Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Stress and the subsequent activation of the HPA axis facilitate the acquisition of cocaine self-administration (Tidey and Miczek, 1997; Mantsch et al., 1998); manipulations of the HPA axis resulting in a reduction of function through pharmacological or surgical means can attenuate the acquisition…
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