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:Genotype-dependent differences in the sensitivity to the motivational effects of opioids in mice have also been reported (Cunningham et al. 1992; Semenova et al. 1995; Orsini et al. 2005).
Category: