Merriam-Webster (2010) defines abnormal psychology as a branch of psychology concerned with mental and emotional disorders such as neuroses, psychoses, and mental retardation and with certain incompletely understood normal phenomena such as dreams and hypnosis. Defining what is "abnormal" and "normal" in psychology is a difficult thing to do because each region, culture, and era have their own opinions and views of what is or is not normal within psychology.