The Erroneous Evaluation Slippery Slope

[This month, I’m serializing my 2003 Harvard Law Review article, The Mechanisms of the Slippery Slope.]

Experience with a policy can change people’s empirical judgments about policies of that sort, and this can of course be good. Sometimes, though, people learn the wrong lesson, because they err in evaluating an experiment’s results. For instance, suppose that after A is enacted, good things happen: stringent enforcement of a drug ban is followed by reduced drug use; an educational reform is followed by higher test scores; a new gun law is followed by lower crime rates.

People might infer that A caused the improvement, even if the true cause was different. Crime or drug use might have fallen because of demographic shifts. Test scores might have risen because of the delayed effects of past policy changes. The furor that led to enacting this policy might also have produced other policies (such as more efficient policing), and those policies might have caused the improvement. But because A‘s enactment was correlated with the improvement, people might incorrectly assume that A caused the improvement, and thus support a still more aggressive drug enforcement strategy, educational reform, or gun control law (B).

Those who are skeptical about A can argue that correlation doesn’t necessarily mean causation, and that post hoc ergo propter hoc (“after, therefore because of”) is a fallacy. But, as with the is-ought fallacy, the fact that philosophers have had to keep condemning this fallacy for over 2000 years shows that it’s not an easy attitude to root out.

Moreover, as with the is-ought fallacy, post hoc ergo propter hoc may correspond to an often non-fallacious heuristic. People might be rational to generally assume that when a legal change is followed by a goo