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:Executive function skills rely heavily on frontal lobe development (Goldman-Rakic, 1987; Casey et al., 1997; Rubia et al., 2003), which has been noted to peak in structural maturation during adolescence (Giedd et al., 1999).
Category: