Commenter Afrosapiens wrote:

First thing, contrary to what HBDers want us to believe, these cranial capacity gaps are extremely small. Too small to explain a one SD gap in IQ or “g”.

First of all, racial differences in brain size are not that small.  The most high quality data ever obtained on black-white brain size differences in the U.S. was a 1980 study of autopsied brains at the Case-Western Reserve University’s Medical School in Cleveland, Ohio.  The purpose of the study was to obtain norms for black and white brain weight so that when an autopsy is performed, one could determine if the brain weight was abnormal since this can tell you a lot about the cause of death.

Now one problem with this data is that the sample varies in age from 25 to age 80. This is not a problem for comparing the mean brain of blacks and whites because both groups died at a mean age of 60, however having such a wide range of ages likely greatly inflated the standard deviation and thus minimize racial differences expressed in SD units.  Since I don’t know the brain weight standard deviations for same age groups,  I decided to convert autopsied brain weight (estimated at age 25) to in vivo volume (following Rushton 1997) and use the standard deviations of Caucasian enlisted army men and women (91 cm3 and 90cm3 respectively, see table 1 of Rushton’s data ) since skull measurements, especially in young adults are much less age sensitive than brain weights in mixed-age adults. To make this conversion, one must first divide by 1.09 because brain weight increases by 9% post-mortem so the in-vivo weight is 9% lower.  Then, converting brain weight to brain volume is simply a matter of multiplying by 1.036.

group n height(cm) brain weight (g) sd of brain weight (g) brain weight height adjusted (g) estimated brain weight at age 25 (g) estimated in-vivobrain volume at age 25 (cm3) cranial capacity sd taken from rushton’s data-set (cm3 )
white males 413 175 1392 130 1392 1570 1492 91
black males 228 173 1286 138 1290 1375 1307
white females 395 162 1252 125 1252 1339 1273 90
black females 222 162 1158 119 1158 1291 1227

The data seems to indicate that the average white male has an age 25 in-vivo brain size that is 185 cm3 bigger than the average black male. A difference of 2.03 SD.

Meanwhile the average white female has an age 25 in-vivo brain size that is 46 cm3 bigger than the average black female.  A difference of 0.51 SD.

Averaging across sexes, white brains are 1.27 SD larger than black brains.  Assuming about a 0.35 within race correlation between IQ and brain size in adults, then racial differences in brain size can arguably explain 0.35(1.27 SD) = 0.44 SD or 7 IQ points.  So virtually half of the 15 point black-white IQ gap is arguably explained by racial differences in brain size.

Further, if racial differences in brain size are just a by-product of selection for intelligence, rather than directly selected for, then one could even argue that genetic IQ differences should be much greater than brain size differences since the latter were just a side-effect of selection, not the primary target.

Of course there are many other biological properties that affect IQ besides overall size such as nerve cell myelination, brain glucose metabolism, number of brain folds, etc, and I don’t know much about how the races differ on these traits, if at all.  It’s even possible that some of these traits greatly favour blacks.

It also remains possible that racial differences in brain size are entirely environmental in origin, particularly if one believes that black mothers provide worse prenatal environments.  Comparing the brain sizes of mulattos with white mothers to mulattoes with black mothers could resolve this question since both would presumably be racial identical but only the latter would have black prenatal environments.