The Specialization of the Hemispheres: Two Brains or One?
For better or worse, humans have very large brains. The largest part of the brain is called the cerebrum. An X-ray of the cerebrum reveals that it's divided into two similar "loaves" called the cerebral hemispheres. (The two halves of the cere- brum are connected by a "bridge" of tough, fibrous tissue called the corpus callosum.) When you hear people talk about your right brain and your left brain, this is what they're talk- ing about. The interesting thing about the two sides of the cerebrum is that they're specialized. The right brain is nor- mally involved in applying overall patterns to objects and ideas. The left brain is more adept at linear thinking involving language. Interestingly, the main nerve pathways in the brain cross over. In other words, in most people, the left hand responds to right-brain signals, and the right hand responds to left-brain signals.
The capacity for certain cerebral activities to be "assigned" to either the left or right brain is called lateralization. The capacity for lateralization is thought to have evolved in humankind very recently in terms of the evolutionary time scale-a mere one million years ago. (The origins of Earth's species through mutations and adaptive radiation, from simple to increasingly complex, date back about 500 million years.)
Your text properly gives you an overview of research indicat- ing typical differences in lateralization among males and females. The statistical evidence is sound. Women tend to distribute functions across hemisphere, and the female corpus callosum (connecting the two hemispheres) does contain more crossover connecting paths. However, recent research suggests that it isn't safe to assume that females are inevitably more right-brained while males are inevitably more inclined to be left-brained. It turns out that variation in lateralization among individuals-male or female-is considerable.