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A growing number of researchers exploring the physiology and neurology of happiness are starting to answer those questions. Perhaps most fundamental of all is what happiness is, in a clinical sense. At this point, nobody can really say with precision. The word happiness, Davidson observes, "is kind of a placeholder for a constellation of positive emotional states. It's a state of well-being where individuals are typically not motivated to change their state. They're motivated to preserve it. It's associated with an active embracing of the world, but the precise characteristics and boundaries have really yet to be seriously characterized in scientific research."
Still, subjects can reliably tell researchers when they're feeling good, and two brain-imaging technologies--functional magnetic resonance imaging (MRI), which maps blood flow to active parts of the brain, and electroencephalograms, which sense the electrical activity of neuronal circuits--consistently point to the left prefrontal cortex as a prime locus of happiness.
That raises the chicken-and-egg question of whether the prefrontal cortex creates the sensation of happiness or whether it merely reflects one's more general emotional state. Davidson thinks the answer is both: "We're confident that this part of the brain is a proximal cause of at least certain kinds of happiness." That suggests some people are genetically predisposed to be happy by virtue of their busy prefrontal cortexes, and research in infants confirms it. Davidson measured left prefrontal activity in babies less than a year old and then subjected them to a test in which their mothers left the room briefly. "Some babies will just cry hysterically the instant the mom leaves," he says. "Others are more resilient." It turns out that the babies with the higher left prefrontal activity are the ones who don't cry. "We were actually able to predict which infants would cry in response to that brief but significant stress."
In short, as parents know instinctively, some babies are just born happy. But neuroscientists have also learned over the past decade that the brain is highly plastic. It rewires itself in response to experience, and that's especially true before the age of puberty. One might naively assume, therefore, that negative experiences might destroy a happy personality--and if they're extreme and frequent enough, that might be true. Davidson has learned, however, that mild to moderate doses of negative experience are beneficial. (In animal studies, he compared groups that had been moderately stressed when young to those that never were and found the former better able to recover from stress as adults. In human studies, in which deliberately inducing stress on kids would be unethical, he based his conclusions on self-reported stories of stressful childhoods.) The reason, he believes, is that stressful events give us practice at bouncing back from unpleasant emotions. They're like an exercise to strengthen our happiness muscles or a vaccination against melancholy.