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 is safe, however, to conclude that in humans, reciprocity is considered a major value given the importance attached to norms prescribing it (Gouldner 1960) and the function of gratitude (McCullough et al. 2001; Trivers 1971).
Category: