I think the wooly part of this is that cognition is a bit slippery to measure, as we often are measuring the effects of applied cognition and never "innate intelligence" within people.
The problem with IQ tests, as an example, is they are biased to a specific culture of "test taking", and early iterations of the IQ test suffered from cultural knowledge being required to fully understand some problems, thereby making it a test of test-taking ability and cultural familiarity as well as some abstract thinking, but the problem is that abstract thinking is always expressed through something like speech, writing, athleticism, taking tests, and so on. Proficiency at these things require intelligence to coordinate and be well at them, but proficiency at test-taking is NOT particularly compelling as a definitive meaning of "smart".
Now, what's heritable and what's developable is also complex, but I think I'll sum up my opinion by quoting some athletic thing I read once that won't get out of my head:
"Hard work beats talent if talent doesn't work hard"
Thus, to quote the article, I'm inclined to agree with their suggestions that it's a matter of what children practice, and some of that might be influenced by genes:
"genotype-environment correlation: as children grow up, they increasingly select, modify and even create their own experiences in part based on their genetic propensities"
This means that culture, parental influence/support/neglect, and peers all have multivariate effects on the habits and hobbies that young people form, which in turn feed into the skills they express. A family of middling athleticism that is involved in these things foster at least the best of what a person can be, as does a family that has rich and wide reading material available to kids gives them a chance to learn a lot and integrate even more knowledge.
Epigenetic stresses probably have an effect, so I expect the opposite can be true. I hear about generational trauma and the ways in which mentally sick people hurt their kids in a way that their children become stressed and cope in ways that can lead to furtherance of a cycle of abuse in the following generation. Periodic abuse in a family dynamic can potentially repress or impair generations of people from the resulting suffering. Anyway, I guess it cuts both ways, eh? We become what we practice and think most often about.
