In the third edition of this series, I use the DeSilva (2021) data-set explore human brain size from 11,700 years ago to about the first World War. The data-set has 704 crania in this period and these range from 907 cc (Morton specimen from the early 20th century) to 1,859 cc (Bristol 2 specimen from about 750 years ago).
The total sample in this era has a mean of 1324 cc with an SD of 156. This is a major reduction from the 1459 cc mean found in the Upper Paleolithic so I must side with those scientists who say the brain has shrunk since the ice age (at least phenotypically).
Why did the brain suddenly start shrinking after 4 million years of growth?
The most obvious reason is malnutrition. The transition from hunters to farmers damaged our height, health and life span, so why not our brain size (and intelligence) too? But the good news is these declines reflect our phenotypic plasticity. The fact that we still have the genetic potential to be tall, smart and big brained is proven by the speed with which these traits have come roaring back with the advent of 20th century nutrition. Indeed my analysis of 21st century military records suggests that at least in America, brain size has reached 1418 cc. This is almost as big as our Paleolithic ancestors and what little shortfall exists, can probably be explained by us being more genetically gracile than our muscle bound stone age forerunners who needed those extra 40 cc to coordinate their ample bulk. It’s well known that brain size better predicts intelligence when body size is adjusted for.