[note: JL has pointed out that subjective measures of skin color, as used in this data set, are much more unreliable than I initially conjectured. Apparently there’s also a race by interviewer effect (Hill, 2002). Contrary to what some may conclude, this added unreliability means that the true associations between color and IQ is higher than found. In the same way, the unreliability of self-identified race as an index of true biogeographic-ancestry also means that the true association is higher — i.e., the found correlations need to be corrected upward.]
Continuing with my most recent line of investigation, I looked at the association between skin color and aptitude tests in the NLSY97. You can read a description of that study here. It’s a follow up to the NLSY79 which, if you read the BC, you should be familiar with. The NLSY97 participants took a number of aptitude tests. Out of them, I selected the SAT verbal and qaunt, the Armed Services Vocational Aptitude Battery verbal and quant, and the Peabody Individual Achievement Test quant. The color-score correlation of 0.12 found for the ASVAB is close to the mean of all correlations. The correlations, again, are just those between color and scores within the self-identifying Black population. As I couldn’t find the relevant variables, I was unable to separate out immigrant Blacks from native Blacks; when I figure out how to do that — so as to follow up with a previous investigation, I will update this post. I didn’t feel like converting the ASVAB scores into IQ scores. For reference, the upper and lower 1/5th of the distribution differ by about 0.4 SD. (The scores are percentiles x 1000.)
Using the ASVAB scores as our index of IQ, this study puts the N-weighted correlation at 0.15 (N= 3694). (Ya, I actually look up those dusty old studies.)
Peterson and Lanier (1929)/r=0.18/n=83
Peterson and Lanier (1929)/r=0.3/n=75
Scarr et al. (1977)/r=0.155/n=288
ADD Health (unpublished)/r=0.17/n=1131
And the average Cohen’s d between the upper and lower 4rths of the spectrum is about 0.5 n = >6,000 (Shuey’s pre-60’s data here)
Feguson (1919)/d= about 0.7/n=657
Feguson (1919)/d= about .9 SD/n=667
Kock and Simmons (1926)/d= about 0.15/n=1078
Klineberg (1928)/d= about 0.15/n=200
Young (1929)/ d= about 0.8 and 0.33/n=277
Peterson and Lanier (1929)/d = about 0.66/n=83
Peterson and Lanier (1929)/d= about 0.2 SD/m=83
Bruce (1940)/d=about 0.25/n=72
Codwell (1947)/d= about 0.33/n=480
Lynn (2002)/d= about 0.5/n=430
NLSY97 (unpublished)/d= about 0.4/n=1433
ADD Health (unpublished)/d=about 0.5/n=1131
It’s interesting that both the found correlations and mean differences show a good deal of cross temporal consistency.
I discussed what the predicted correlation for a genetic hypothesis would be elsewhere. It would be a function of a) the reliability of the measures of IQ and skin color (maybe 0.9 and ?), b) the correlation between color and African ancestry in the US Black population (about 0.45), c) and the correlation between IQ and White ancestry in the Black population, given the 1) restriction of range in the distribution of White ancestry in the Black population (? maybe 0.5), 2) the within population heritability of IQ (0.4 to 0.8, age depending), and 3) some proposed between population heritability (0.5 to 1). Contra Nisbett, for any hypothesis it would be below 0.20.
The fact that there is a correlation, of course, does not reduce the environmental-genetic uncertainty much, as the correlation could always be explained environmentally. Though, given that between family differences explain little variance in IQ, such explanations are constrained. (For example, Guo and Stearns (2002) found that between family effects explained only 17% of Black IQ variability in the ADD health data, data in which there was roughly a 1/2 SD difference between lighter and darker Black kids (ages ranging approximately from 12 to 18).) If not between family effects, then what? “Colorism” seems to be an obvious possibility — but it must work through some environmental mechanisms.
Whatever the case, from a Popperian perspective, a genetic hypothesis just survived another of my falsification attempts. As they say, that which does not falsify, makes stronger.