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:Trivers (1971) argued that the evolution of a trait for benefiting genetically unrelated individuals is also possible as long as the benefactor selects recipients that are likely to provide return benefits (i.e., has a good reputation as a reciprocator) and hence the costs of helping can be…
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