons why the locality objection does not have the dialectical
force that many commentators have believed, and why it fails to
threaten the argument from the previous section.
First, just because heritability is a population statistic, it does not
follow that heritability estimates cannot provide any information about
the relative contributions of genes and environment to individual trait
differences in other populations than the one that has been sampled, or
in other measurement circumstances. Determining the extent to which
heritability estimates are generalizable is ultimately an empirical
question, meaning that it cannot be answered by reection or concep-
tual analysis alone (Bouchard & Loehlin, 2001, p. 247).
Second, there is some empirical evidence supporting the generaliz-
ability of heritability. For example, high heritability values for general
intelligence have been found in different countries, with their own
particular cultures and environmental conditions, from different conti-
nents (Knopik, Neiderhiser, DeFries, & Plomin, 2017, pp. 170–173).
Moreover, it is to be expected that heritability measures from similar
contexts and similar measurement conditions will not be altogether
unlike each other. For a more detailed discussion of this issue, see Ses-
ardic (2005, pp. 78–86).
Third, even if it were true that heritability measures never can pro-
vide any information or indication about how heritable a trait is in other
populations than the one that has been sampled, or in other measure-
ment circumstances, nothing follows concerning the issue of causal
interpretation. Just because the ratio of V
G
to V
P
may be context-
dependent, it can still be the case that it says something about the
causal contribution of genetic variance to phenotypic variance in the
population from which the sample has been gathered (Tal, 2009, pp. 90–91).
In general: the nongeneralizability of an associative relationship does
not imply that the relationship is not causal in the context where the
association is present.
Fourth, it should be noted that Lewontin actually disagrees with this
claim. He tells his readers that one should avoid “confusing the spatio-
temporally local analysis of variance with the global analysis of causes”
(Lewontin, 1974, pp. 410, italics added), the latter of which requires
knowledge about “functional relations” (Lewontin, 1974, p. 403) that
hold true of “the entire spectrum of causal relations” (Lewontin, 1974, p.
407). However, this is a very radical claim—one that would (if it were
true) undermine many, if not most, causal claims made in the sciences.
For example, in scientic disciplines such a medicine, biology and
psychology, we are interested in understanding how things work under
relatively normal parametric conditions. This does not mean that such
disciplines cannot discover causal relations, but rather that the causal
relations that we know to hold true in most contexts may break down in
more rarely occurring, or (e.g.) counterfactual, contexts. For example,
just because we haven't investigated the functional relationship between
sugar consumption and diabetes under “the entire spectrum” of envi-
ronmental conditions, it would be irrational to claim that we cannot
know that sugar consumption causes diabetes. A relatively local analysis
of causes can be compatible with ignorance about global function (cf.
Haldane, 1938, p. 34).
Fifth, it is wrong to assume that knowledge about causation requires
knowledge about “function” or “mechanism”. For example, we know
that having a third copy of chromosome 21 causally contributes to lower
IQ, even though there is a lot about function or mechanism that we don't
know. Moreover, blaming heritability estimation for not providing
insight into mechanism or function is like blaming the Beck Depression
Inventory (used for the measurement of depression severity) for failing
to say anything about why people become depressed. The point is simply
that heritability measures can provide causal information without
saying, or even purporting to say, anything about how genes inuence
phenotype development.
Taken together, these reasons demonstrate that Lewontin's locality
objection does not undermine the argument of this paper.
5. The quincunx analogy
A GWAS is performed by searching for single-nucleotide poly-
morphisms (SNPs)—i.e., substitutions of single nucleotides—associated
with a trait of interest. In cases of synonymous substitutions, the mu-
tations do not lead to any phenotypic difference, but in other cases they
do. The purpose of a GWAS is to identify SNPs that are associated with a
certain trait by separating those who have the trait and those who do not
have it into different groups, and by identifying variants that are more
common in the group with the trait of interest. Now the SNPs that are
identied are not necessarily associated with the trait themselves, as it is
possible that they are located close to regions of the genome that are
associated with the trait, and that usually are inherited collectively—in
which case the variants in question are said to be in linkage disequi-
librium. That said, there are ways of extenuating this problem, and
weighted sums of SNPs that are found to be associated with the trait take
the form of polygenic scores (PGSs) that function as predictors of the
trait. Moreover, the variants on the basis of which a PGS is calculated are
sometimes said to be causal (e.g., Yang, Zeng, Goddard, Wray, &
Visscher, 2017, p. 1305).
In a recent paper, Kaplan and Turkheimer (2021, p. 61ff.) have
argued that GWAS face similar problems as those of the ANOVA
approach to heritability estimation already pointed out by Lewontin
(1974). They present an argument by analogy, focusing on Galton's
quincunx (better known as the “Galton board”). The quincunx is a ver-
tical board with rows of pins. When a ball is dropped into the top of the
quincunx, it bounces either left or right when it hits the pins, eventually
landing in one of the bins at the bottom of the machine (see Galton,
1894, for a more detailed description). The pins, we are told, are
10
Cf. Lewontin (1974, p. 409; 2006, p. 537).
J. Egeland
Personality and Individual Dierences 200 (2023) 111896
5
“difference makers”, in the sense that their relative placement is asso-
ciated with certain outcomes (i.e., the distribution of balls in the bins at
the bottom); but, for the purposes of their thought experiment, it is
assumed that the walls of the quincunx are opaque, so that it is impos-
sible to observe which path the ball travels.
Next, Kaplan and Turkheimer ask us to imagine a population of
quincunxes with identiable varieties of pin placements. If a particular
pin placement at a particular location is more frequent among those
quincunxes in which the ball landed to the left, then it is likely to have a
left-bias: it will be predictive of left-biased outcomes. Moreover, the
situation is analogous to GWAS in the following ways: (1) a particular
variant of pin placement at a particular location in the quincunx func-
tions in the same way as an individual SNP in the genome: just as pin
placements are associated with certain ball distributions, so are SNPs
associated with certain phenotypic traits; (2) the predictions made on
the basis of the bias that exists in a population of quincunxes corre-
sponds to PGSs: they are indicative of ball distributions and phenotypic
outcomes respectively.
In their discussion of the analogy, Kaplan and Turkheimer make a
number of points—most of which have been made by previous com-
mentators (e.g., concerning G-E interaction, G-E covariation, and rea-
sons why PGSs in one population may not be equally predictive in other
populations) and are not directly related to the analogy. But the most
important insight, we are told, is this: knowledge about how particular
pin placements (or SNPs) are associated with certain ball distributions
(or phenotypic traits), does not contribute to our “understanding where
a particular ball ended up”. And the reason is that
Since a ball only interacts with a small minority of the pins in any
particular trial, most of the time the pin in question will have been
entirely irrelevant to the ball's path. Even when it was relevant,
however, if all we know is that, at that location, there is a pin variant
with tendency towards one direction, we can't know (except in cases
of 100% bias) if the ball in fact took the more typical path, or if it
took the path that was less likely (Kaplan & Turkheimer, 2021, p.
64).
And this is important since it supports the idea that GWAS cannot pro-
vide any relevant information about causation, or even about individual
differences:
The associations discovered by GWAS (and related technologies) are
unlikely to provide any meaningful basis for explaining variation in
individuals; still less do they themselves reliably point towards
causes of individual outcomes (Kaplan & Turkheimer, 2021, p. 60).
There are, however, two problems with this argument. The rst problem
is that it is not necessary that a particular pin/SNP causally contributes
to the outcome in the case of a particular ball/person, or that we know
with 100% certainty what the causal path taken is, in order for us to
know that certain pin placements/SNPs satisfy important properties (i.
e., association, manipulability, and counterfactual dependence) that
increase the probability that they are causally related to certain out-
comes. Precise knowledge about which causal path has been taken is not
a necessary condition for gleaning information to the effect it is some-
what probable that some such path has been taken. When we learn that
certain SNP variants are associated with a certain trait, it is rational to
somewhat increase our credence that they are causal—not to conclude
that they cannot provide information relevant for understanding the
outcomes observed since it is impossible to know exactly whether, or
how, a particular SNP has contributed to a particular phenotype.
Now one may reasonably question whether Kaplan and Turkheimer
really do think that being provided with causally relevant information
requires knowledge about exactly what the causal “path” from SNP to
phenotype looks like. Here are a couple of quotes showing that they do:
To understand the causal role that a gene plays in the development of
a trait is therefore to understand when (under what conditions) and
how it is transcribed, and how the products are used across the
development of the trait in question (Kaplan & Turkheimer, 2021, p.
61).
If we understand Lewontin's projects to be about understanding how
individuals develop the traits that they have, and understanding why
the distribution of traits in a particular population is the way that it
actually is, the kinds of results given to us by GWAS/PGS will be of no
more value to us than the kinds of ANOVA-based quantitative ge-
netics research that he was criticizing. The analogy of the quincunx
helps us to see why (Kaplan & Turkheimer, 2021, p. 68).
