In a section of her national bestseller “The Highly Sensitive Person,” Elaine Aron Ph.D discusses why ‘HSP’s’ are a distinct group, separate from the nonsensitive.
You Are Truly a Different Breed (pg 27)
“Jerome Kagan, a psychologist at Harvard, has devoted much of his career to study this trait. For him it is as observable a difference as hair and eye color. Of course, he calls it other names – inhibitedness, shyness, or timidity in children – and I cannot agree with his terms. But I understand that from the outside, and especially in a laboratory setting, the children he studies do seem mainly inhibited, shy, or timid. Just remember as I discuss Kagan that sensitivity is the real trait and that a child standing still and observing others may be quite uninhibited inside his or her processing of all the nuances of what is being seen.
Kagan has been following the development of twenty-two children with the trait. He is also studying nineteen who seemed to be very “uninhibited.” According to their parents, as infants the “inhibited” children had had more allergies, insomnia, colic, and constipation that the average child. As young children, seen in the laboratory for the first time, their heartbeat rates are generally higher and under stress show less change. (Heart rate can’t change much if it is already high.) Also when under stress, their pupils dilate sooner, and their vocal cords are more tense, making their voice change to a higher pitch. (Many HSP’s are relieved to know why their voice can become so strange sounding when they are aroused.)
The body fluids (blood, urine, saliva) of sensitive children show indications of high levels of norepinephrine present in their brains, especially after the children were exposed to various forms of stress in the laboratory. Norepinephrine is associated with arousal; in fact, it is the brain’s version of adrenaline. Sensitive children’s body fluids contain more cortisol, both when under stress and when at home. Cortisol is the hormone present when one is in a more or less constant state of arousal or wariness. Remember cortisol; it comes up again.
Kagan then studied infants to see which ones would grow into “inhibited” children. He found that about 20 percent of all babies are “highly reactive” when exposed to various stimuli: They pump and flex their limbs vigorously, arch their backs as if irritated and trying to get away, and frequently cry. A year later, two-thirds of the study’s reactive babies were “inhibited” children and showed high levels of fear in new situations. Only 10 percent showed low levels. So the trait is roughly observable from birth, as was the case with Rob.
All of this suggests what I have already said- that sensitive children come with a built-in tendency to react more strongly to external stimuli. But Kagan and others are discovering the details that make that so. For example, Kagan found that babies who later showed this trait also had cooler foreheads on the right side of their head, which indicates greater activity on the right side of the brain. (The blood is drawn away from the surface toward the activity.) Other studies have also found that many HSP’s have more activity in the right hemisphere of the brain, especially in those who stay sensitive from birth into childhood – that is, were clearly born that way.
Kagan’s conclusion is that persons with the trait of sensitivity or inhibitedness are a special breed. They are genetically quite different, although still utterly human,…”