Modern empirical social science consists largely of attempts of answering questions of the form "What is the causal effect of X or Y?" As a result, social scientists rely on a number of empirical techniques aimed at disentangling causal relationships from mere correlations.
One such technique is Judea Pearl's (1995, 2000) front- door criterion, which relies for identification on the presence of a single, strictly exogenous mechanism on the causal path between the treatment and outcome. Social scientists in general--and economists in particular--have been resistant to the idea of adding the front-door criterion to the standard empirical toolkit, largely due to the difficulty posed by finding the required mechanism. To help overcome that resistance, we first explain how to use the front-door criterion in a regression context.
We then present three empirical illustrations of the front-door criterion. Finally, and most importantly, we look at what happens when some of the assumptions underpinning the front-door criterion are violated.