The brain is the control center of the body, even for processes about which the individual has no awareness. A baby's head is large in relation to the rest of its body because the brain at birth is almost the same size as in adulthood, but the brain both grows and shrinks throughout life. Brain size is only one of several factors contributing to a person's intelligence, however. Male and female brains differ in brain structure, proportion and organization.
Every sensation must reach the brain before a person can perceive it; every muscle movement starts in the brain. The left and right cerebral hemispheres are similar but not identical. Each side controls the sensations and movements of the opposite side of the body, but the left hemisphere is more analytical and handles spoken and written language in most people, while the right hemisphere deals more with spatial ability, creativity, and remembering music and song lyrics. Lower brain structures handle more automatic functions such as the sleep-wake cycle, balance and breathing.
The brain is especially flexible in infancy and childhood, though some plasticity remains throughout life. Babies lose half their neurons in the first year after birth, but the brain still grows about three times the baby's size during that time. New nerve cell connections form as the individual learns new things. Brain density, based on the number of brain cell connections, is a more accurate measure of intelligence than overall brain size.
Men's brains have a sexual pursuit area that is 2.5 times larger than the sexual pursuit area in women's brains. Men's left and right cerebral hemispheres differ more; women's cerebral hemispheres tend to share more functions. This lets women's brains more easily take over lost brain functions after a stroke.