My great-grandmother once told me that babies keep only one trait throughout the course of their lifetime—their temperament. It would seem her folk wisdom was not far off the mark. A study published in the Sept. 6 issue of Molecular Psychiatry, demonstrates that a male infant’s reaction to unexpected stimuli, a biological measure of temperament, can predict amygdala activity later in adulthood.
More than two decades ago, Jerome Kagan and Nancy Snidman from the Infant Study Lab at Harvard University discovered that not only did children who were shy and inhibited at two years of age preserve that tendency through 11 years of age, they also demonstrated a distinctive heart rate, blood pressure and pupil dilation compared with more extroverted children. Carl Schwartz, then a resident in psychiatry at Harvard Medical School, wondered whether that behavioral profile would continue through adolescence.
“We already knew that serious depression and anxiety starts to take off in early adulthood and adolescence,” Schwartz says. “I was interested to see if that inhibition continued later on in life—and perhaps if there was some link between that personality type and later clinical issues.”
To address the question, Schwartz, Kagan, and colleagues followed 135 children for more than 18 years. Beginning at four months of age, each infant was introduced to a set of novel yet non-threatening stimuli including a brightly colored mobile, the smell of butyl alcohol, and unfamiliar voices. The researchers found that male infants who were highly reactive, responding with more crying and limb movement to new stimuli, later as adults showed increased blood flow in the amygdala, a brain region linked to novelty and emotion, in response to unfamiliar faces. This, they argue, suggests that temperament is an inherited trait governed by neurobiology. (They did not see this correlation in females.)
“There is something about the neurobiology of high reactive children, and we aren’t quite sure what that is yet, that is inherited and makes the amygdala hyper-excitable,” says Kagan, a member of the Dana Alliance for Brain Initiatives. “That’s something that stays with these kids over the course of their life. Even though their personality might have changed and their behaviors might have changed, that neurobiology has not.” While Kagan is careful to point out that the neurobiology of temperament is not deterministic, he does think it is quite hard to alter. “Think of different breeds of dogs. They are all part of the same species, but have very different personalities and temperaments,” he says. “It’s very difficult to take a pit bull and give him the temperament of a Labrador. It’s not impossible, but it’s very difficult.”
Schwartz readily admits he is not sure why the study resulted in a gender difference. He hopes to examine the question in more detail in future research—as well as look at potential genetic markers for more reactive amygdalas.
But Doreen Arcus, a former student of Kagan’s who is a professor of psychology at the University of Massachusetts Lowell, cautions that the neurobiology of temperament is not exempt from environmental influence. “There’s a great analogy Jerry Kagan taught me 20 years ago. He asked me, ‘Which is the more powerful animal, the tiger or the shark?’ The answer depends on what environment you’re talking about,” she says. “The fact that the amygdala is more reactive to novelty is several steps away from how the person actually functions in the world. To really understand temperament, we need to examine how it interacts with social context as well.”
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Site: Centre for Sudies on Human Stress