QUOTE : A number of researchers think that there are two systems in the brain and that it is the balance of these two that creates sensitivity. One system, the “behavioral activation” (or “approach,” or “facilitation,” system) is hooked up to the parts of the brain that take in messages from the senses and send out orders to the limbs to get moving. This system is designed to move us toward things, especially new ones. It is probably meant to keep us eagerly searching for the good things in life, like fresh food and companionship, all of which we need for survival. When the activation system is operating, we are curious, bold, and impulsive. The other system is called the “behavioral inhibition” (or “withdrawal,” or “avoidance,” system). (You can already tell by the names which is the “good” one according to our culture.) This system is said to move us away from things, making us attentive to dangers. It makes us alert, cautious, and watchful for signs. Not surprisingly, this system is hooked up to all the parts of the brain Kagan noted to be more active in his “inhibited” children.