The introduction to Part II of the book says: "Part II presents our best estimate of how much intelligence has to do with America's most pressing social problems. The short answer is 'quite a lot,' and the reason is that different levels of cognitive ability are associated with different patterns of social behavior. High cognitive ability is generally associated with socially desirable behaviors, low cognitive ability with socially undesirable ones."
But Herrnstein and Murray add what looks like a disclaimer, that associated with does not mean coincident with. (As statisticians are fond of saying, correlation does not mean causation.) "What this means in English is that you cannot predict what a given person [my emphasis] will do from his IQ score -- a point that we have made in part I and will make again, for it needs repeating. On the other hand, despite the low association at the individual level, large differences in social behavior separate groups of people when the groups differ intellectually on the average."
If you go back to my example given in the second essay (Feb. 1, 2010), "What looks random maybe isn't," you'll see that I tried to emphasize that no single flip of the little sink strainer can be predicted (seat or not seat itself), but after 187 trials of ten flips each, a pattern emerges. What is unpredictable singly can be accurately estimated en masse.
Now here comes what looks like a side-step. Where can one get anough data to do decent analysis on, let alone draw valid conclusions from?
The authors point to a long-term study called the National Longitudinal Survey of Youth, begun in 1979 with 12,686 persons aged 14 to 22 at the time. (Check out http://www.bls.gov/nls/ for more info. The page is maintained by the US Dept. of Labor, Bureau of Labor Statistics.) Herrnstein and Murray call the study "the mother lode" and add that it is unique because it combines in one study all the elements that had to be studied piecemeal beforehand -- and presumably the elements had to be gathered from many different studies.
(I had somewhat the same problem when dealing with estimated traffic demand at a proposed new interchange. The highway volume data came from the state; the cross-street volume data came from a county or city. The data had to be integrated into one data set before any analysis was even possible.)
The next thing Herrnstein and Murray do, after explaining where they get their best data, is to define cognitive classes. They divide the population into five classes, as seen here, and what isn't in the picture is in the text: classes I and V are each only 5% of the population; classes II and IV are each 20%, and class III is 50% of the population.
Then comes an important note to the reader: "You -- meaning the self-selected person who has read this far into the book [p. 121] -- live in a world that probably looks nothing like the figure. In all likelihood, almost all of your friends and professional associates belong in that top Class I slice. Your friends and associates whom you consider to be unusually slow are probably somewhere in Class II. Those whom you consider to be unusally bright are probably somewhere in the upper fraction of the 99th centile, a very thin slice of the overall distribution. In defining Class I, which we will use as an operational definition of the more amorphous group called the "cognitive elite," as being the top 5 percent, we are being quite inclusive. It does, after all, embrace some 12 1/2 million people." [Note this was 14 years ago.]
Very important endnote: In the light of eternity, how smart we are matters little compared to what we do with what we've got. "The greatest of these is love." Remember the parable of the Talents. "Much is required from him to whom much is given." It's the same as a great quip I saw on a blog, maybe one of yours: the most glorious monstrance in the world is nothing compared to Who is in it.