THE MURDER of nine black Americans in Charleston, South Carolina by a young white man hoping to start a "race war" has renewed calls for stricter gun control, as well as the removal of the Confederate battle flag from the grounds of the South Carolina state house. It has also renewed a pressing question: why are mass shootings so common in America? One popular answer is that there are simply too many guns in America, and that it is far too easy to get one. But what can be done about this? As Lexington rightly noted, not much. But why not? Joseph Heath, a philosophy professor at the University of Toronto, published a penetrating meditation last week on the ways ideology can distort sincere attempts at social-scientific explanation. He has nothing at all to say about gun control. But he does offer a series of insights about the follies to which experts are prone when attempting to analyse social problems. In particular, Mr Heath points to the hazards of "wanting a policy lever" and "picking one side of a correlation", both of which I think are very much in play in the debate over gun violence.

"Often when we study social problems," Mr Heath writes, "there is an almost irresistible temptation to study what we would like the cause of those problems to be (for whatever reason), to the neglect of the actual causes. When this goes uncorrected, you can get the phenomenon of 'politically correct' explanations for various social problems." Many of these explanations trickle down from the Ivory Tower into public consciousness through the media, as well as through direct instruction in colleges and university, becoming the "conventional wisdom" that shapes our political debates.

Most academics prefer that the cause of a given social problem be one government can do something about. The desire for a "policy lever", as Mr Heath calls it, often leads scholars, and the people who cite their work, to overlook or de-emphasise, perhaps unconsciously, equally probable causes that are less likely to warrant intervention. Mr Heath, himself a man of the left, mentions that left-leaning scholars have a "tendency to overestimate the causal effects of inequality" because redistributing wealth is something governments can do. If inequality caused every other social pathology, we could then fix them all with redistribution, ideally.

I think we see similar reasoning at work in response to mass shootings. Many of us want to see lax gun-control behind these massacres, because gun control is a policy lever we can pull—at least in principle. A lack of reasonable gun control, many of us feel, is what the cause of the problem ought to be.

"At some point, we as a country will have to reckon with the fact that this type of mass violence does not happen in other advanced countries," Mr Obama observed in his remarks on the Charleston murders. "And it is in our power to do something about it", he continued, but then hesitated. "I say that recognising the politics in this town foreclose a lot of those avenues right now".

It is certainly true that other advanced countries do not have this sort of problem. It is also true that other advanced countries have been much more willing to implement strong restrictions on gun-ownership. But how are these truths related?

The question brings us to Mr Heath's insights on "picking one side of a correlation". It might be the case that a lack of strong gun-control is the reason America has such a high incidence of mass shootings. If you're eager for a policy lever, this is the direction you'd like the causation to go. But if A and B are correlated, A might cause B, or B might cause A. Or A and B might stand in a relationship of "mutual reinforcement", or there might be a further factor, C, that causes them both—a "common cause". We must be careful not to hastily pick one politically convenient side of the correlation, or neglect the possibility of more complex pictures of reciprocal causation or of hidden common causes. It's illuminating, I think, to run through each possibility.

Why gun control is doomed A lack of strong gun control might cause frequent mass shootings. But might frequent mass shootings cause a lack of strong gun-control as well? Sure. In response to terrifying gun crime, some people want to buy guns to defend themselves. These people may therefore feel that new gun-control measures threaten their safety. Try to see it their way, for a moment. Tinkering around the edges of the rules governing the legal acquisition of firearms can't and won't change the fact that many, many millions of guns are floating around out there. Moreover, the sort of person who might murder you isn't the sort of person who cares about the law. If it's not possible to disarm the bad people, you might need to arm yourself. According to this line of thinking, it's perverse to make armed self-defence harder for good people who follow the rules. It simply puts them at a strategic disadvantage relative to bad people who don't. Now, if this line of reasoning is prevalent enough—and it is quite prevalent in America—mass shootings may galvanise resistance to new restrictions on gun ownership. They intensify the view that people need to arm themselves in self-defence. And this plays into a possible "mutual reinforcement" story. Suppose, as seems reasonable, that lax gun restrictions are partly responsible for frequent mass shootings in America. Then it would be the case that mass shootings create resistance to the reforms that would reduce mass shootings. This suggests that America might be stuck in a more or less stable loop in which permissive gun laws facilitate frequent mass shootings, which in turn reinforce a felt need to preserve permissive gun laws, and so on. I don't know if this is true, but I suspect that there's something to it. If so many Americans didn't feel so threatened by America's violent tendencies, they'd be less likely to want to arm themselves in response. The data on the question are ambiguous. The rate of gun deaths has declined significantly in the past twenty years, but people don't seem to be aware of it. Meanwhile, mass shootings are on the rise. The percentage of Americans who believe that having a gun in the house makes them safer (63%) has doubled since 2000. Yet this shift in attitude doesn't seem to show up in the rate of gun-ownership, which has been very stable over the past two decades. Still, Americans seem to be unusually likely to want to build up a personal arsenal in response to the threat of gun violence rather than, say, becoming determined to the round up all the guns and dump them in the ocean. Why? This question moves us into the frustrating domain of vague cultural explanations. It all has something to do with the violent rebellion of the American founding, with the anarchy of the American frontier, with the threat of hostile natives and the fear of slave revolts. We don't know why the will to gun-up persists so strongly in America, but it does. We don't know why gun-ownership seems more like a precious, basic right to Americans than it does to the citizens of other developed countries, but it does. We don't know why Americans are so obsessed with movies, television and games about the glamour of killing people (and animals and monsters and aliens and robot) en masse with guns, but we are. And we don't know why every year or so a young white American male grabs some guns and slaughters a roomful of completely innocent people, but it just keeps on happening. Better gun control might make it harder for guns to enter the hands of these sociopaths, but as Lexington notes, and the president laments, gun control is probably not going to happen.