“Oh, no. That fact alone – not smiling much as an infant – is insufficient” to predict such outcomes later on, said Jerome Kagan, a psychologist at Harvard University who has studied child development for decades.
But Kagan’s research has shown that infants as young as four months old do show clear differences in temperament, probably based on the inherited biology of the brain, and that these differences can persist. About 20 percent of healthy, Caucasian babies raised in a nice, nurturing environment are nonetheless easily over-stimulated – kids that Kagan calls “high reactives.” When these babies are shown an interesting sight or sound, they thrash their arms and legs around and cry, perhaps, Kagan’s work suggests, because a sensitive structure in the brain called the amygdala is more excitable than normal.
Babies who are “high reactives” at four months often go on to smile and laugh less at one, two and three years of age, Kagan said. They do smile sometimes, but “they are less joyous. Their mothers see these children as ‘serious.’ By 10 or 11, these children may feel a “private mood of high tension” and often describe themselves “as thinking too much, or not feeling relaxed.” At age 11, these children also show a larger brain response to sounds in part of the brain that is “primed” by the amygdala, suggesting that this extra excitability is retained.
Because “high reactive” babies are somewhat tense to start with, if their life circumstances “are oppressive or full of personal loss” as well, they “would be at a slightly higher risk of depression than the average person,” Kagan said. “But you’d lose money if you said that every time you saw a high reactive baby, he or she would be depressed by age 20.”
The bottom line, said Dr. Robert Sege, associate chief of general pediatrics at Tufts-New England Medical Center, is that smiling is a baby’s way of initiating a “conversation” with parents. “If you’re worried about your baby not smiling, smile more at your baby. That’s the cure. The baby’s brain doesn’t grow in isolation.”