For the biologist who doubts an old hypothesis or wishes to test out a new one, there is the biological laboratory. There, under conditions over which he can exercise the most rigid control, he can vary the light, the air, the food, which his plants or his animals receive, from the moment of birth throughout their lifetime. Keeping all the conditions but one constant, he can make accurate measurement of the effect of the one. This is the ideal method of science, the method of the controlled experiment, through which all hypotheses may be submitted to a strict objective test. ... Unfortunately such methods of experiment are denied to us when our materials are humanity and the whole fabric of a social order.
Margaret Mead (1901 - 1978)