The relevance of psychology in nursing is that the mind and the body are interwoven, so in order to treat a person's physical health, a person's mental health must also be treated. Nursing training involves psychology training so that nurses can have a foundation and a working knowledge of human behavior.
When nurses understand the basics of human behavior, they are better able to treat their patients. Regardless of which field a nurse goes into, she will need to use psychology while practicing nursing. Nurses need to be able to understand the emotional reactions from patients and be able to treat their patient's emotional needs as well as their physical needs for optimal health. Psychology is also needed to gain the trust of the patient. When patients trust the nurses that are treating them, they are more likely to be open with their symptoms and problems.
There is also an entire field devoted to psychology called psychiatric nursing. Psychiatric nurses work in the mental health field with patients who have diagnoses such as schizophrenia, suicidal tendencies, depression or various addictions. These nurses often have to work with families and the parents of patients in order to teach them how to cope with the patient's illness. This mental health nursing looks at the emotional as well as behavioral parts of the person's illness to form a treatment plan.