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:…have been based on unselected populations of animals although it might be more productive to study this issue in animals that show an innate predisposition to express high or low levels of anxiety
18 Introduction
(Brush, 2003; Landgraf & Wigger, 2002; Lister, 1990; Steimer & Driscoll, 2003).
Category: