Heritability is an idea originated in breeding, and it is interesting mainly for breeders. It is defined as the part of the variation for a trait in a population that is due to genetic variation (in contrast to enviromental variation).
The heritability is useful for a breeder because if the variation observed in a population is mainly due to enviromental factors the trait won't be improved by selecting the best individuals in the population.
However, the concept is usually misunderstood. First, it always depends on the population studied. One trait could have high heritability in one population, and low in another because different populations can have different genetic variation. Moreover, for a particular population it also depends on the environment because is the fraction of variation not due to the enviroment, and that, of course, depends on how variable is the environment.
Moreover, calculating the heritability, in practice, is very difficult even in controlled environemnts and populations, and when it is calculated most of the time is restricted to heritability in the narrow sense, taking into account only aditive effects. (This basically means ignoring all medelian gene interactions).
What the heritability does not mean is how much a trait depends on the genes. That idea does not even makes sense because any trait will depend on many genes even when it has no heritability. For instance, the number of fingers in humans has a very low heritability, because most of the variation is due to the enviroment (e.g. accidents), but the number of fingers clearly depends of many genes.
It can tell you something about existing interventions in a variable. In the US, for instance, we spend years of effort and upwards of hundreds of thousands of dollars on primary/secondary school education, and we know that we do a very poor job of making sure that different students have similar education experiences.
So, if SAT scores have low heritability in the US currently, then we would expect that we could figure out which education experiences tend to lead to higher SAT scores and try to do a better job of making sure everyone gets those kinds of experiences. If, on the other hand, heritability is high, then throwing more effort/money at how we do education currently should not be expected to improve SAT scores, and we either need to rethink how we do education, or rethink whether SAT scores measure what we want.
If, on the other hand, heritability is high, then throwing more effort/money at how we do education currently should not be expected to improve SAT scores
I agree with spkoc that this conclusion doesn't necessarily follow from high heritability. I think it would follow from high and stable heritability across multiple attempted interventions.
An exaggerated story for the point I'm about to make: imagine you've never tried to improve SAT scores, and you measure the heritability. You find that, in this particular environment, genetic variance explains 100% of ...
The evidence indicates that throwing more effort/money at how we do education does not improve IQ scores (for which SAT scores are a thinly-veiled proxy, except that every decade or so they make cosmetic changes to the SAT methodology) or student outcomes. Attempts to rethink education have failed. And IQ is generally useful enough that it is strongly correlated with outcomes we want.
If you're used to the tech sector with rapid change every decade, moving into the human services sector is going to be a very depressing experience. The low-...
But isn't this exactly the mainstream intuition that the OP dissolves? My understanding:
a) Heritability measures don't seem to make sense for really complex traits like intelligence.
b) Heritability measures are not stable outside the environmental conditions in which they were measured.
For instance, some people have sickle cell anemia, which helps them better survive malaria(but otherwise is slightly harmful). If you measure heritability of infant mortality in environment with malaria and then in environment without malaria you get opposite effects. ...
My view is that people should basically talk about heritability less and interventions more. In most practical circumstances, what we're interested in is how much potential we have to change a trait. For example, you might want to reduce youth obesity. If that's your goal, I don't think heritability helps you much. High heritability doesn't mean that there aren't any interventions that can change obesity-- it just means that the current environments that people are already exposed to don't create much variance. Similarly, low heritability means the environment produces a lot of variance, but it doesn't tell you anything specific you can actually do!
If you goal is to find interventions, all heritability gives you is some kind of vague clue as to how promising it might be to look at natural environmental variation to try to find interventions.
On the other hand, there is some non-applied scientific value in heritability. For example, though religiosity is heritable, the specific religion people join appears to be almost totally un-heritable. I think it's OK to read this in the straightforward way, i.e. as "genes don't predispose us to be Christian / Muslim / Shinto / whatever". I don't have any particular application for that fact, but it's certainly interesting.
Similarly, schizophrenia has sky-high heritability (like 80%) meaning that current environments don't have a huge impact on where schizophrenia appears. That's also interesting even if not immediately useful.
what can I infer?
If you are missing a finger from an accident and you want children with the normal 5 digits, you can use heritability to work out that finding a wife with 6 fingers isn't going to get you back to normal.
Another one is if you are gay and you want gay children, the low heritability of sexual orientation indicates that finding a gay gamete donor won't help much.
On the other hand if you are short and want taller children, you might put a lot of effort into finding a wife who is taller than average (heritability of height tells you that this will work).
My understanding is that heritability is a measure of predictive ability. Meaning if a trait is 80% heritable and you want to guess whether or not Bob has that trait then you'll be 80% more accurate if you know whether or not Bob's parents have the trait than if you didn't have that information. Likewise, for a very low heritability trait like having 2 legs, knowing whether or not Bob's parents have two legs doesn't improve your guess much if it improves it at all.
As you mentioned, environmental factors can at times subsume the genetic factors (e.g. heritability of height can be subsumed with very low nutrition). So if environmental factors for the dataset that you're trying to predict from are significantly different from the factors which you used to determine heritability, then the heritability estimate may not be as accurate and it should be reassessed for the different factors.
You can still make very concrete predictions about traits based on heritability even though very different environmental circumstances could reduce how heritable the trait is. Heritability of IQ has been determined in an environment exceedingly similar to that faced by kids in the modern schooling system. Let's say IQ has shown to be 80% heritable in circumstances not dissimilar to the US school system (as far as I am aware this is the current state of the art). Now if you want to predict the IQ of 20,000 parents of 10,000 US schoolchildren you'll do 80% better if you know the kids IQ than if you were just guessing randomly. Similarly, if you know Bob is smart you should update your prior estimate that Bob's parents are also smart significantly in favor of their intelligence.
if a trait is 80% heritable and you want to guess whether or not Bob has that trait then you'll be 80% more accurate if you know whether or not Bob's parents have the trait than if you didn't have that information.
I think this is more or less correct for narrow-sense heritability (most commonly used when breeding animals) but not quite right for broad-sense heritability (most commonly used with humans). If you're talking about broad-sense heritability, the problem is that you'd need to know not just if the parents have the trait, but also which genes Bo...
Heritability is an explanation. It's R-squared. As useful as "percent of variance explained" can be in some situations, and as useless in others.
I'm skeptical of this "percentage of variance explained" expression. In this setting, the only candate for explanation is causal explanation. But as far as I understand one cannot infer causation from statistical data (and heritability seems to be a purely statistical measure) without any additional counterfactual information.
The simplest and most useful answer is that heritability tells you the amount of variation that environmental factors don't control*. Traits with very high heritability** are generally going to be worse targets for intervention than traits with low heritability.
*In the range of environments over which the data was collected. The heritability of a trait as measured in Somalia or North Korea may be much lower that as measured in America. You can interpret this as meaning that there is much more hope for useful intervention in Somalia or North Korea, although the practical difficulties may be considerable.
**Some relevant traits are nearly 100% heritable, unfortunately. This includes executive function, which governs working memory and impulse control. Any non-biochemical intervention aimed at improving these traits is unlikely to succeed.
My take is that the scientific concept of "heritability" has some problems in its construction: the exact definition (Var(genotype)/Var(phenotype)), while useful in some regard, does not match the intuition of the word.
Maybe the quantity should be called "relative heritability", "heritability relative to population" or "proportion of population variance explained", like many other quantities that similarly have form A/B where both A and B are (population) parameters or their estimates.
Addendum 1.
"Heritable variance"? See also Feldman, Lewontin 1975 https://scholar.google.com/scholar?cluster=10462607332604262282
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:
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:
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":
From reading this section, I gather that:
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)