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Gun Control and the “Post Hoc Ergo Propter Hoc” Fallacy
It always irks me when I read an article about the supposed effect of a policy change, say a gun ban in the UK or Australia, by looking at the effect on some variable (gun deaths, mass shootings, etc.) after the legislation was passed. The problem is that you can mistakenly conclude a causal relationship between the policy and its impact when none exists. It’s the post hoc ergo propter hoc fallacy, “after this, therefore, because of this.”
As an economics major, I learned that you still need a control and treatment group when dealing with time series, just like you would with cross-sectional data. Other majors, I’m sure, were taught the same thing. We’re familiar with the example of a drug trial, for example, where you find two groups of people that are identical in almost every way and give one group the drug (say, to reduce blood pressure). Ideally, if you see the blood pressure levels drop in the treatment group then you’re confident it’s the result of the drug.
But what about when we’re looking at something like gun deaths or mass shootings?
Well, that’s time series data, a measurement over time rather than a cross-section at a particular point in time. Taking the example of a gun ban, we want to know the effect that change in policy on a variable measured over time (e.g., gun deaths).
Here, the treatment is the policy change itself, just like the blood pressure drug in the previous example. But your “control group” is the time period before the policy changed. In fact, one of the first things a researcher wants to do is look at the data and see if there is an apparent change at the time the policy is enacted.
In the chart above, we can see that workplace deaths steadily decreased over time. If you ignored that and looked at what happened after OSHA was passed you’d mistakenly conclude that the law was successful at increasing workplace safety because, well, deaths declined afterwards. In reality, the law was passed after a trend was well underway.
So, whenever you read an article that reads, “after law X was enacted, variable Y changed” in a way that suggests it was because of the law, the first question you should ask is: what was happening before law X was passed? In the case of Australia, which is often cited as an example of where a gun ban worked, gun-related homicides began a downward trend in 1969, well before the ban was enacted in 1996. You can see how pervasive the fallacy is because most articles you’ll see focus on what happened to the level of gun deaths after a ban was enacted.
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Published in Guns
Thank you, BK. I’m so glad you’re writing things like this!
Very helpful. Thank you.
Oops! Bereket, it’s Post Hoc, Ergo Propter Hoc fallacy, not Ad Hoc! I learned this in high school in the 1960s, and it has stayed with me.
Indeed! Chalk it up to temporary insanity. Thanks!
I’ve made exactly the same mistake—so thrilled I remembered “propter” that I completely missed “ad.” Whoops.
I somehow sensed the truth of what you’ve written, but now that I see it in black and white, it makes perfect sense! Thanks, Bereket. This kind of information is so critical to the decisions that we make as well as the assessments of their effects.
So helpful. Thank you.
None Dare Call Them Science Deniers
I’ve been re-watching old (they’re all old) episodes of “West Wing” – Its amazing how sensible 90’s flaming liberalism seems now.
There is an episode of the same title.
Your OSHA chart is not unique. There is also a chart of post war poverty. How it was being steadily eradicated – until 1965 – when poverty levels stabilized. The great society war on poverty, has been largely apocryphal.
By way of additional data, this is what the overall murder stats in Australia were doing at the same time. (Sorry, but I couldn’t find a graph with as long a period as Bereket’s)
(source)
I’ve had similar arguments regarding civil rights. Hearts and minds changed, then people demanded the laws change.
The lefties in my life think the laws changed hearts and minds.
Can I chalk up to the fingers flying faster than the thought should let them? That happens to me all the time.
Let’s just keep changing the methodology for measuring gun violence, changing methodology obviously works wonders for work place deaths. We could change it several times a year and do away with gun violence entirely. We also know methodology changes did away with inflation. We don’t even have to address the post ad hoc proctology liberals are offering.
In this case, post hoc ergo ad hoc may be more accurate. The “gun control worked” fallacy is that its proponents have already made up their minds what the effect of the policy will be, before they have seen any data. Then they cast around, after the fact, for “data” that supports their pre-existing belief. The term “ad hoc” seems very appropriate for this kind of misbegotten “research.” I’m pretty confident that, with this kind of reasoning, you could prove that gun control laws reduce littering, extend the life expectancy of cancer patients, and produce self-driving cars.
Well, as I like to remind myself, if we pass laws against all guns, then guns will become as rare and as hard to acquire as marijuana has been for the last 50 years.
There should be a few facts tossed into the debate. The United States isnt #1 for firearm homicides – not even in the Americas.
Now, you can look at that list, and see that many countries with higher firearm homicides than the US, have much stricter gun laws than the US. (the list is from Wikipedia, and is compiled for firearm related homicides only)
The second fact I wanted to throw out there, is that if you compare the US with other G7 or G8 countries. (excluding Russia) you’ll find that the US has higher violent crime rates in all categories. Since these countries have roughly the same Knife control laws you’d think that they’d have roughly the same number of stabbings and slashing.
So to simplify this down, the US has a violence problem – not just a gun problem. The political problem is that violence as a whole is too nebulous, with little or no political solutions. So gun grabbing is seen as the magic bullet.
Another category where the US is #1, is the use of psychoactive drugs. In fact many of the mass shooters have had prescriptions or have also been recreational users of such drugs. I think this is an area where profound studies could be conducted.
I wonder where we would be if you took out gang members killing other gang members? 114th?
And where do suicides figure in those “firearm homicides”?
The question occurs to me because today one of our state reps was on the radio, talking about proposing gun legislation next year to “reduce gun violence” by which he said he meant making it harder for people to use a firearm to commit suicide.
Excellent post. There’s a parallel in traffic safety, which had been improving for decades before the feds got into the act and started regulating auto design. See the second graph and ensuing discussion here: http://libertycornerii.blogspot.com/2008/01/cell-phones-and-driving-once-more.html
The list I had used from Wikipedia was supposed to be violent crime statistics. Suicide isnt a violent crime. (mostly because the perpetrator is not available to prosecute)
I heard that more than 50% of murders in the US, are committed in less than 2% of the counties. Wouldnt it be easy to national guardsmen on every corner in the most dangerous areas? Not to investigate or engage criminals but rather to stop crime by having overwhelming force at hand?
That’s just crazy talk!
Yes, its crazy…
But when Rahm Emanuel gets indicted, and the City of Chicago is under federal management, what happens next?
If you take out the major cities, all run by Democrats and all with anti gun laws, the US is in the G7 group.
Yes, I wanted to compare the US with other industrial nations – you’ll note there isn’t one ranked higher than the US on the list.
The point of the list, is there isnt a clear correlation between gun control and violence. Not just in the US, where someone with a gun can simply drive up the interstate into a ‘gun free’ city, but worldwide this invalidates the controller’s arguments.