I can appreciate that this is an emotional issue for some people, but if we limit ourselves to evidence and science, it looks like we have yet another issue in which John McCain doesn’t know what he’s talking about.

At a town hall meeting Friday in Texas, Sen. John McCain, R-Ariz., declared that “there’s strong evidence” that thimerosal, a mercury-based preservative that was once in many childhood vaccines, is responsible for the increased diagnoses of autism in the U.S. — a position in stark contrast with the view of the medical establishment. [...] McCain said, per ABC News’ Bret Hovell, that “It’s indisputable that (autism) is on the rise amongst children, the question is what’s causing it. And we go back and forth and there’s strong evidence that indicates that it’s got to do with a preservative in vaccines.”

As is too often the case with the senator, his comments were ill-informed and misleading.

ABC News’ Jake Tapper ran a helpful report that cut through the spin and presented people with the evidence -- all of which contradicts McCain's remarks.

This is not just a problem of another Republican leader who has little use for evidence and reason. As Kevin Drum noted, McCain’s confusion, when shared with large audiences, can have public-health consequences.

The odds of thimerosal being responsible for autism are now slim and none, and perpetuating this myth does real damage — both to the cause of autism research and to the millions of parents who hear this and decide to keep their children from receiving the normal complement of childhood vaccines. So what happened here? Why did McCain perpetuate this rubbish without even a smidgen of doubt in his voice? Was he pandering to some constituency or other? Was he just making shit up because he didn’t really know anything about the subject? Was he misinformed by own staff about this? Unfortunately, my guess is that the correct answer here is “making shit up,” a quality that McCain has shown an unfortunate weakness for in the past.

I know reporters love the guy, but the reality is that John McCain tends to say whatever thought pops into his head, without much regard for whether it’s true or makes sense. As Mark Kleiman added, “The best one can say for McCain’s behavior is that it marks him as a fool, willing to flap his jaw about important topics based on ignorance.”