Human Brain Size

At birth, the average infant has a brain that weighs about three-fourths of a pound. In adulthood, the average human brain clocks in at about three pounds (slightly more for the average male brain). Compare that with the brain of a sperm whale that can average 18 pounds, or that of an elephant the weight about 11 pounds. It is also interesting to view the human brain in terms of relative size in relation to body size. The human brain is estimated to be about three percent of the person’s body mass (although it uses about twenty percent of all the energy generated). The shrew’s brain accounts for about ten percent of its body mass. The Encephalization Quotient is a measure of brain size relative to body size. According to Michael Balter, the cat has an Encephalization Quotient of about one, chimps register about two and a half, dolphins around five, and humans at nearly 7.5. And then there are rats and rabbits that are way down on the Encephalization Quotient at 0.4 or below.