Proportion of Pre- and Postnatal Brain Growth

The human brain size compared to weight is larger than other primates, but there are additional differences in the developmental trajectory as well.

For the rhesus macaque, the brain at birth is about 48-68% the adult brain size; for chimpanzees it is 36-46% and for humans, it is only 25-29% the adult size.1 This means that humans experience much more postnatal brain growth than both chimpanzees and rhesus macaques, although they are more similar to chimpanzees in this regard. Outside of primates, there are mammals born with much less prenatal brain growth, such as with brown bears and polar bears. These animals are born with a brain that is only 2-8% of the adult brain size.1

The rate of prenatal brain growth compared to body growth is approximately the same for humans and chimpanzees, but following birth, brain growth versus body growth decreases substantially for chimpanzees until they reach adult brain size. For humans, the postnatal brain growth versus body growth continues at a similar rate to prenatal growth until the adult brain size is reached.2 This difference accounts for the increase in the adult brain versus body size in humans compared to chimpanzees.