In my last article I used the demographic approach to estimate a reader’s IQ. This is used when you lack psychometric data, but you know other details statistically related to test scores. This method is most often used when dementia is suspected but the patient has no psychometric history, thus the only way to check if cognition has declined is to estimate the expected level of past IQ. Such estimates are made from demographic variables like education, occupation and race, but I like to add physical variables like head size, height, body mass index etc, hence I renamed it bio-demographics.
In part 2 we take a historiometric approach. Like the biodemographic approach it’s also used when we don’t have test scores per se, but we do have a cognitive history amenable to quantitative analysis. For example if I know you learned to read at age 5, while the average kid can’t learn until 6, I might estimate your IQ is 120, since you were cognitively functioning at 120% your age level.
Another example might be when Homo Erectus was said to have the mind of a modern European 7-year-old because the tools they made could not be learned by children younger. This might also be called cognitive archeology, a term James Flynn used.
The reader wrote:
I was precocious only with respect to verbal ability. My kindergarten teacher evidently told my mother that I had the most extensive vocabulary of any child she had taught in a 20 year period. Her typical class probably consisted of 20 students, so I take this to mean that I had the most advanced vocabulary out of ~400. Half of the students at this school were white, and the other half black.
Of course being at the one in 400+ level at this particular school is not necessarily equivalent to being at the one in 400+ level for Americans as a whole, especially since the racial distribution of this school is not typical.
Since half the school is white, we can guess he was at the one on 200+ level among the white students, however because of systemic racism, upper class whites send their kids to white schools, leaving the children of the lower class to attend largely black schools.
On page 63 of Charles Murray’s Coming Apart he notes that whites with only a high school diploma average IQ 99 (U.S. norms) and those with even less average IQ 87. Splitting the difference, IQ 93 was perhaps the mean of white parents of students at this school.
But given the 0.5 IQ correlation between parents and their children, we’d expect the children of these white parents to progress 50% to the national white mean (IQ 103), thus the white students likely averaged IQ 98.
Assuming a standard deviation of about 15 we’d expect the one in 200+ level (+2.5 SD) to be IQ 136+.
So our historiometric estimate of the reader’s IQ (136+) is somewhat higher than the bio-demographic one (127).