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:…clear differences in behavior toward family members and friends (Hamilton 1964; Roberts 2010) and is consistent with previous studies that have found an increase in closeness to parents occurring during the transition to university (Kenny 1987; Pipp et al. 1985; Sullivan and Sullivan 1980).
Category: