A psychologist is a practitioner of psychology, the systematic investigation of the mind, including behavior, cognition, and affect. The two major categories of psychologists are those who conduct scientific research and those who work in an applied psychology area. Applied psychologists are further subdivided into a number of mental health and other professions, the most well-recognized being clinical and counseling psychologists. Doctoral level trained psychologists are also the experts in the provision/administration and interpretation of psychological tests and assessment.

Contrast with psychiatrist

main: Psychiatrist In most western countries, including the U.S., licensed/registered psychologists hold a doctorate in their field, while licensed psychiatrists hold a medical degree with a specialty in psychiatry. Psychiatrists are physicians who have earned an MBBS/M.D. or a D.O. professional degree, whereas psychologists have earned a doctoral degree Ph.D., Psy.D., or Ed.D. Psychiatrists generally spend shorter periods of contact time with clients/patients, and the principal method of treatment is psychopharmacology. Conversely, clinical and counseling psychologists generally rely upon psychological assessment and the use of psychotherapy to relieve psychological distress. It is not uncommon for people suffering from mental illness to combine these services to maximize their impact. Psychologists in the U.S. military and two states (New Mexico and Louisiana) have prescription privileges, combining psychosocial and psychopharmacological interventions. Many psychologists conduct research-based, standardized cognitive and projective testing to guide the diagnosis of intellectual disabilities, behavioral/mood disorders, and personality disorders; these test results also inform treatment approaches. Administration and interpretation of the majority of these standardized test instruments require specialized Doctoral level training in psychology.

United States and Canada

The American Psychological Association has issued a guideline that only those who hold a Ph.D. or Psy.D. degree in psychology or a closely related field should call themselves psychologists. This of course applies to both research psychology and applied psychology.

Licensed psychologists, in virtually every jurisdiction and state in Canada and the U.S., are required to have obtained academic doctoral degrees (typically a Ph.D. or Psy.D.). These academic programs typically take four to six years of postgraduate work to complete and offer some combination of rigorous training in scientific research and clinical practice. As part of this training, students contribute to the body of scientific knowledge in the form of a dissertation. In this sense, the Ph.D. in professional psychology is a hybrid academic/professional degree (with a greater focus on academics related to clinical practice), and university programs in professional psychology are not only academic but are also training programs typically characterized by rigor and intensity. A Ph.D. psychologist is trained to be both a scientist and practitioner of the profession. Some psychologists have a professional degree in psychology (Psy.D.), which focuses more on clinical practice, and includes similarly rigorous coursework, supervised professional training, internship, and developing the ability to conduct and interpret academic research. The majority of Psy.D. programs also require a dissertation, which can include quantitative or qualitative research, theoretical scholarship, program evaluation or development, critical literature analysis, or clinical application and analysis.