The concept of heritability gets misunderstood a lot, so there are several articles discussing what it doesn't mean. But reading through all of them leaves me confused about what it does mean in practical terms, outside the technical definition.
For example, as I myself wrote in an old comment about common misunderstandings:
Caution: heritability, as in the statistical concept, is defined in a way that has some rather counter-intuitive implications. One might think that if happiness is 50% heritable, then happiness must be 50% "hardwired". This is incorrect, and in fact the concept of heritability is theoretically incapable of making such a claim.
The definition of heritability is straightforward enough: the amount of genetic variance in a trait, divided by the overall variance in the trait. Now, nearly all humans are born with two feet, so you might expect the trait of "having two feet" to have 100% heritability. In fact, it has close to 0% heritability! This is because the vast majority of people who have lost their feet have done so because of accidents or other environmental factors, not due to a gene for one-footedness. So nearly all of the variance in the amount of feet in humans is caused by environmental factors, making the heritability zero.
Another example is that if we have a trait that is strongly affected by the environment, but we manage to make the environment more uniform, then the heritability of the trait goes up. For instance, both childhood nutrition and genetics have a strong effect on a person's height. In today's society, we have relatively good social security nets helping give most kids at least a basic level of nutrition, a basic level which may not have been available for everyone in the past. So in the past there was more environmental variance involved in determining a person's height. Therefore the trait "height" may have been less hereditary in the past than now.
The heritability of some trait is always defined in relation to some specific population in some specific environment. There's no such thing as an "overall" heritability, valid in any environment. The heritability of a trait does not tell us whether that trait can be affected by outside interventions.
Some articles that go deeper into the details and math of this include "Heritability is a ratio, not a measure of determinism" (dynomight.net) and "Heritability in the genomics era - concepts and misconceptions" (Nature Reviews Genetics).
However, all of these examples of what heritability doesn't mean have left me very confused about what it does mean. I know that if a trait is 80% heritable, I cannot infer that it is "80% genetically determined", but what can I infer? That 80% of the observed variance in that trait is genetic, yes, but what's the practical thing of interest that having this information allow me to predict, that I couldn't predict before? In particular, what does knowing the heritability of traits such as IQ, subjective well-being, or Big5 scores tell me?
Looking at the Wikipedia article for heritability, I see very little that would help answer this question; the closest that I can find is the "controversies" section, which says that there are people who think the concept shouldn't be used at all:
Heritability estimates' prominent critics, such as Steven Rose, Jay Joseph, and Richard Bentall, focus largely on heritability estimates in behavioral sciences and social sciences. Bentall has claimed that such heritability scores are typically calculated counterintuitively to derive numerically high scores, that heritability is misinterpreted as genetic determination, and that this alleged bias distracts from other factors that researches have found more causally important, such as childhood abuse causing later psychosis. Heritability estimates are also inherently limited because they do not convey any information regarding whether genes or environment play a larger role in the development of the trait under study. For this reason, David Moore and David Shenk describe the term "heritability" in the context of behavior genetics as "...one of the most misleading in the history of science" and argue that it has no value except in very rare cases. When studying complex human traits, it is impossible to use heritability analysis to determine the relative contributions of genes and environment, as such traits result from multiple causes interacting. In particular, Feldman and Lewontin emphasize that heritability is itself a function of environmental variation. However, some researchers argue that it is possible to disentangle the two.
The controversy over heritability estimates is largely via their basis in twin studies. The scarce success of molecular-genetic studies to corroborate such population-genetic studies' conclusions is the missing heritability problem. Eric Turkheimer has argued that newer molecular methods have vindicated the conventional interpretation of twin studies, although it remains mostly unclear how to explain the relations between genes and behaviors. According to Turkheimer, both genes and environment are heritable, genetic contribution varies by environment, and a focus on heritability distracts from other important factors. Overall, however, heritability is a concept widely applicable.
Out of those references, the one that sounded the most useful in telling me what heritability might actually mean was the one associated with the sentence "Overall, however, heritability is a concept widely applicable". This is the previously mentioned "Heritability in the genomics era - concepts and misconceptions" (Nature Reviews Neuroscience), which includes a section on "applications":
The parameter of heritability is so enduring and useful because it allows the meaningful comparison of traits within and across populations, it enables predictions about the response to both artificial and natural selection, it determines the efficiency of gene-mapping studies and it is a key parameter in determining the efficiency of prediction of the genetic risk of disease.
From reading this section, I gather that:
- If I wanted to breed plants or animals that were high on a particular trait, having the heritability estimate for that trait could be useful
- The heritability of a trait can be used to help infer how much statistical power gene-mapping studies targeting that trait need
- If I was trying to predict the genetic risk of something like schizophrenia, then... I don't quite understand this part, but apparently having the heritability estimate would help me know how reliable my prediction was going to be
Usefulness for breeding programs is hopefully an irrelevant consideration when we're talking about humans, which leaves me with the two others; and those also seem to suggest that knowing the heritability of a trait isn't useful on its own, and will only be something that helps me do or evaluate a gene-mapping or genetic risk prediction study better.
This seems to suggest that knowing the heritability of a trait such as IQ, subjective well-being or a Big5 score tells me essentially nothing by itself; is this correct?
(cross-posted to the Psychology & Neuroscience Stack Exchange)