While Loughner seems to be a psychotic of no recognizable political persuasion, too crazy to have a coherent political grievance against Giffords, inflammatory rhetoric that demonizes the opposition , which has become endemic on the right, probably does increase the likelihood of political violence. Threats against politicians generally are up, and threats against Obama are off the charts. The relationship between the political climate and a given act of violence reminds me a little of the relationship between global warming and a given hurricane: you can't say that the warming caused the particular hurricane or made it worse, but you can infer that due to the warmer climate there will be more violent storms. For evidence of the effect of violent rhetoric, see http://bit.ly/hgCFSi.
That link is to a John Sides post at The Monkey Cage:
Farrell had the same thought inspired by the same post:Is there any evidence that vitriol leads to violence? Yes. See this paper (pdf) by Nathan Kalmoe, a doctoral student in political science at the University of Michigan:
Does violent political rhetoric fuel support for political violence? Political leaders regularly infuse communication with metaphors of fighting and war. Building from theoretical foundations in media violence research, I field a nationally-representative survey experiment in which subjects are randomly assigned to different forms of the same political advertisements. I find that even mild violent language increases support for political violence among citizens with aggressive predispositions, especially among young adults. [check out the experiment; it's interesting.]
...the scientific consensus about climate change suggests changes to average temperatures (and changes to the associated likelihood of certain weather events), but it is usually going to be next to impossible to tell whether any given event is 'caused' by climate change (it may simply be the result of random fluctuation). Testing arguments about climate change involves multiple data points and the usual problems of statistical inference etc.Of course, many of those denying any deleterious effect to inflammatory rhetoric would also deny that global warming is anything to worry about.
Similarly, it is probably a bad idea to attribute any particular violent action to an overall climate of violent rhetoric without some strong evidence of a direct causal relationship. E.g., if the assassin had quoted some of the violent rhetoric that has been widely criticized as an inspiration, had listened to Michael Savage's radio shows several hours a day or whatever, one would not be able to prove a causal relationship, but it would not be an unreasonable inference. There does not seem to be evidence of that sort in this case. John points to some evidence that is suggestive of a broader statistical relationship between violent rhetoric and attitudes towards violence. This is obviously much weaker than the kind of evidence that climate scientists have gathered pointing to global warming. But, to the extent that it does point to a possible relationship between violent rhetoric and violent action, it is to a probabilistic relationship.