by 4 months, reliable differences emerge. Babies who react positively are more likely to become busy, active, boisterous toddlers—some would say a handful, others refer to them as exuberant. In contrast, those who find the stimulation to be too much are more likely to be fearful and shy as toddlers and preschoolers. This evidence suggests that, for some children, negative reactivity to novel stimuli as infants evolves into a shy, inhibited, anxious temperamental pattern by toddlerhood.
This is not a trivial phenomenon. About 20 percent of healthy, European-American samples display negative reactivity to novel stimuli as young infants (Kagan et al., 1998). About one-third of the reactive infants studied by Kagan and his colleagues remained highly fearful of unfamiliar events at 14 and 21 months, and 13 percent of these infants continued to show subdued and shy behavior with unfamiliar adults and peers at 4 ½ years of age (Kagan et al., 1998). At age 6, the inhibited children continued to be socially wary and reticent during their interactions with peers and an adult experimenter, and they exhibited signs of physiological stress (Kagan et al., 1987). Researchers who have focused on the socially reticent behavior of these children have found contemporaneous associations with maternal reports of both shyness and internalizing behavior problems (Coplan et al., 1994). Recently, inhibited temperamental patterns have been associated with a physiological pattern of resting right frontal EEG activation (Calkins et al., 1996; Fox et al., 1995, 1996; Schmidt and Fox, 1994), which appears to be associated with a tendency to respond to stressful events with negative affect or depressive symptomatology (Davidson, 1992). Fox and his colleagues (in press) have recently linked this physiological pattern at 9 months of age to continuity in behavioral inhibition up to age 4. We refer to the same physiological pattern when we discuss the developmental consequences of maternal depression in Chapter 9.
Once caregivers and babies have ridden the roller-coaster of rapid developmental change through the first 3 months of life and the baby 's behavioral style or temperament seems to be easier to discern, the next task is to support the baby's developing abilities to regulate his or her emotions and behavior.
Although learning to sleep through the night might seem far afield from controlling outbursts of emotion, learning to wait before acting, self-monitoring, and acquiring the ability to organize segments of behavior sequentially—all of which are embraced by the term “self-regulation”—they all involve various forms of self-monitoring and response inhibition that, in turn, reflect the growing maturity of the brain, as we discussed at the beginning of this chapter. The infant's emerging ability to replace crying with other forms of communication is just the first step along a developmental progression that recruits the child's increasing competencies into more and more mature self-regulatory functioning.