Heritability is very hard for a layperson (such as myself) to wrap his head around, because the meaning assumed by the layperson ("how much a trait is determine

Heritability is very hard for a layperson (such as myself) to wrap his head around, because the meaning assumed by the layperson ("how much a trait is determined by genetics rather than by the environment") is superficially correct, and accurately sums up the intuitive idea that various technical definitions of heritability are based on, but scientific estimates of heritability usually differ from the intuitive definition in significant ways. For example, one common definition of heritability, the one given by Wikipedia as a matter of fact, has a very important twist: "The heritability of a trait within a population is the proportion of observable differences in a trait between individuals within a population that is due to genetic differences." In this definition, heritability explicitly depends on the choice of population. For example, if North and South Korea are considered as a single population, you will get a different result than if they are considered separately. It's easy to misuse population-specific measures of heritability to make inappropriate cross-population comparisons. For example, if intelligence is highly heritable among native-born Dutch and highly heritable among native-born Croats, it might seem reasonable to conclude that IQ differences between Dutch and Croats must be genetic. Yet the same argument would show that the difference in height between North Korea (where height is presumably highly heritable within the population) and South Korea (ditto) must be genetic.