Why people do the things they do is an age-old question. However, psychology - the science concerned with behavior, both human and non-human animals - is only about 125 years old. Despite its youth, it is a broad discipline, essentially spanning subject matter from biology to sociology. Biology studies the structures and functions of living organisms. Sociology examines how groups function in society. Psychologists study the intersection of two critical relationships: one between brain function and behavior, and one between the environment and behavior.