heritability
Heritability answers a slippery question: of all the differences we see in a trait across a group, what share traces back to genetic differences rather than the environment? It is a ratio, usually written between 0 and 1, where a high value means most of the variation between people lines up with their genes, and a low value means most of it comes from circumstances.
The crucial subtlety is that heritability is about variation in a population, not about any one individual. A heritability of 0.8 for height does not mean 80% of your height is genetic and 20% is diet; it means that, in this group living in these conditions, about 80% of the differences among people track genetic differences. It says nothing about why a single person is the height they are.
Heritability is always tied to a particular population at a particular time. Change the environment — say, end famine so everyone is well fed — and the environmental share of variation can shrink, which paradoxically raises heritability even though genes have not changed. It also cannot tell you whether a trait can be changed by intervention, nor explain differences between populations. These limits are why heritability is so often misread.
Twin and family studies estimate the heritability of adult height in well-fed populations at roughly 0.8, meaning most height differences among those people align with genetic differences — yet average heights have risen over a century as nutrition improved.
High heritability still leaves room for environmental change in the average.
Two flavours exist: broad-sense heritability counts all genetic effects, while narrow-sense heritability counts only the additive part that parents reliably pass on. Breeders care most about the narrow sense.