The term natural history group refers to subjects in a drug trial that receive no treatment of any kind and whose illness is, as a consequence, left to run its "natural" course. The term stems from the natural history

Natural history of disease

The natural history of disease refers to a description of the uninterrupted progression of a disease in an individual from the moment of exposure to causal agents until recovery or death. Knowledge of the natural history of disease ranks alongside causal understanding in importance for disease...

 of an illness

Illness

Illness is a state of poor health. Illness is sometimes considered another word for disease. Others maintain that fine distinctions exist...

, which is the course and outcome of that illness in the absence of treatment.

First usage

In 1863, Austin Flint

Austin Flint

Austin Flint was an American physician, born at Petersham, Massachusetts He was educated at Amherst and Harvard and graduated at the latter in 1833. After practicing at Boston and Northampton, he moved to Buffalo, N. Y., in 1836...

 (1812–1886) in his report of the first-ever trial that directly compared the efficacy of a placebo treatment with that of an active treatment, spoke of "the natural history of [an untreated] disease".

Third arm

The natural history group is often referred to as the third arm of a controlled drug trial, from the simple notion that a trial has three arms.

The observed outcomes within this group are then compared with the outcomes manifested by a group that has been given the active drug, and with that manifested by a second group who have been given a dummy, placebo

Placebo

A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...

drug (thus, the natural history group is the trial's "third arm").