"Some people have more highly activated left prefrontal cortexes and some people have more highly activated right prefrontal cortexes. (This has nothing to do with the question of hemispheric dominance that determines whether you are right-handed or left-handed, which occurs in other areas of the brain.) The majority of people have higher left-side activation. People with higher right-side activation tend to experience more negative emotion than people with higher left-side activation. Right-side activation also predicts how easily someone's immune system will become depressed. The right-brain activation is also correlated with high baseline levels of cortisol, the stress hormone. Though the settled patterns of activation do not stabilize until adulthood, babies with greater right-side activation will become frantic when their mothers leave a room; babies with strong left-side activation will be more likely to explore the room without apparent distress. In babies, however, the balance is subject to change. "The likelihood," Davidson says, "is that there's more plasticity in the system in the early years of life, more opportunity for the environment to sculpt this circuitry."