The below passage is from the 1911 book The Jews: A Study of Race and Environment
By Maurice Fishberg (hat-tip to blogger n/a):
One of the methods of determining the volume of the brain case, and approximately the weight of the brain, is the determination of the cranial capacity. Very few direct measurements of this kind have been taken, because only few Jewish skulls have found their way into anthropological museums, where they could be studied carefully. But from the few studies of this character that have been made, it appears that the Jews are somewhat at a disadvantage. Lombroso’s studies of the Jews in Turin, Italy, which were made in an indirect fashion, showed that the Jews have a smaller cranial capacity than the Catholics of that city.2 Weinberg collected measurements of seventeen Jewish skulls in various museums of Europe, which were made properly, and are not approximations. The average cranial capacity was 1421 c.cm., which is about thirty to forty c.cm. below the average cranial capacity of the population of Europe. Of course the small number of skulls thus measured is not sufficient to draw positive conclusions.
But the average cranial capacity is nowhere near 40 cc (cubic centimeters) above 1421 (1461 cc) in Europe, and certainly wasn’t in 1911. For example based on Beals, Smith, and Dodd’s (1984), collation of 20,000 skull size measures, Lynn (2006) reported the following crania data for Europeans and other peoples:
One reason for the huge discrepancy might be the method used. For example most of Beals, Smith and Dodd’s data Lynn says he used was obtained by mechanical packing with mustard seed. However they note that historically, the main method was filling the cranium with shot, but this method became obsolete because it yielded results that were 6% too high.
Given that Fishberg’s book was written in 1911, he likely is reporting on shot values which thus need to be reduced by 6%. Doing so reduces the Jewish value to 1341 cc which is indeed below the European mean.
But keep in mind that Ashkenazi Jews are descended from a hybrid population that was about 40% Middle Eastern and 60% mostly European (Atzmon et al, 2010). Given 1369 cc for Europeans and 1293 cc for South Asians & North Africans (SANA), the ancestral population likely averaged (0.6)1293 + (0.4)1369 = 1324 cc so there’s been an increase of perhaps 17 cc.
What was the average IQ of the ancestral population? Richard Lynn put Europeans at 99 and SANAs at 84, but he also put sub-Saharan Africans at an absurdly low 67 but noted that living in the Third World depressed their IQs by 13 points. Most SANA don’t quite live in the Third World, so lets say their IQs are depressed by half that, and raise them to 92 (note: no such adjustment is made for brain size because that data mostly comes from before 1940 when even the First World was malnourished).
Expected IQ of ancestral population: 0.6(92) + 0.4(99) = 55.2 + 36.8 = 95
On a scale where British, American & Australian whites average IQ 100 (SD = 15), today’s Ashkenazi Jews average 110. In other words, their IQ seems to have increased by about 1 standard deviation in the last 700 years, probably because of intense natural selection to earn a living in high IQ occupations (Cochran, Hardy & Harpending, 2006).
Assuming a 0.4 correlation between IQ and brain size (Gignac & Bates, 2017), selection for a 1 SD increase in intelligence should cause about (0.4)(1 SD) = 0.4 SD increase in IQ. Given the within sex standard deviation for cranial capacity is about 91 cc, we should expect the brain size of Ashkenazi Jews to have increased by 0.4(91 cc) = 36 cc
So the actual brain size increase has been about half as much as the actual increase but given these are all very rough numbers, we shouldn’t expect perfect agreement.