Final answer:
Social psychology is the subfield of psychology that explores how interpersonal and group relationships influence human behavior, making it the relevant discipline for studying why people have attitudes towards out-group members.
Step-by-step explanation:
Social psychology is the subfield of psychology that examines how people affect one another and explores the influence of interpersonal and group relationships on human behavior. It focuses on the power of the situation and how individuals change their behavior to align with the social situation at hand. In the context of examining why people have attitudes towards out-group members, social psychology would be the relevant subdiscipline of psychology to study this phenomenon.