And this is the source of the second problem. Since Kaplan and Tur-
kheimer indeed are followers of Lewontin's projects, they assume that a
necessary condition for causal knowledge (and even just being provided
with causally relevant information about genetic causation) is knowl-
edge of the exact mechanism by which genes contribute to the devel-
opment of a trait. However, as we have seen in the previous section, this
is clearly setting the bar too high, and reection on relevant examples
illustrates why: Lacking an awareness of the mechanism by which
caffeine functions as an adenosine antagonist, blocking the action of
adenosine on its receptors, does not prevent a child from learning (either
by testimony or experience) that caffeine consumption has the effect of
reducing drowsiness. Inferring that it does from the Lewontonian posi-
tion would appear to be a reductio against said position.
Moreover, insisting that knowledge of mechanism is necessary for
knowledge of causation is really to endorse a sort of skepticism about the
behavioral sciences, as it would threaten many, if not most, of their
claims to causal knowledge. Or as Turkheimer, Goldsmith, and Gottes-
man (1995, p. 149) once rhetorically asked: “If knowledge of mechanism
were required prior to investigation of relationships between predictor
and outcome, how much of behavioral science would be disallowed?”
However, this is clearly an unacceptable consequence. We know that
trisomy 21 (i.e., down syndrome) causes lower IQ, even though we do
not “understand when (under what conditions) and how [the relevant
genes are] transcribed, and how the products are used across the
development of the trait in question”.
6. Conclusion
This paper has argued that heritability measures can provide caus-
ally relevant information. Since a non-nil heritability measure tells us
that the relationship between genetic variance and phenotypic variance
satises certain characteristic properties of causation, it increases the
probability that the relation is causal. Furthermore, the argument was
defended against Lewontin's locality objection and Kaplan and Tur-
kheimer's recent quincunx analogy.
However, critics of heritability estimation may very well claim that
the most important objections—namely, G-E interaction and G-E cova-
riation—haven't been addressed. This is true, and I want to make a few
closing remarks with respect to these objections. First, when G-E inter-
action and G-E covariation are relatively small or moderate, they do not
swamp out the main effects of genotypes and environments (Sesardic,
2005, ch. 2–3).
11
Second, the question of how additive the relations
between genotypes and environments are for human traits is ultimately
an empirical one, and only very few signicant G-E interaction effects
have been discovered and replicated (Gauderman et al., 2017; McGue &
11
It should be noted that Lynch and Bourrat (2017) recently have argued that both active and
reactive G-E covariance should be included in the V
G
term. They argue that this is the only
consistent way of interpreting heritability estimates in a causal manner.
J. Egeland
Personality and Individual Dierences 200 (2023) 111896
6
Carey, 2017). Knopik et al. (2017, p. ch. 8) provide a useful summary of
the literature, explaining that a large proportion of reported G ×E ef-
fects do not replicate (cf. the litterature review by Duncan & Keller,
2011), that most replicated effects have to do with non-cognitive
traits,
12
and that genuine G ×E effects usually are small enough that
they do not obscure the main effects of genotypes and environment.
13
Third, even if it turns out that nonadditivity is the rule rather than the
exception,
14
it does not undermine the argument of this paper. Since the
argument only claims that heritability measures can provide causally
relevant information—not that they provide causally relevant informa-
tion under all (or even most) measurement conditions, or that they al-
ways justify causal inferences—they are only likely to do so in cases
where we do not know that G-E interaction or G-E covariation does not
leave room for readily interpretable main effects.
Lastly, it is worth mentioning that recent work in sociogenomics
evidencing IGEs—i.e., effects whereby phenotype expression is inu-
enced by the genotypes of other conspecics—may complicate the issue
of heritability estimation and causal inference. Indeed, observable ef-
fects of genetic nurture (Kong et al., 2018) and social epistasis (Dom-
ingue et al., 2018) may increase H
2
values for certain traits, even though
this is not solely due to the individuals' own genotypes. Some may argue
that this weakens the plausibility of genetic inferences, since genetic
effects must be endogenous. However, a problem with this position is
that an important lesson of the gene-centered view of evolution is that
our common-sense conceptual distinctions between the individual on
the one hand, and the social on the other, may not be entirely adequate
for making sense of biological reality. Just as the genotype of an indi-
vidual can have effects on extended phenotypes, there does not appear to
be any scientic reason as to why it should not be possible for the
phenotype of an individual to be inuenced by its extended genotype, or
why this should not count as genuine genetic causation. The organismal
world may not always “respect” our intuitive conceptual distinctions,
developed for dealing with non-scientic, everyday matters, but, as the
history of science teaches us, our conceptual framework and the types of
sense-making that it enables can be reformed in order to improve our
understanding.
CRediT authorship contribution statement
Jonathan Egeland is the sole author of this paper.
For helpful comments, he thanks Pierrick Bourrat and Neven
Sesardi´
c.
Declaration of competing interest
On behalf of all authors, the corresponding author states that there is
no conict of interest.
Data availability
No data was used for the research described in the article.
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J. Egeland
