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:The results also indicated that mecamylamine treatment decreased nicotine self-administration in all subjects, consistent with previous studies (Martin et al. 1990; Donny et al. 1995; Watkins et al. 1999).
Category: