THE NEUROCHEMISTRY OF TIMIDITY
The difference between cautious Tom and bold Ralph, Kagan believes, lies in the excitability of a neural circuit centered on the amygdala. Kagan proposes that people like Tom, who are prone to fearfulness, are born with a neurochemistry that makes this circuit easily aroused, and so they avoid the unfamiliar, shy away from uncertainty, and suffer anxiety. Those who, like Ralph, have a nervous system calibrated with a much higher threshold for amygdala arousal, are less easily frightened, more naturally outgoing, and eager to explore new places and meet new people.
An early clue to which pattern a child has inherited is how difficult and irritable she is as an infant, and how distressed she becomes when confronted with something or someone unfamiliar. While about one in five infants falls into the timid category, about two in five have the bold temperament—at least at birth.
Part of Kagan's evidence comes from observations of cats that are unusually timid. About one in seven housecats has a pattern of fearfulness akin to the timid children's: they draw away from novelty (instead of exhibiting a cat's legendary curiosity), they are reluctant to explore new territory, and they attack only the smallest rodents, being too timid to take on larger ones that their more courageous feline peers would pursue with gusto. Direct brain probes have found that portions of the amygdala are unusually excitable in these timid cats, especially when, for instance, they hear a threatening howl from another cat.
The cats' timidity blossoms at about one month of age, which is the point when their amygdala matures enough to take control of the brain circuitry to approach or avoid. One month in kitten brain maturation is akin to eight months in a human infant; it is at eight or nine months, Kagan notes, that "stranger" fear appears in babies—if the baby's mother leaves a room and there is a stranger present, the result is tears. Timid children, Kagan postulates, may have inherited chronically high levels of norepinephrine or other brain chemicals that activate the amygdala and so create a low threshold of excitability, making the amygdala more easily triggered.
One sign of this heightened sensitivity is that, for example, when young men and women who were quite shy in childhood are measured in a laboratory while exposed to stresses such as harsh smells, their heart rate stays elevated much longer than for their more outgoing peers—a sign that surging norepinephrine is keeping their amygdala excited and, through connected neural circuits, their sympathetic nervous system aroused.4 Kagan finds that timid children have higher levels of reactivity across the range of sympathetic nervous system indices, from higher resting blood pressure and greater dilation of the pupils, to higher levels of norepinephrine markers in their urine.
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Silence is another barometer of timidity. Whenever Kagan's team observed shy and bold children in a natural setting—in their kindergarten classes, with other children they did not know, or talking with an interviewer—the timid children talked less. One timid kindergartener would say nothing when other children spoke to her, and spent most of her day just watching the others play. Kagan speculates that a timid silence in the face of novelty or a perceived threat is a sign of the activity of a neural circuit running between the forebrain, the amygdala, and nearby limbic structures that control the ability to vocalize (these same circuits make us "choke up" under stress).
These sensitive children are at high risk for developing an anxiety disorder such as panic attacks, starting as early as sixth or seventh grade. In one study of 754 boys and girls in those grades, 44 were found to have already suffered at least one episode of panic, or to have had several preliminary symptoms. These anxiety episodes were usually triggered by the ordinary alarms of early adolescence, such as a first date or a big exam—alarms that most children handle without developing more serious problems. But teenagers who were timid by temperament and who had been unusually frightened by new situations got panic symptoms such as heart palpitations, shortness of breath, or a choking feeling, along with the feeling that something horrible was going to happen to them, like going crazy or dying. The researchers believe that while the episodes were not significant enough to rate the psychiatric diagnosis "panic disorder," they signal that these teenagers would be at greater risk for developing the disorder as the years went on; many adults who suffer panic attacks say the attacks began during their teen years.5
The onset of the anxiety attacks was closely tied to puberty. Girls with few signs of puberty reported no such attacks, but of those who had gone through puberty about 8 percent said they had experienced panic. Once they have had such an attack, they are prone to developing the dread of a recurrence that leads people with panic disorder to shrink from life.
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