Advancing Knowledge for a Better Tomorrow

September 2022

An Introduction to Heritability

Genes are ubiquitous. They affect just about every human and animal quality, physical and non-physical. For convenience, call all of these “traits” When we measure the heritability of any trait from observed data, it is substantial.

By heritability we mean the extent to which differences in a trait are the result of differences in genes. We are interested here in variation, so to speak, not in normal or average values. It is possible for something to be completely under genetic control without seeming heritable, since there might be no natural variation in it. However if something is heritable it must be under a corresponding degree of genetic control. Heritability is usually expressed as a percentage, ranging from none (0) to total (100), e.g. 73%, Since the future of genomics will be driven by the all-pervasiveness nature of genetic effects, some more explanation is called for.

Heritabilities are estimated from pedigree studies, first initiated in the 19th century. Here we do not need to know what the operative genes are, at the DNA level, we can still infer that they must exist. Among humans, these pedigree studies include twins—fraternal or identical, raised together or raised apart—siblings and more distant relatives, and adoption studies of biologically unrelated siblings. The underlying contrasts include genes being held constant, and non-genetic effects being held constant. The data can all be combined into one large“structural equation model”to provide an estimate of heritability which represents the complete weight of known evidence.

For many physical traits, heritability approaches 1, so that differences in the trait are almost wholly the result of genetic differences. For behavioural traits like intelligence it is as high as 0.7. For personality and just about every other behavioural trait is it approximately 0.5, sometime less, sometimes more.

If anything, currently-known heritabilities are underestimates, since we only have natural experiments (differences observed in the real world) available to make these inferences. If we had experimental control we could introduce variation, of an artificial kind if necessary, as desired and see what the results are, thereby gaining a much more accurate idea of genetic influence (though perhaps not in real-world conditions). For human subjects this is usually not possible due to ethical constraints—although those may fluctuate over time.

What is not heritable is strictly speaking non-genetic. That includes all the influences commonly understood as the“environment” a very loose term for nurture, but which also includes many sources of error, including measurement error, construct invalidity of the trait measures (which may only obliquely get at the underlying trait), and random developmental error. It is important not to simply conflate the popular sense of“environment”here with non-genetic influences. Those are effects which do not trace to the genes of the individual subjects considered, but they do include the genes of other people. Environments also correlate with genes, since those elicit responses; they are constructed actively by genes, since humans and animals are niche developers and not helpless victims like potted plants in greenhouses.

To see just how pervasive heritability is, an example is useful. Humans have accidents. They cut their thumbs off sawing wood, they crash their cars, they fall into holes in the sidewalk while texting on their phones, they step on rakes on the lawn while lollygagging at their attractive neighbours. The number of lifetime accidents is a trait like any other, it can be measured and its heritability can therefore be computed from pedigree studies. The data shows that it is substantially heritable, despite the initial impression that these must be random events. Twins will have not-dissimilar rates of accidents over their lifespan. Clearly what underlies this trait is something like risk-taking and lifestyle choice, and genes influence both of those.

Knowing that traits have a genetic basis is some distance from knowing which genes, precisely, are responsible. Indeed, we knew about heritability, and even had preliminary estimates for it when genes were still just a vague and wholly indefinite hypothesis, latent and never observed directly.