To quote the authors' preface: "This book is about differences in intellectual capacity among people and groups and what those differences mean for America's future."
In Part I, "The Emergence of a Cognitive Elite," the authors argue that in past ages and cultures, most notably western Europe, social class was determined largely by lineage and money rather than by intelligence. Therefore one would probably find a pretty normal distribution of intelligence at all class levels: that is, one could find smart peasants and stupid aristocrats.
". . . a large majority of the smart people in Cheop's (sic) Egypt, dynastic China, Elizabethan England, and Teddy Roosevelt's America were engaged in ordinary pursuits, mingling, working, and living with everyone else. Many were housewives. Most of the rest were farmers, smiths, millers, bakers, carpenters, and shopkeepers. Social and economic stratification was extreme, but cognitive stratification was minor." (p. 27)
One of the major premises of the book is that what they call cognitive stratification is a product of a high-tech society. Before the 20th century, ". . . the number of very bright people was so much greater than the number of specialized jobs for which high intelligence was indispensable." (ibid.) But during that century, assert the authors, a class structure based on intelligence emerged.
Chapter 1, "Cognitive Class and Education, 1900-1990," gives some information that I find highly interesting. The authors point out that from 1900-1990 there was a fifteen-fold increase in the proportion of people getting college degrees, and that students wishing to enter college were being more effectively selected for high IQ. They also mention that "Starting in the 1950s, a handful of institutions became magnets for the very brightest of each year's new class. In these schools, the cognitive level of the students rose far above the rest of the college population." (p. 29) They show with graphs (which one needs the smattering of statistics to interpret for oneself) that:
1, during the 20th century, the "prevalence" of the college degree went from about 2% to about 33% of the population;
2, starting about 1950, more of the top (high-school) students went to college;
3, between the 1920s and the 1960s, college attendance became more closely correlated to IQ, and
4, the cognitive sorting continued throughout one's college career.
"By the early 1960s," say the authors, "the entire top echelon of American universities had been transformed. The screens filtering their students from the masses had not been lowered but transformed. Instead of the old screen -- woven of class, religion, region, and old school ties -- the new screen was cognitive ability, and its mesh was already exceeding fine." (p. 42)
(I recall mentioning elsewhere that both my parents graduated from high school in 1931, and though they were both very bright people, for them college was out of the question. They went immediately to work, and mentioned many times they were lucky to find it. They pushed education at us six kids, with the result that all six went to college, we have five BA's and an AA, and four Master's degrees; more than that, two of my nieces have doctorates. I find it interesting that I would consider the girls part of the "cognitive elite"; my siblings and I, for the most part, definitely are not.)