Some Gun Control Maths

(updated with another example 4/10/15)

For some reason I am fascinated by the debate over US gun control.  I have the good fortune to live in a country where it isn’t an issue but it I still find myself reading about it extensively. I think it is partly the role that statistics plays in the argument. I really liked this article in Slate which pointed out how the statistics are used to bolster existing positions and not to settle things. Also the way the issue acts as a touchstone of the division between red and blue USA.  

Nevertheless that are a few bits of basic maths that occurred to me recently that do seem relevant (even if they bolster my prejudices).

Do Large US Cities Account for the US High Homicide Rate?

There is myth going around that if you take out the homicides from a few (usually four) large cities that

(a ) are liberal controlled

(b ) have relatively strong gun laws

then the rate for the rest of the USA is not much different than most Western European countries. This is clearly wrong. The named cities vary, but if you take the four largest: New York, Los Angeles, Chicago and Houston then the total population is about 17 million and there were a total of 1233 homicides in those 4 cities in 2014. The population of the USA is 320 million and there were about 14,000 murders in 2014. So the rate including the largest cities is: 14,000/320,000,000 = 4.4 per 100,000. The rate excluding the largest cities is 4.2 per 100,000. It really doesn’t matter which four cities you choose, or if you choose 5 or 6, the numbers aren’t nearly big enough to have a noticeable effect on the national rate.


Mental Health

Those who argue against gun control often argue that the way to tackle gun violence is through a better mental health strategy.  They point to the significant proportion of mass killers who have some kind of mental health issue. These are of course a small proportion of all homicides (and there is no evidence that killers in general are more likely to be mentally ill than average) but it is still really important to try and prevent them.  This study suggests about a quarter of mass murderers have some kind of psychiatric history – a much larger proportion are unusual in being a bit of a loner or similar but have no diagnosed mental illness. I am not clear what kind of mental health strategy is being proposed (politicians tend to stick on phrases like “we need to look at mental health”) but presumably it involves identifying those who have mental health issues, giving some kind of treatment, and (as mental health treatment is lengthy and unreliable) limiting them in some way – at a minimum limiting their access to guns or more drastically their freedom in general. Now here’s the rub – over 43 million US adults suffer from some kind of mental illness (this does not include substance abuse such as drugs and alcohol) and 10 million of those suffer from serious mental illness. Furthermore it is extremely hard to tell which of those 10-40 million might be violent (even though very few of them will be). So what is the strategy?   Prevent between 10-40 million people having access to guns most of whom will never do anything violent – even though this would still let about 75% of mass murders get access? How could this be done without introducing some kind of gun control?  I am all for better treatment for the mentally ill – but don’t be fooled into thinking this is going to do anything to reduce homicides unless you introduce gun control for everyone.


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