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.
In 1863, Austin Flint (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".
[1] The natural history group is often referred to as the third arm of a controlled drug trial, from the simple notion that a trial constructed in this way has three, rather than two arms (the "active" and "placebo" groups).
Most of our knowledge of the placebo effect comes from the laboratory setting where the experiments are designed to shed light on its neurobiological aspects.
Studying the placebo effect in the laboratory setting gives us the opportunity to control psychological and physiological variables, and to rule out possible confounding factors for the placebo effect.