Many Americans resist that explanation, in the belief that the ignorant masses will use group differences in test scores as justification for racial discrimination. But other Americans understand that averages are only averages, and that group discrimination is irrational.
And when one shifts focus to personal experience, Clark's findings make sense. Consider your own extended family or others with which you are familiar.
There's likely a range of physical differences and intellectual interests even between siblings and parents and children. But there are also patterns and resemblances, as you look back and forward a few generations.
And the differences between extended families will tend to be perpetuated by what social scientists call assortative mating -- the tendency, perhaps more pronounced lately, of people to marry people with similar characteristics.
So is there anything to the notion of America as once a land of upward mobility? Yes. As Clark notes, immigrant groups have risen rapidly from restrictions in countries of origin, to success in America.
The Eastern European Jews who arrived in the Ellis Island years (1892-1914) are a spectacular example, and there are others. The group most disproportionately producing American physicians today, Clark reveals, are Egyptian Coptic Christians.
And statistical predictability isn't individual destiny. "Whatever success you do attain will still be achieved only through struggle, effort and initiative."
Children from unprivileged households do sometimes achieve great success in this country, as in modern Sweden and even medieval England. The 44th president, like the 16th, is proof of that. Upward mobility is possible, even if not probable.
But readers may still be uncomfortable with the likelihood that, in Clark's words, "a completely meritocratic society would most likely be one with limited social mobility."
What to do about this? Clark recommends Scandinavian-style economic redistribution. But that may not work well in our more heterogeneous society.
Another approach is to affirm the dignity of honest work and modest success, to remember that, as George Eliot wrote, "the growing good of the world is partly dependent on unhistoric acts." A society gains strength not just from its elites, but from the cumulative achievements of mostly ordinary individuals.