They know that some members of a racially or ethnically defined group that on average scores low on IQ tests will score far above average. They know that some members of a group that scores high on such tests will score far below average.
From that observation, ordinary Americans readily conclude that it is irrational to discriminate according to race or ethnicity or religion and that it is rational to judge individuals on their own merits.
Proof of this comes from our last two presidential elections. Most Americans know or can readily guess that blacks on average score below whites (and further below Asians) on intelligence tests.
But they also know -- even his most vociferous critics don't deny this -- that President Barack Obama, like all recent presidents and serious presidential candidates, is well above average in intelligence. They would not have elected him president, twice, if they thought otherwise.
So the fact that there are differences in average IQ scores, or in some other testable characteristic, between races does not undercut the case against group discrimination, at least for the large majority of Americans.
But it does undercut the case for racial quotas and preferences. It undercuts the case for the "disparate impact" legal doctrine that the Supreme Court concocted in a 1971 case on hiring discrimination.
The Court acted when memories were still fresh of resistance to racial desegregation orders in the South. But the doctrine is out of date 43 years later.
"Disparate impact" doctrine assumes that in a fair society we would find the same racial or ethnic or religious mix in every school, every occupation and every neighborhood. But that's nonsense, as anyone acquainted with American life knows.
Americans are quite capable of treating individuals fairly even while acknowledging group differences that, as science shows, are the result of recent, copious and regional natural evolution.