August 6th, 2019


guns and causation

I'm reading Judea Pearl's _The Book of Why_ (Caltech book club), about causal inference and types of causation. Just got through necessary vs. sufficient causation. It seems to me that captures part of the gun debate: widespread gun ownership is necessary for frequent mass shootings (no guns, no shootings) but not sufficient (we can imagine lots of guns without shootings... like most of the US's own history).

Gun control advocates point out that taking the guns away will stop the shootings; gun rights advocates argue guns aren't really to blame.

Pearl's own illustrative example is a house fire: a match (or some other ignition) is both necessary and sufficient, oxygen in the air is necessary (no oxygen no fire) but not sufficient (just adding oxygen doesn't start a fire). That includes an assumption that oxygen is 'normal', present whether or not the match is (thus enabling the match to be sufficient, because we can assume the oxygen is there).

Are guns like oxygen? For a lot of the US, yes, in being ubiquitous, considered normal, and mostly not killing people. Removing oxygen to prevent fires is usually overkill, and they would say removing guns is too.

Other modern crises:

* greenhouse gases are reasonably necessary and sufficient for global warming. (No gases, no warming; adding greenhouses gases to an 1800 AD background suffices to cause warming.) (As a side note, Pearl points out that climate models allow climate scientists to generate counterfactual 'data' quite easily, by tweaking model parameters.)

* cars and deaths: cars are necessary for 40,000 dead Americans a year (no cars, no such deaths... alternative transport modes aren't nearly so dangerous) and sufficient (adding cars, or rather a car-oriented and -dependent culture[1] is the main reason we have the deaths).

[1] Japan actually has 3/4 as many cars per capita as the US, but 1/3 as many road fatalities per capita; the cars are used less, as well as being smaller and slower in common use. Many fewer deaths per vehicle too, but similar deaths per vehicle-km.

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car deaths per km

This is worth calling out from the previous post: look at and sort by the various columns. In particular the third one, road deaths per billion vehicle-km. US is 7.3, Japan is 6.4, not hugely better. Most rich countries are better, down to 3.4 (UK) or Norway (3.0) Many rich countries are least 1/3 better than the US (5.1 or lower).

So when we talk about the 40,000 car crash deaths a year in the US, and how preventable they are, there are two dimensions: reducing the amount driven, by increasing density and mass transit and bikeability, and improving the safety of cars as they are driven, by I don't know what means exactly but roads can clearly have only 40% the death rate of US ones.

Between the two, well, Canada and Australia (large car-loving countries like the US) have less than half the road deaths per capita of the US, so 20,000 American deaths/year are easily preventable. Looking at the UK or Nordic countries, 30,000 deaths/year are preventable.

See the comment count unavailable DW comments at