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:It has been demonstrated in numerous experiments that anonymous pre-play communication increases cooperation in social dilemmas and improves bargaining efficiency (Sally, 1995; Camerer, 2003; Valley et al., 2002; Ellingsen and Johannesson, 2004).
Category: