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:Moreover, although C57 mice were shown to be the most sensitive mouse strain to the rewarding properties of morphine, with contrasting effects in the DBA strain (Orsini et al. 2005), the opposite effects have also been demonstrated in a conditioned place preference paradigm (Cunningham et al.
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