the presence of recognizable caregivers (Ainsworth and Bell, 1970; Sroufe, 1979). These likely responses to short-term stressors play an important part in the emotional development of children and are not expected to have long-term adverse effects. However, as previously mentioned, stress responses inhibit normal growth and developmental processes that are an essential part of a healthy childhood, thus long-term or repeated exposures to stressors is likely to have negative effects on normal development (IOM, 2000).
Animal studies show that infants are particularly susceptible to stressful events, such as neglect, that have the potential to permanently alter the HPA system, resulting in hyperactive stress responses (Meaney et al., 1996; Denenberg, 1999). Decreased maternal attention such as licking and grooming have also been implicated in the development of more stress-reactive animals (Liu et al., 1997). Introducing an infant that is genetically predisposed to be more stress-reactive into the care of an adoptive mother that is genetically predisposed to be less stress-reactive causes the infant to develop with a higher than expected stress tolerance, implying a role of nurture in addition to the genetic predisposition that determines the characteristics of stress response.
Primate studies demonstrate the importance of maternal presence during early life stages. Monkeys that are separated from their biological mothers at a very young age and reared with a cloth surrogate, but provided with daily peer interactions, are less socially inept than monkeys reared in complete isolation. However, the monkeys reared with the cloth surrogate still produce a number of physiological indicators that point toward anxiety and fear (Suomi, 1991). When faced with stress, these animals produce higher levels of stress response neurochemicals such as glucocorticoids and catecholamines. Other studies indicate that monkeys reared without a cloth surrogate and only in the presence of infant peers exhibit parallel hyperactive stress responses to those reared with the surrogate (Champoux et al., 1989; Champoux et al., 1992).
As discussed, early life inputs, such as maternal presence and attention, can be crucial to the normal development of the stress response system as children grow into adults. These key inputs can keep stress response activity in check and result in the maturation of a response system that is capable of rapidly shutting down responses when the stressor has been removed. However, lack of this positive input can create a system that is hyperactive and unable to modulate responses to stimuli (NRC/IOM, 2000).
The 2000 National Research Council/Institute of Medicine report, From Neurons to Neighborhoods: The Science of Early Childood Development, highlights the importance and difficulty of crossing between disciplines to understand the multiple factors that influence early childhood development. The report recommends pursuing integrative science that includes: