White matter, made of the long arms of neurons encased in a protective film of fat, helps distribute processing throughout the brain. It gives males superiority at spatial reasoning. White matter also carries fibers that inhibit "information spread" in the cortex. That allows a single-mindedness that spatial problems require, especially difficult ones. The harder a spatial task, Gur finds, the more circumscribed the right-sided brain activation in males, but not in females. The white matter advantage of males, he believes, suppresses activation of areas that could interfere with work.

Men do have an outstanding spatial sense and a mechanical ability. They excel at predicting the path of a moving object. It seems when testosterone floods the male at puberty, boys generally outstrip girls in geometry and mechanical drawing (but doesn’t the latter require an ability to do fine, dexterous tasks, a skill that Fisher claims for women?) This spatial ability is in the brain architecture created by fetal testosterone. It also evolved from millions of years ago when men were tracking “zebras and wildebeests,” thereby giving them the advantage in stringing telephone cable and getting to the moon.

Scientists at Johns Hopkins Medical Institution have discovered "striking" differences between men and women in a part of the brain linked with ability to estimate time, judge speed, visualize things three-dimensionally and solve mathematical problems. The differences, the researchers say, may underlie well-known trends that vary by sex, such as the fact that more men than women are architects, mathematicians and racing-car drivers.

In a study reported in the journal Cerebral Cortex, the researchers show that a brain region called the inferior parietal lobule (IPL) is significantly larger overall in men than in women. The area is part of the cerebral cortex and appears on both sides of the brain, just above ear-level. There is also a symmetry difference, with men having a larger left IPL than right. In the women studied, it's the right IPL that is somewhat larger, though the difference between the two sides of the brain is less obvious than in men, says psychiatrist Godfrey Pearlson, M.D., who headed the project.