You never know whether you've pinned down the causation, but comparing those two statistics is the strongest single piece of evidence I can think of. Also, you don't have to be absolutely certain to still use that evidence to strongly update against your previous beliefs. What else are you supposed to do with it? Ignore it because you can't be sure?
It makes sense, too. An abundance of guns lets heated arguments escalate easily into deadly outcomes and in general adds a kill-other-human option to your toolkit. As humans are wont to do, when being presented with an option consistently, some of them will use it. Furthering a frontier mentality in a vicious circle. In a culture where ubiquitous guns are common-place, I'd feel safer with one too. Problem of the commons combined with a cavalier attitude about being provided with the means of easily kill other humans. It's quite ridiculous from a mainstream European point of view, truth be told.
Mindkilling indeed, I was flabbergasted when I read your suggestion of having teachers carry guns.
EDIT: No, I don't see often scrawny and reclusive types getting remotely the same kill count, or often even going through with it, using a knife or other means. The barrier of getting up and personal makes the resolve needed another category entirely, in addition to the outcome being much less predictable. Even illegaly acquiring a gun will be beyond at least the many "spur of the moment" gun killings.
EDIT 2: http://www.nytimes.com/2013/01/06/sunday-review/more-guns-more-killing.html
I don't know how to keep this topic away from http://lesswrong.com/lw/gw/politics_is_the_mindkiller/ , so I'm just going to exhort everyone to try to keep this about rationality and not about politics as usual. I myself have strong opinions here, which I'm deliberately squelching.
So I got to thinking about the issue of gun control in the wake of a recent school shooting in the US, specifically from the POV of minimizing presumed-innocents getting randomly shot. Please limit discussion to that *specific* issue, or we'll be here all year.
My question is not so much "Is strict gun control or lots of guns better for us [in the sole context of minimizing presumed-innocents getting randomly shot]?", although I'm certainly interested in knowing the answer to that, but I think if that was answerable we as a culture wouldn't still be arguing about it.
Let's try a different question, though: how would we know?
That is, what non-magical statistical evidence could someone give that would actually settle the question reasonably well (let's say, at about the same level as "smoking causes cancer", or so)?
As a first pass I looked at http://en.wikipedia.org/wiki/List_of_countries_by_intentional_homicide_rate and http://en.wikipedia.org/wiki/List_of_countries_by_firearm-related_death_rate and I noted that the US, which is famously kind of all about the guns, has significantly higher rates than other first-world countries. I had gone into this with a deliberate desire to win, in the less wrong sense, so I accepted that this strongly speaks against my personal beliefs (my default stance is that all teachers should have concealed carry permits and mandatory shooting range time requirements), and was about to update (well, utterly obliterate) those beliefs, when I went "Now, hold on. In the context of first world countries, the US has relatively lax gun control, and we seem to rather enjoy killing each other. How do I know those are causally related, though? Is it not just as likely that, for example, we have all the homicidally crazy people, and that that leads to both of those things? It doesn't seem to be the case that, say, in the UK, you have large-scale secret hoarding of guns; if that was the case, they'd be closer to use in gun-related homicides, I would think. But just because it didn't happen in the UK doesn't mean it wouldn't happen here."
At that point I realized that I don't know, even in theory, how to tell what the answer to my question is, or what evidence would be strong evidence for one position or the other. I am not strong enough as a rationalist or a statistician.
So, I thought I'd ask LW, which is full of people better at those things than I am. :)
Have at.
-Robin