Physiological psychology is sometimes related to psychiatry, and in fact may end up becoming the parent branch which contains psychiatry. This term is not universally accepted as being official jargon, but the concept behind it is rather clear and it does unify several previously similar areas of research.

As background, psychiatrists are the only doctors of psychology who must have completed medical training (at least in the United States). They are certified as are other medical doctors, and they are able to write prescriptions (subject to standard DEA regulations and limitations). Psychiatrists tend to handle the psychological disorders that are perceived as being caused by physical irregularities; as an example, bipolar disorder is caused by abnormal concentrations of neurotransmitters and is therefore often treated with antidepressant medications that moderate the production or removal of neurotransmitters.

At the risk of being overly broad in defining it as such, physiological psychology refers to the study of how physical conditions of the human body affect an individual's subjective experience. If a study were to be conducted to find out which region of the brain is active when a person is exercising free will, then this would be on the shared border of cognitive and physiological psychology. Investigating which chemicals are released in which location when a person feels "love" or "anger" would also fall under the umbrella of physiological psychology. These three examples were chosen in part because such research has already been done. Neurobiology and neurology also possess a portion of research and applied methods which could be classified as physiological psychology.