Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
Evolution of the brain
The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominids and finally to "Homo sapiens". Human brain size has been trending upwards since 2 million years ago, with a 3 factor increase. Early australopithecine brains were a little larger than chimpanzee brains. The increase has been seen as larger human brain volume as we progressed along the human timeline of evolution (see Homininae), starting from about 600 cm in "Homo habilis" up to 1600 cm in "Homo neanderthalensis" (male averages). 
The increase in brain size topped with Neanderthals; 
Brain size of "Homo sapiens" varies significantly between population (races), with male averages ranging between about 1,200 to 1,450 cm.