A reader sent me the following email:
I have been reading your blog off-and-on for the last 5-6 years and I’ve seen that you’ve started to estimate reader IQs from their histories and test scores and I was curious what you would estimate mine to be. I’ve written a history that I think contains the majority of the most pertinent information for your approximation. I’m sorry if I wrote too much but I wanted to give as much information as possible.
Biography: I am black male in my early 30s who is currently in his 4th year of a PhD program in psychology (my program is not elite at all and is roughly #100 in the US grad school ranking for my field) after getting my BA in the same from a top 60 college. I grew up upper-middle class and have lived in multiple countries but am mono-lingual and have spent most of my life in the US. I could read the bible at the age of 5 and I was able to read at a college level by the time I was in middle school. My math ability has lagged behind notably, however. I was better than most of my peers at math but I was not truly exceptional by any means. My family is religious but I have been an atheist since I was 14.
About 520,000 black Americans become old enough to get a PhD every year, but of these, only 2,512, did so in 2019. This puts the median black PhD in the top 1,256, or one in 414 level (+2.8 SD level). Assuming a 0.68 correlation between IQ and years of education, we’d expect your IQ to be 0.68(+2.8 SD) = +1.9 SD above the black mean.
Given that U.S. blacks have a mean WAIS-IV IQ of 87.7 with an SD 14.4 (U.S. norms), +1.9 SD = IQ 115.
According to page 374 of Charles Murray’s Coming Apart, blacks with PhDs or professional degrees average IQ 112.2 (U.S. norms)
You also write:
Physical characteristics and health: I am 5’10 and my head circumference is around 60.5cm. I think this should be taken with a grain of salt though since my head is very long and fairly narrow which should deflate actual brain volume. I was very overweight until I was around 20 years old at which point, I lost a lot of weight and have vacillated between 160-185 since and am roughly 180lbs right now. I have a history of depression and anxiety and have diagnosed epilepsy.
605 mm circumference implies a 192.5 mm cranial diameter, however because your circumference was measured in-vivo, I subtract 11 mm for fat and skin around the skull. This reduces it to 181.5 mm which implies a true circumference of 570.2 mm. Below is the curve predicting brain size from head circumference:
Source: Figue 3 in Jørgensen, J. B., & Quaade, F. (1956). External cranial volume as an estimate of cranial capacity. American Journal of Physical Anthropology, 14(4), 661–664.
The above curve predicts you’d have a cranial capacity of about 1676 cc according to an equation I created from said curve:
cranial capacity = 0.0080(head circumference)^2 – 1.9(head circumference) + 158
However because of ethnic and gender differences in head shape, I would multiply this figure by 0.915, reducing it to 1534 cc. This is still over 227 cc above the mean for young black men circa 1980 and probably about 174 cc above young black men today, putting you in the 97 percentile (+1.91 SD). Such a large brain may help explain your epilepsy because of the enhanced electrical activity.
Assuming a 0.4 correlation between IQ and within-sex brain size, this predicts an IQ of 0.4(+1.9) = +0.76 SD above the black mean, or IQ 99.
Estimated IQ of black American PhD: 115
Estimated IQ of black American with very large (1534 cc) brain size: 99
Estimated IQ of black American with both: 119
So 119 would be what I’d expect based on your two most salient biodemographic IQ correlates, but in part 2 we’ll discuss your actual test scores and see how close this prediction comes.