I'm not sure if this one is going to be too obtuse. I'll attempt to explore some of the history of thought behind the idea of dealing with people as aggregated classes, rather than as unique individuals.
The British classical economists, people like Adam Smith, David Ricardo etc, tried to think and work in aggregated "wholes"
so for example, they didn't think in terms of a glass of water, or an ounce of gold, they would refer to "water" and "Gold" in the aggregate, meaning all of the water and all of the gold in the world. Prices were not the price of milk today in the corner shop on such and such a corner, it was "The Price Level", People weren't thought of as individuals, they were thought of in terms of "nations" or "labour" or "landlords". Their methodology completely abstracted from the individual.
There are a couple of routes from those guys and their methodology, straight into the present day.
the less contentious, is via Marshall, prof of economics at Cambridge at the beginning of the 20th century, and his best known but perhaps most misleading student, John Maynard Keynes (his dad was also called John, hence use of middle name).
The more contentious, even than Keynes! is Karl Marx, whose three volumes of "Kapital", were pretty much devoted to expounding an economic theory (labour theory of value) and its implications (including exploitation). The basis which he built on, was arguably a fairly faithful, if muddled, regurgitation of David Ricardo.
Marx' other great forebear, was arguably Hegel. Hegel's "internal relation" concept, held that all things in the universe are related as a whole, and a change in one area or aspect of the universe, changed all other areas of the universe too. One implication of this is that it appears that Hegel denied the possibility of seperate sciences.
Hegel was also very influential on the thinking of the German Historical School economists, and Historicism was in turn the basis of the "Institutionalists" who dominated early 20th century American economic thought.
Both Marx and i think Hegel too, also drew on a much older idea dating at least from Joachim of Fiora (communism has a much longer history than Marx and his "Scientific socialism") This held that an aggregated borg like collective "humans" had been "alienated" from their connection with the universe by the creation.
I suspect that these "collective" influences are part of the explanation of the presentation of people as aggregated groups, such as "blacks are" or "whites are".
There is a lovely refutation of this line of thinking in Bohm-Barwerke's "Karl Marx and the close of his system" (a better translation of the title would be "Karl Marx and the completion of his system" as Bohm-Barwerke published in 1898, after the posthumous publication of the third and final volume of Kapital).
Bohm-Barwereke was criticizing Marx assertion that differences in rates of profit cancel each other out either side of the mean, hence the mean price can be used. The same refutation / reductio ad absurdum applies to all of the aggregations.
We might just as well try in this way to prove the proposition that animals of all kinds, elephants and May flies included, have the same length of life; for while it is true that elephants live on an average one hundred years [not sure about that! @] and May flies only a single day, yet between these two quantities we can strike an average of fifty years. By as much time as the elephants live longer than the flies,
the flies live shorter than the elephants. The deviations from this average "mutually cancel each other," and consequently on the whole and on the average the law that all kinds of animals have the same length of life is established!
Bohm-Barwerke page38
However, Marx is not so easily refuted; Alexander Grey explains why
To witness Bohm-Bawerk or Mr. [H.W.B.] Joseph carving up Marx is but a
pedestrian pleasure; for these are but pedestrian writers, who are so pedestrian as
to clutch at the plain meaning of words, not realising that what Marx really meant
[Cole] has no necessary connection with what Marx undeniably said. To witness
Marx surrounded by his friends is, however, a joy of an entirely different order.
For 'it is fairly clear that none of them really knows what Marx really meant; they
are even in considerable doubt as to what he was talking about; there are hints that
Marx himself did not know what he was doing. In particular, there is no one to tell
us what Marx thought he meant by 'value'. And indeed, what all these conjectures
reveal is somewhat astounding, and, one would like to think unique. Capital is, in
one sense, a three-volume treatise, expounding a theory of value and its manifold
applications. Yet Marx never condescends to say what he means by 'value', which
accordingly is what anyone cares to make it as he follows the unfolding scroll
from 1867 to 1894. Nor does anyone know to what world all this applies. Is it to
the world in which Marx wrote? Or to an abstract, 'pure' capitalist world existing
ideally in the imagination, and nowhere else? [Croce] Or (odd as the suggestion
may appear) was Marx (probably unconsciously) thinking in terms of medieval
conditions? [Wilbrandt] No one knows. Are we concerned with Wissenschaft,
slogans, myths, or incantations? Marx, it has been said, was a prophet - and
perhaps this suggestion provides the best approach. One does not apply to Jer-
emiah and Ezekiel the tests to which less inspired men are subjected. Perhaps the
mistake the world and most of the critics have made is just that they have not
sufficiently regarded Marx as a prophet - a man above logic, uttering cryptic and
incomprehensible words, which every man may interpret as he chooses.
Alexander Grey; "the socialist tradition from Moses to Lenin"
For methodological individualism, we need to go to Aristotle and via the Scholastics, directly to Locke and Jefferson (Jefferson had read Juan de Mariana's "history of Spain" and possibly other works too).
A seperate route was via Franz Brentano (arguably a modern Scholastic, he was also Freud's philosophy teacher and a pioneer in his own right in psychology) and into the Austrian School of economics, Of which Eugen von Bohm-Barwerke was one of the early thinkers.
To sum this comment up in one line;
I suspect that the people who are thinking individuals and people who are thinking aggregated wholes are talking past each other, in different paradigms.