The hard part is not the evidence, it's dealing with the social result. Do we as a culture decide "Yes, it might be true, but it's too harmful, so we will act as if it's false?"

Give extra tutoring to those lower on the scale, and withhold it from those higher, to try to even the balance?

Give special advantage to those lower? Is there a way to do that without disadvantaging others? What sort of advantage?

Something else?

Each of those options has pros and cons.

It should be discussed, but like I said, it's politically difficult, so people try not to talk about it.