The expected value of any net impact assessment of any large scale social program is zero.

The Iron Law arises from the experience that few impact assessments of large scale⁠2 social programs have found that the programs in question had any net impact. The law also means that, based on the evaluation efforts of the last twenty years, the best a priori estimate of the net impact assessment of any program is zero, i.e., that the program will have no effect.