It’s fairly substantial but there is no linear relationship between iq and any life outcome whatsoever. It just so happens that children with high iq’s are also good at abstract reasoning. One really doesn’t need a rigorous proof to infer that highly intelligent people do better at complex subjects and that this ability is probably mostly innate and thus genetic. People that deny this usually make up excuses regarding advantageous early childhood educational interventions, but of course then you get people like Stefan Banach and Gauss who were essentially self taught geniuses. Green, Faraday etc the same, self taught geniuses. Their enthusiasm is probably not a good counter argument, as enthusiasm would imply that rather than education, motivation is the deciding factor for intellectual excellence, but many people are motivated and fail to bear from fruit from their endeavors at university. Similarly you can force the question: whence cometh enthusiasm? Again, it seems some are timid and others aggressive, some are industrious and others indolent, and so we might be induced to state genetics as our argument for why some might be “motivated” to pursue science and math. All of this is distracting from the underlying point that people notice they are or are not stupid, and they choose to act on in this by self selecting into relevant advantageous environments. IQ is a sometimes useful, culturally relevant, and often controversial proxy for intelligence; the innateness of intelligence requires none of psychometrics to be plausible.