Rosenhan and Seligman - Defining Abnormality
Statistical Infrequency - For instance, one may say that an individual who has an IQ above or below the average level of IQ in society is abnormal.
Violation of Social Norms - A person's thinking or behaviour is classified as abnormal if it violates the unwritten rules of what is expected or acceptable behaviour in a social group.
Failure to Function Adequately - An individual may be unable to perform the behaviours necessary for day-to-day living, e.g. self-care, holding down a job.
Deviation from Ideal Mental Health - Unable to meet the varying demands of day-to-day situations, negative view of self and the world.