[nostrademons]

I look at it this way:

Intelligence is nothing more or less than a certain set of behaviors. Things like curiosity, experimentation, analysis & reflection, precision of thought, voracious reading, etc. It's genetic because behavior is genetic. But genetic != immutable: it's possible to change your intelligence by changing your behavior.

It's really habits that matter. Somebody could be born predisposed towards learning lots of things, and then gradually become unintelligent because they believe it just doesn't matter. Or they could be born frighteningly intelligent and then gradually go crackpot by falling victim to confirmation bias and only seeking out information that fits their pre-existing mental models (Noam Chomsky and Stephen Wolfram fit this description, and IMNSHO a prominent YC founder is in danger of going down this road). Or they could be born normal, but have parents that encourage them to try things out and explore the world around them. Or they could be born dumb, but choose to adopt habits that involve taking in prodigious amounts of information.