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134 EFFECTS OF EARLY MALTREATMENT AND STRESSIt is now clear that in humans, as in rodents, there is a period in our developmentwhen it is difficult to activate the HPA axis, and that this period is one duringwhich stimuli from the caregiver maintain the axis in a relatively quiescentstate (Gunnar, 2003). As shown in figure 6-2, taking the infant to the doctor forher well-baby checkups and childhood inoculations provokes a significant elevationin cortisol at 2 months of age. This elevation is comparable to that seen atbirth to mild pain stressors (see review, Gunnar, 1992). There is a reduction inresponse at 4 and 6 months, and then at 15 months, on average, no significantincreases in cortisol are observed. Other researchers have shown a similar pattern(c.f. Lewis & Ramsay, 1995), with work by Jacobson and colleagues (Jacobson,Bihun, Chiodo, & Berube, 1994), who studied babies receiving their exam andinoculations at 12 months, suggesting that this process is complete by the end ofthe first year. Studies of other psychosocial stressors confirm this pattern, withbrief separations being capable of elevating cortisol among 9-month-olds but not13-month-olds (Gunnar, Mangelsdorf, Larson, & Hertsgaard, 1989).In all these studies, the infant has been with the parent during the stressor period,and this appears to be critical in producing this type of hyporesponsivity (see review,Gunnar & Donzella, 2002). Furthermore, there are marked individual differences,with many babies showing no stressor-induced elevations in cortisol, whereassome babies continue to show significant cortisol elevations. Echoing the rat research,whether or not elevations in cortisol are provoked appears to depend, in part at least,on the nature of the relationship between the infant and the caregiver. In relationshipsin which the infant has experienced sensitive and responsive care, the pres-Delta cortisol g/dl0.70.60.50.40.30.20.1024 6 12Age (mos)15 18Figure 6-2 Salivary cortisol increases (posttest–pretest) studied longitudinally in responseto well-baby exams and inoculations in a sample of typically developing infants. Figureadapted from Gunnar et al., 1996.