xvi, 132 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / The Differential Susceptibility Hypothesis and the related Biological Sensitivity to Context theory contend that individuals with "susceptible" traits reap the benefits of positive rearing environments and exhibit better outcomes compared to their less susceptible peers. Studies have largely focused on physiological reactivity as an index of this susceptibility in children and adults, and most have measured physiology by grand mean changes from baseline to a stressor. The goal of this dissertation was to examine baseline Respiratory Sinus Arrhythmia (RSA) and RSA stress reactivity by taking advantage of analytical techniques modeling growth over time, as well as individual differences in this growth-using latent Growth Modeling (LGM) and Growth Mixture Modeling (GMM), respectively. Maternal sensitivity at 5 months and the quality of the attachment environment at 17 months were used as indicators of environmental conditions that might interact with infant susceptibility. Problem behavior and social competence were assessed at 17 months as measures of child well-being. Consistent with the theory of differential susceptibility, there were no significant differences in problem behavior or social competence among infants with low baseline RSA, but infants with high baseline RSA exhibited the lowest levels of problem behavior if reared in an environment that fostered security and more competence if their mothers exhibited greater sensitivity. Contrary to hypotheses, LGM analyses revealed that withdrawal of infant RSA appeared to buffer the impact of being reared in an environment that fostered disorganization, as infants with disorganized attachment histories exhibited the lowest number of problem behaviors. Two distinct groups of children were identified by GMM analyses: a class of infants with low RSA that decreased across the still-face episode, and a class of infants with high RSA that increased across this episode. Class by maternal sensitivity interactions were significantly predictive of social competence, with the high increasing class emerging as the group most susceptible to environmental influences, consistent with the differential susceptibility hypothesis. This dissertation adds importantly to both the sharpening and extension of theories of differential susceptibility. / Committee in charge: Jennifer Ablow, Chairperson, Psychology; Philip Fisher, Member, Psychology; Jeffrey Measelle, Member, Psychology; Jane Squires, Outside Member, Special Education and Clinical Sciences