The average man's brain weighs about 3 pounds, (1.35 kg); the average woman's, 2 pounds, 10 ounces (1.21 kg). Because men have bigger bodies on average, the relation of brain to body weight is about the same in both sexes. In any case, there is no evidence that a large brain means high intelligence. For example, the brain of the brilliant French author Anatole France was a mere 2 pounds, 4 ounces (1.02 kg).
Months before birth, a surge of the male hormone testosterone apparently affects the action of certain brain chemicals in the male fetus. Some researchers think this predisposes boys to react more strongly than girls to stress and perhaps causes the greater aggressiveness many boys begin to display at an early age. High testosterone levels in the male fetus may also be a factor in the greater frequency of left-handedness, dyslexia, and stuttering in boys. The same hormone has been linked to a slight male edge in mathematical and spatial skills, but this is speculative and controversial.
Some researchers think that certain kinds of verbal superiority shown by girls, on average, may be linked to the fact that sections of a woman's corpus callosum are usually somewhat thicker than a man's. The corpus callosum is the main bundle of nerve fibers that cross between the right and left halves of the brain.
If women do have extra connections between the two hemispheres, it may help them develop alternate pathways for specialized brain functions. If the left half of a woman's brain is damaged by stroke, for example, she is more likely to overcome severe speech difficulties than a man with a similar injury, suggesting that her verbal ability is not as strictly localized in the left brain as the man's is.