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:Studies have shown that response times are faster on trials when there is an expectation of a reward than on trials when there is none (Murray et al. 2008; Pessiglione et al. 2006), reflecting an increased motivation to obtain rewards.
Category: