There is no correlation between brain size and intelligence. Individuals with the smallest brains (as small as 46 cubic inches [750 cubic centimeters]) and the largest brains (as large as 128 cubic inches [2,100 cubic centimeters]) have the same functional intelligence.
Brain cells grow in size and degree of myelination as a child grows from birth to adulthood. Although the number of neurons does not increase after infancy, the number of glial cells does increase. An adult brain is approximately three times as heavy as it was at birth. Between ages 20 and 60, the brain loses approximately 0.033 to 0.10 ounces (1 to 3 grams) a year as neurons die and are not replaced. After age 60 the annual rate of shrinkage increases to 0.10 to 0.143 ounces (3 to 4 grams) per year.
