natural experiment

A scientific experiment is one that is set up to prove a theory and therefore is strictly controlled so that the result of the outcome is clear. The experimental group is usually set against a control group, so that the factor that is essential in the experiment is not present in the control.  An example is giving a pill to one group and a placebo to the other.  If the pill has a significant effect in the first group and the placebo has no effect in the second, then the contribution of the pill is clear.

 It is difficult to achieve this kind of control in a natural environment where people more often content themselves with observing what happens and then drawing whatever conclusions they can from their observations.  But in a natural experiment it is possible to identify a life situation in which there is a particular factor at work, and compare that to another life situation in which that factor is not present.  It is like setting up an experiment with randomly selected people in it.  A comparison of one situation with another should determine the effect of the factor that interests you.

 The term is not new but has come to the fore because of the Nobel Prize in Economic Sciences awarded to economists Joshua Angrist, Guido Imbens and David Card,  who have shown conclusively that causation can be inferred from such natural experiments.