We're a social species. We're almost totally dependent on others during infancy, but during childhood and adolescence we gradually move towards the autonomous but interdependent adulthood that a democratic society requires.

Human brain development reflects our extended maturation. Our basic birth brain weighs about a pound. It doubles in size during the first year, and much of this increase drives the developing competence of the sensorimotor systems that process environmental awareness and interaction. Our brain's third pound, which develops over the next 20 or so years, drives the development of cultural competence—our ability to interact successfully with other humans, perhaps the most important biological subset of our environment.

What we have to master during our maturation is the ability to successfully identify and appropriately respond to the novel and familiar dangers and opportunities that constantly confront us. A previous column indicated that separate but highly interconnected brain systems process these sets of cognitive challenges. For example, the sensory lobes at the back of our brain identify the nature and dynamics of a challenge and the frontal lobes develop an appropriate response to it. The right hemisphere (in most people) identifies and responds to novel challenges and the left hemisphere to familiar challenges.