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An idiosyncrasy is an unusual feature of a person (though there are also other uses, see below). The term is often used to express eccentricity
Eccentricity (behavior)
In popular usage, eccentricity refers to unusual or odd behavior on the part of an individual. This behavior would typically be perceived as unusual or unnecessary, without being demonstrably maladaptive...

 or peculiarity.
A synonym may be quirk.


The term idiosyncrasy originates from Ancient Greek , idiosynkrasía, "a peculiar temperament", "habit of body" ' onMouseout='HidePop("70924")' href="">symbol
A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

s or words. Idiosyncratic symbols mean one thing for a particular person, as a blade could mean war
War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

, but to someone else, it could symbolize a surgery
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...

By the same principle, linguists (such as Ferdinand de Saussure
Ferdinand de Saussure
Ferdinand de Saussure was a Swiss linguist whose ideas laid a foundation for many significant developments in linguistics in the 20th century. He is widely considered one of the fathers of 20th-century linguistics...

) state that words are not only arbitrary, but also largely idiosyncratic signs.


Idiosyncrasy defined the way physicians conceived disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

s in the 19th century. They considered each disease as a unique condition, related to each patient. This understanding began to change in the 1870s, when discoveries made by researchers in Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 permitted the advent of a 'scientific medicine', a precursor to the Evidence-Based Medicine
Evidence-based medicine
Evidence-based medicine or evidence-based practice aims to apply the best available evidence gained from the scientific method to clinical decision making. It seeks to assess the strength of evidence of the risks and benefits of treatments and diagnostic tests...

 that is the standard of practice today.


In contemporary medicine (as of 2007), the term idiosyncratic drug reaction
Idiosyncratic drug reaction
Idiosyncratic drug reactions, also known as type B reactions, are drug reactions that occur rarely and unpredictably amongst the population. This is not to be mistaken with idiopathic, which implies that the cause is not known...

 denotes a non-immunological hypersensitivity
Hypersensitivity refers to undesirable reactions produced by the normal immune system, including allergies and autoimmunity. These reactions may be damaging, uncomfortable, or occasionally fatal. Hypersensitivity reactions require a pre-sensitized state of the host. The four-group classification...

 to a substance, without connection to pharmacological toxicity. Idiosyncratic stresses here the fact that other individuals would react differently, or not at all, and that the reaction is an individual one based on a specific condition of the one who suffers it. Most commonly, this is caused by an enzymopathy, congenital or acquired, so that the triggering substance cannot be processed properly in the organism and causes symptoms by accumulating or blocking other substances to be processed. An idiosyncrasy causing symptoms like an allergy is also called pseudoanaphylaxis
Anaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". It typically results in a number of symptoms including throat swelling, an itchy rash, and low blood pressure...



In psychiatry
Psychiatry is the medical specialty devoted to the study and treatment of mental disorders. These mental disorders include various affective, behavioural, cognitive and perceptual abnormalities...

, the term means a specific and unique mental condition of a patient, often accompanied by neologisms. In psychoanalysis
Psychoanalysis is a psychological theory developed in the late 19th and early 20th centuries by Austrian neurologist Sigmund Freud. Psychoanalysis has expanded, been criticized and developed in different directions, mostly by some of Freud's former students, such as Alfred Adler and Carl Gustav...

 and behaviorism
Behaviorism , also called the learning perspective , is a philosophy of psychology based on the proposition that all things that organisms do—including acting, thinking, and feeling—can and should be regarded as behaviors, and that psychological disorders are best treated by altering behavior...

, it is used for the personal way a given individual reacts, perceives and experiences a common situation: a certain dish made of meat may cause nostalgic memories in one person and disgust in another. These reactions are called idiosyncratic.

Idiosyncrasy in economics

In portfolio theory, risks of price changes due to the unique circumstances of a specific security, as opposed to the overall market, are called idiosyncratic risk. This risk can be virtually eliminated from a portfolio through diversification -- holding multiple securities means the movements of individual securities "cancel out". It is also often called unsystematic or specific risk. In complete markets, there is no compensation for idiosyncratic risk -- that is, a security's idiosyncratic risk does not matter for its price. For instance, in a complete market in which the Capital Asset Pricing Model
Capital asset pricing model
In finance, the capital asset pricing model is used to determine a theoretically appropriate required rate of return of an asset, if that asset is to be added to an already well-diversified portfolio, given that asset's non-diversifiable risk...

 holds, the price of a security is determined by the amount of systematic risk
Systematic risk
In finance, systematic risk, sometimes called market risk, aggregate risk, or undiversifiable risk, is the risk associated with aggregate market returns....

 in its returns.

In econometrics
Econometrics has been defined as "the application of mathematics and statistical methods to economic data" and described as the branch of economics "that aims to give empirical content to economic relations." More precisely, it is "the quantitative analysis of actual economic phenomena based on...

, idiosyncratic error is used to describe error from panel data
Panel data
In statistics and econometrics, the term panel data refers to multi-dimensional data. Panel data contains observations on multiple phenomena observed over multiple time periods for the same firms or individuals....

that both changes over time and across units (individuals, firms, cities, etc.)