No-one does real intelligence studies in humans because it's such a third-rail in academia. Think "The Bell Curve". Some scientists seem to believe that intelligence is intrinsic. The analogy with dog breeds, with their different temperaments and capabilities, with individual variation, applies to humans as well.
See Richard Haier: IQ Tests, Human Intelligence, and Group Differences | Lex Fridman Podcast https://www.youtube.com/watch?v=hppbxV9C63g