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:…to effectiveness, they must also allow for selectivity in providing benefits (e.g., on the basis of learned attitudes toward more or less ‘‘deserving’’ recipients of benefits), as is required by genetic cost-benefit models such as kin selection (Hamilton 1964) or reciprocal altruism (Trivers 1971).
Category: