On proportionality bias

This week The Rest is History dropped an excellent five-part series on Jack the Ripper. I managed to listen to all of it in two days. I strongly recommend it if you have any interest in that case, Victorian England, or the history of crime investigation, but also because Tom Holland and Dominic Sandbrook several times mention—albeit not by name—a useful concept for thinking about how we look for causes in past events: proportionality bias.

I’ve written about this briefly before, in a review of Rob Brotherton’s Suspicious Minds, on the cognitive aspects of conspiracism. Briefly, proportionality bias is our built-in tendency, when trying to explain something important, to look for something equally important as the cause. A cause that is, subjectively, incommensurate with the results is unsatisfying and tempts us toward identifying something else—usually something we are already suspicious of. Again, this is a built-in tendency, not a pathology, but while proportionality bias may be a helpful heuristic on the personal level, it does not scale to world events.

The go-to example is the JFK assassination. Many still can’t accept that the president of one of the most powerful states in history—a man involved in international intrigues at the highest levels including espionage, covert assassination programs, proxy wars, and near nuclear war—was killed by an unsympathetic loser who happened to have temporary work in a building his limousine happened to drive past. It happens to be true but is narratively unsatisfying. The conclusion that it must have been something or someone else comes first, and not necessarily consciously; arguments against Oswald are only meant to confirm it.

Something similar has been happening among certain types with regard to Charlie Kirk. It can’t have been a terminally online loser who killed him out of some half-formed, privately nursed political grievance; it must have been (insert fantasy villain of your choice, though one, the subject of a million crank fixations, keeps coming up).

In the case of Jack the Ripper, this is how we get theories of vast royal machinations or secret Masonic rituals—or some combination of the two—as the force behind the Whitechapel murders. That it was likely a still-unidentified local pervert who was skilled with a knife would, to many, prove disappointing. The imagination would prefer a dark cabal of willing actors, or at least a colorful weirdo, to the anonymous forces of poverty, deviance, and insanity.

Again, Holland and Sandbrook do a good job explaining this, even tying it back to their JFK series. The one point I would add to the above is the role that subjectivity of proportionality bias.

When I reviewed Brotherton’s book, I mentioned the assassinations of JFK and Archduke Franz Ferdinand in June 1914. The former is still the subject of fevered speculation and conspiracy theory; the latter, though part of a flourishing conspiratorial worldview in the 1920s, not so much. Why? Because JFK’s murder still feels important to people in way Franz Ferdinand’s, having become something we learn about in middle school rather than discuss casually, does not.

By the same token, Jack the Ripper is important—to us, for reasons Holland and Sandbrook explain well. In the same way that protesters will show up to vandalize the statues of high-profile targets like Robert E Lee or Christopher Columbus but not for Magellan or Raphael Semmes, or that ancient aliens enthusiasts will fixate on famous landmarks like the Pyramids but not much more complicated ancient construction projects like Roman aqueducts, preexisting cultural cachet matters perhaps more than the arguments themselves. This should, by itself, raise our suspicions.