My thought is that we assume humans have the same capacity on average, because while there might be differences, we don’t know which way they’ll go so they should ‘wash out’ as statistical noise.
In another comment, I mentioned that I think this is actually only fair to assume while we don’t know much about the individual humans. We could break this symmetry pretty easily.
FWIW, the analogue to my response here would be to say we can expect all chickens to have approximately the same capacity as each other, even if individuals chickens differ. The claim isn’t about humans per se, but about similarities borne out of genetics.
Since humans also differ from each other genetically, isn’t the distinction here just a matter of degree?
In another comment, I mentioned that I think this is actually only fair to assume while we don’t know much about the individual humans. We could break this symmetry pretty easily.
Since humans also differ from each other genetically, isn’t the distinction here just a matter of degree?