Primer The heritability fallacy David S. Moore1* and David Shenk2 The term ‘heritability,’ as it is used today in human behavioral genetics, is one of the most misleading in the history of science. Contrary to popular belief, the measurable heritability of a trait does not tell us how ‘genetically inheritable’ that trait is. Further, it does not inform us about what causes a trait, the relative influ- ence of genes in the development of a trait, or the relative influence of the envi- ronment in the development of a trait. Because we already know that genetic factors have significant influence on the development of all human traits, mea- sures of heritability are of little value, except in very rare cases. We, therefore, suggest that continued use of the term does enormous damage to the public understanding of how human beings develop their individual traits and identities. © 2016 Wiley Periodicals, Inc. How to cite this article: WIREs Cogn Sci 2016. doi: 10.1002/wcs.1400 INTRODUCTION represent something new and rather narrow. At that time, geneticists had a strictly deterministic under- f someone were to tell you that research has proven standing of how genes influence the formation of Ithat human intelligence is highly ‘heritable,’ what traits. They considered the relationship between genes would you think that means? Most people would and the environment to be akin to the relationship fi probably assume that it means people inherit a signi - between a plant seed and the rain that waters it: cant percentage of their intelligence directly, via their Genes were thought to contain specific, blueprint-like fi parents’ genes. In fact, though, the scienti c terms instructions for the formation of traits, whereas the ‘heritable’ and ‘heritability’ actually have very little to environment provided the nutrients and other salubri- do with genetic inheritance. This is confusing, because ous conditions that would allow those instructions to ‘heritability’ sounds like it means the same thing as unfold. According to this earlier way of thinking, a ‘inheritability.’ The confusion about what ‘heritabil- person’s DNA has specific instructions for blue eyes, fi ity’ actually measures signi cantly adds to a deep mis- or athletic arms, or a mathematical mind; the environ- understanding about how, exactly, our genomes ment merely allows for emphasis or de-emphasis of contribute to our observable characteristics (see Char- those already-designed traits. (If this sounds familiar, ney, Genes, behavior, and behavior genetics, WIREs it’s probably because it strongly resembles what many Cogn Sci, also in the collection How We Develop). of us were taught about genetics in grade school.) The term heritability was first given this new mean- ’ 1 THE APPROPRIATION ing in J. L. Lush s1937bookAnimal Breeding Plans. In that text, Lush proposed a calculation for what he called OF ‘HERITABLE’ ‘heritability’ that neatly codified the then-popular deter- For hundreds of years, the word ‘heritable’ was used ministic viewpoint. Because, Lush argued, an animal’s without confusion as a synonym for ‘hereditary.’ But phenotype (i.e., its observable traits, such as intelligence, in the early 20th century, the word was repurposed to height, eye color, etc.) is a function of genetic instructions plus the finishing influence of the environment, we should be able to statistically separate the influence of each.2 *Correspondence to: [email protected] Relying on mathematical guidelines from the geneticist 1Pitzer College and Claremont Graduate University, Claremont, CA, USA Sewall Wright, Lush proposed that in any given group: 2DeLTA Center, University of Iowa, Iowa City, IA, USA Vp phenotypic variation = Vg genetic variation fl fl Con ict of interest: The authors have declared no con icts of inter- ðÞ+ Ve environmentalðÞ variation est for this article. ðÞ © 2016 Wiley Periodicals, Inc. Primer wires.wiley.com/cogsci FIGURE 1 | Natural variability in human eye colors. Source: Sturm and Frudakis.3 Lush asserted that the Vg portion of that total them to correlations for that trait among fraternal can reasonably be termed ‘heritability,’ as it twins. Because identical twins share 100% of their revealed the portion of the trait variation that DNA and fraternal twins share, on average, only could be accounted for by variation in genes. The 50% of their DNA, this statistical comparison intention was ‘to quantify the level of predictability yields a crisp number that seems to highlight that of passage of a biologically interesting phenotype portion of a trait caused by genetic instruction. from parent to offspring’. In this way, the new Even though behavioral geneticists are sometimes technical use of ‘heritability’ accurately reflected careful to point out that each ‘heritability’ measure that period’s understanding2 of genetic determinism. only actually applies to variations in a specific Still, it was a curious appropriation of the term, group, they often use the term ‘heritability’ in a because—even by the admission of its proponents— way that conveys a sense of direct genetic influ- it was meant only to represent how variation in ence on traits. Also, because the term (if not its DNA relates to variation in traits across a popu- new-fangled scientific meaning) is so familiar to lation, not to be a measure of the actual influ- the public, the casual misinterpretation of the ence of genes on the development of any given term’s narrower meaning has been rampant in the trait. For example, in a large group of people popular press. For many decades now, we have with eyes of different colors (Figure 1), ‘Vg’ only been entertained with journalistic accounts of twin represents the extent to which variation in the studies suggesting that ‘personality is heritable’,4 group’sDNAaccountsforvariationindifferent ‘criminals are born, not made’,5 and ‘cheating eye colors in that group—not whether or how genes play [a] large role in female infidelity’.6 DNA is responsible for the development of eye Twin studies have reaffirmed the strong public color. In that sense, it was a highly misleading impression that some physical and personality new use of the term (even in the context of traits can be passed directly from parent to child determinism) that was bound to cause confusion: through DNA. While understandable, this impres- And indeed it did. sion is flatly incorrect, as brightly illustrated by three significant flaws in some scientists’ use of— and thus the public’s understanding of—the term TWIN STUDIES ‘heritability.’ The new use of the term caught on. Since that time, behavioral geneticists have conducted hun- THE GROUP VS. INDIVIDUAL FLAW dreds of studies derived from the Lush-Wright Although the original motivation for the concept of concept of heritability. The most prominent heritability was ‘to quantify the level of predictabil- method to determine heritability in human beings ity of passage of a biologically interesting pheno- has been statistical comparisons of identical and type from parent to offspring,’2 it is essential to fraternal twins. In what seems like a straightfor- realize that the equation Vp = Vg + Ve has no rele- ward approach, researchers compute correlations vance at all when it comes to individual develop- for a trait among identical twins and compare ment. The Vp in that equation refers to variation © 2016 Wiley Periodicals, Inc. WIREs Cognitive Science The heritability fallacy across individuals in a population, not to causal because there was zero variation in paint and con- processes that occur within any individual person. struction materials within this neighborhood. A Although the term ‘heritability’ is often misunder- statistical investigation focused on variation rather stood as speaking to the degree to which a certain than causation is, if not very narrowly interpreted, trait’s appearance in an individual is caused by likely to lead to a mistaken conclusion about what genetic factors, the logic underlying the equation caused the fires and about what can be done to nullifies that conceptualization. ‘Heritability,’ prevent them in the future. explains author Matt Ridley, ‘is a slippery concept, To see this, consider a newer neighborhood much misunderstood. For a start, it is a population right across the river in which every single home was average, meaningless for any individual person: you built with non-flammable aluminum and painted cannot say that Hermia has more heritable intelli- with fire-retardant paint. Even if this neighborhood gence than Helena. When somebody says that herit- has just as many space heaters, not a single home ability of height is 90 percent, he does not and here could ever catch fire. cannot mean that 90 percent of my inches come So what causes individual fires? The answer is from genes and 10 percent from my food.’ Instead, complex, and statistical variation surveys cannot he means that 90% of the variation in height in a address the question effectively; although all of the group of people can be accounted for, statistically, variation in fires in the fire-ravaged neighborhood by variations in those people’s DNA. But there is can be accounted for by focusing on space heaters no sense in which the specific DNA variations are alone, the fact remains that multiple factors— causing 90% of a person’s height. As Ridley notes, flammable building materials, the presence of heat ‘there is no heritability in height for the individual.’7 sources, and available oxygen, for example—are This group/individual distinction might seem responsible for collectively causing fires. Thus, focus- like a mere mathematical technicality, but it is not. ing on variation rather than causation can contribute Failing to recognize the distinction introduces a criti- to a misleading sense of how important a particular cal logical flaw that, on its own, completely under- factor might be in contributing to a particular mines the broad popular understanding of the term outcome.