Abnormal psychology is an area of psychology focused on studying abnormal behaviour and mental illness. The terms “abnormal” and “illness” are both terms which carry with them some complexity (not to mention the term “mental”). There is much disagreement regarding which specific behaviours should be classified as abnormal, but we can identify 3 factors on which a judgement of abnormality might be based, namely statistical deviation, deviation from social norms, and judgements of maladaptation.
A statistical deviation is quantified in terms of how often a given behaviour occurs within a society, and can, generally speaking, also apply to behaviours in which individuals excel far beyond normal levels, such as in the case of measurements of intelligence or memory retention – although many individuals working within the field of abnormal psychology might not place emphasis on the study of excellence in relation to assessments of abnormality.
Deviation from social norms can refer to behaviours which are deemed unacceptable, and which deviate from culture-specific norms and expectations.
Maladaptive behaviours can be defined as behaviours indicating that the person in question is unable to function or is undergoing suffering, or where their behaviour may be a source of suffering or disruption in the lives of others.