Intelligence seems to correlate with total number of neurons in the brain.

Different animals' intelligence levels track the number of neurons in their cerebral cortices (cerebellum etc don't count). Neuron number predicts animal intelligence better than most other variables like brain size, brain size divided by body size, "encephalization quotient", etc. This is most obvious in certain bird species that have tiny brains full of tiny neurons and are very smart (eg crows, parrots).

Humans with bigger brains have on average higher IQ. AFAIK nobody has done the obvious next step and seen whether people with higher IQ have more neurons. This could be because the neuron-counting process involves dissolving the brain into a "soup", and maybe this is too mad-science-y for the fun-hating spoilsports who run IRBs. But common sense suggests bigger brains increase IQ because they have more neurons in humans too.

Finally, AIs with more neurons (sometimes described as the related quantity "more parameters") seem common-sensically smarter and perform better on benchmarks. This is part of what people mean by "scaling", ie the reason GoogBookZon is spending $500 billion building a data center the size of the moon.

All of this suggests that intelligence heavily depends on number of neurons, and most scientists think something like this is true.

But how can this be?

https://www.astralcodexten.com/p/why-should-intelligence-be-related