The independence of irrelevant alternatives (“pairwise independence”) can also apply to individuals, it is just that Arrow’s theorem doesn’t directly require it.
However, a better statement of the independence of irrelevant alternatives for Arrow is “if all individuals have preferences satisfying pairwise independence, then the group’s decision function should also satisfy it”.
So for group IIA to matter we should have individual IIA
The independence of irrelevant alternatives (“pairwise independence”) can also apply to individuals, it is just that Arrow’s theorem doesn’t directly require it. However, a better statement of the independence of irrelevant alternatives for Arrow is “if all individuals have preferences satisfying pairwise independence, then the group’s decision function should also satisfy it”. So for group IIA to matter we should have individual IIA