We all know that height is highly heritable (about 0.8 according to my memory). Less well known is that BMI and muscle strength are very heritable too, at 0.5-0.6, according to this. This is somewhat counterintuitive in that common sense in some sense tells us that BMI can be reduced by not eating unhealthy and exercising, and similarly, that everyone can grow muscle from working out with the correct form. Well, it appears that the ability to do so is highly heritable. It conforms to my observation of the sheer difficulty with which overweight people have with gastronomic self control and of the rarity with which physical clumsiness changes within people.
This reminds me of an analogous cognitive characteristic. On the cognitive end, a similarly high level of heritability for vocabulary is similarly counterintuitive (it was to me too, before I had developed some basic psychometric intuition). Isn’t vocabulary memorized, learned? Yes but some people learn it much more naturally than others, and actually retain it. There is, I hypothesize, some neurobiological mechanism that more or less determines the capacity for storing vocabulary. From my personal experience vocabulary is much more about recognition of analogies than about memorization. With the former, the latter is hardly needed to obtain a sizable vocabulary. I also believe analogies are universal, independent of the language in which they are represented; this is largely supported by the fact that the two languages I know well, English and Chinese, share more often than not the same analogies for different definitions of characters/words. A common linguistic construction of semantic units in both, for instance, is the extension of concrete to abstract. So different cultures, with different languages, more or less discovered the same analogies for construction of their respective languages. All this suggests some faculty in the brain, that one has to some degree or another, linked with general ability to discern analogies, that is manifested through vocabulary size, a direct consequence of that ability.
As an FYI, this is personal speculation of a quasi-educated nature. I say quasi because I am constructing an explanation most rational based on my personal knowledge, which is very limited, my having at the present moment not even passing knowledge of linguistics or neuroscience, which I hope to obtain on a gradual basis.