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:This study adds new knowledge of two distal factors, neighborhood affluence and disadvantage, in the social environment that may, in the future, be confirmed as underlying causes (Link & Phelan, 1995) of poor birth outcomes and risky maternal pre-
natal health behaviors.
Category: