We should keep in mind that when we label somebody an asshole in a political context, they're usually trying to be funny. Or "funny". Given that researchers are beginning to understand the very real difference in mental architecture of conservatives, it's not surprising that, as a group, they'd have a very different sense of humor from us (the normal people).

If you want an apples to apples comparison, try Fox "News"'s attempt at achieving comedic parity with The Daily Show, their "Half Hour News Hour" (which Fox strangled in the crib after letting it drag on for 13 episodes). Any of the multitude of clips illustrates the point, but the pilot began with a written manifesto that shines a light on the conservative funny bone:

The Half Hour News Hour is next. It contains satire and comedy. It's not politically correct nor does it contain serious news. It will annoy and offend. If you think you will be go to Foxnews.com[sic] and come back later. Overly sensitive people...you've been warned.

(emphasis, of course, added)

Pretty upfront about it: The essence of conservative humor is annoying and giving offense. And so we, the targets of their offense, frequently perceive conservatives as "assholes".

People who study "neuropolitics" have determined that conservatives are measurably more sensitive to threat; they jump higher when startled, in one memorable experiment. So maybe it's not surprising that where liberal humor involves shrugging it off and turning it back on the attacker with humor, conservative humor more often involves hitting back, and hitting back harder. Generations of demagogues have taught conservatives that they're the victims of liberals, and so conservative humor is basically all about "getting back" at liberals.

Watch a few minutes of the Half Hour News Hour, contrast it with some Jon Stewart, and you'll see just what I mean. Note especially the reliance on actors to create insulting liberal caricatures.

I couldn't tell you whether there's really an equivalent percentage of assholes left or right. But I can tell you for sure that each perceives an equal percentage of assholery in the other. And a whole lot of it may just be misfired humor.