It's not often I agree with David Cameron. But although his reluctance to take part in TV debates is motivated by base partisan considerations, I have some sympathy for it. I'm not at all sure that leadership debates are a good thing.

The problem with them is that the medium shapes the message. Mere verbal communication, without graphs, equations, hyperlinks or tables of data can distort messages; for this reason, I almost never talk about economics with non-economists.

Debates favour glib but silly metaphors over lengthier but correct analysis; in a debate, it's easier to prate about "the nation's crerdit card" than to explain the maths of debt sustainability. They favour shallow certainty over an awareness that social phenomena are complex. And they allow speakers - at least in the studio - to get away with erroneous claims; the Clegg vs Farage debate on Europe contained many inaccuracies.

Televised debates, then, select for confidence and plausbility rather than for deeper intellectual virtues.

This is not necessarily a bad thing in the US. Part of the job of being President and head of state is to look the part, to present a figure that's inspiring and reassuring.

Nevertheless, I suspect it has drawbacks. One is that it might entrench class inequality, by favouring politicians from public school backgrounds. Another is that, in a parliamentary system, presidential qualities are less significant, and others which are not so evident in TV debates are more desirable - such as dull competence or ability to manage a team.

All this is just a hunch. What about evidence?

The hard evidence we have comes from the 2010 leadership debates. These seem to have won Nick Clegg at least goodwill if not (pdf) actual votes. But many voters now regret extending him that support. This might be a particular instance of a wider trend in the west - that confidence in politicians generally has declined in the TV age. This might reflect an X Factor-type phenomenon - that, especially where preferences are ill-defined, popularity won on TV might not be long-lasting.

Let's put this another way. Tony Yates and Simon Wren Lewis know vastly more about economics than Cameron. But would this superiority really come across fully in a TV debate? (I'm not making a partisan point here: righties can compare their favourite rightist thinker to Ed Miliband).

Or try casual introspection. Think about the individuals you've met whom you'd most want to have in positions of power. Are they really the sorts that would come across best in TV debates? I suspect not, in many cases. For that matter, would successful politicians from an earlier era have done well in them? LBJ ducked out of a debate in 1964 for a reason. And would Attlee or Gladstone have become PM if TV debates existed in their day?

Now, all this is tentative. If TV debates were part of a move towards more deliberative democracy, they would be a good thing. They'd also be welcome if they exposed charlatans to the fate experienced by Steven Emerson. The question is, though: do they help select in favour of desirable qualities or against? I'm not sure.