That being said, IQ is highly predictive one's ability to perform a broad range of cognitive tasks (along with many other important life outcomes).
Might IQ be a measure of the aggregate of the benefit of equivalent measures of neurons?
For example, we know that MS patients have autoimmune diseases that attack the myelin sheath on neurons which slows their neuronal processing speed. Perhaps higher IQ people might have more Schwann cells to speed neuronal transmission more than lower IQ people.
Maybe the diameter of neurons or the width of the gap between the synapses. Or the sheer number of neurons and synapses might be different.
IQ could be an indirect measurement through test taking, that approximates a combined measure of all the physical properties of neurons and their arrangements that contribute to higher intelligence.
- there is a meaningful (albeit small) correlation between IQ and reaction times
- people with higher IQs show lower brain activation when performing cognitive tasks compared to people with lower IQs. That is, their brain has to do less work to perform the same task.