A reader asked me to estimate Einstein’s IQ, which I’ve wanted to do for a long time. But first it’s important to note that his brain reportedly weighed 1230 g when he died. However as scholar J.P. Rushton has explained, brain weight increases by 9% post-mortem so I will divide this figure by 1.09 to get the in-vivo weight which gives 1128 g. This sounds small but keep in mind he died at 76 and adult brain size averages 2 grams smaller each year of older age, which means had he died the same year but been 25 instead of 76, his brain would have been 1230 g which equates to 1274 cm3.
Rushton shows that Caucasian enlisted men in the U.S. army have a mean cranial capacity of 1468 (Standard Deviation (SD) = 91) however this data was from 1989, and because of better nutrition, cranial capacity has been increasing by 1.25 cm3 per year, so in 1955, the average Caucasian man should have had a brain size of 1426 cm3, so even adjusting for age and cohort, Einstein’s brain was 1.67 SD below average for a man.
Einstein was also Jewish. The average brain size of Ashkenazi’s Jews does not appear to be known, but given that Ashkenazis average 0.66 SD better than white gentiles on IQ tests, and given that within sex brain size correlates about 0.4 with IQ, we might guess they average brains that are 0.66 SD(0.4) = 0.26 SD larger than white gentiles. If so, that would make Einstein’s brain 1.93 SD smaller than the average Ashkenazi man of his age and cohort. In other words the bottom 3% for his generation and ethnicity.
Given that Einstein is the poster boy for genius, this is a terrible symbolic blow to craniometry. Because most people think anecdotally, not statistically, no matter how many studies report a positive correlation between IQ and brain size, people can always counter “but Einstein had a small brain.”