The Book of Why: The New Science of Cause and Effect (Penguin Science)

by Judea Pearl

In 1986, Reuben Baron and David Kenny articulated a set of principles for detecting and evaluating mediation in a system of equations. The essential principles are, first, that the variables are all related by linear equations, which are estimated by fitting them to the data. Second, direct and indirect effects are computed by fitting two equations to the data: one with the mediator included and one with the mediator excluded. Significant change in the coefficients when the mediator is introduced is taken as evidence of mediation. The simplicity and plausibility of the Baron-Kenny method took the social sciences by storm. As of 2014, their article ranks thirty-third on the list of most frequently cited scientific papers of all time. As of 2017, Google Scholar reports that 73,000 scholarly articles have cited Baron and Kenny. Just think about that! They’ve been cited more times than Albert Einstein, more than Sigmund Freud, more than almost any other famous scientist you can think of. Their article ranks second among all papers in psychology and psychiatry, and yet it’s not about psychology at all. It’s about noncausal mediation.