Journalists and commentators love to quote “informed sources” and the like within the political parties: it makes them sound so much better-connected than the general public, even when the sources are actually getting their information from reading the same published figures as everyone else.

And using internal sources makes sense. Relying on just one side is obviously perilous, but if people in both major parties are telling you the same thing, surely that must add to its credibility? In most circumstances it does, but not always.

To pick just one example among many, Dennis Shanahan in last Saturday’s Australian told us that “The Prime Minister’s seat of Bennelong in Sydney would appear to be safe, at least according to party sources on both sides” – this despite published polls that have consistently showed Howard trailing.

The problem here is that the prospect of a landslide skews the normal incentives of the political parties. Instead of each trying to talk up the difficulties it faces and its own chances of overcoming them, both parties now have a common interest: neither wants to believe what the polls are saying.

For Labor, that’s because it wants to avoid any impression of overconfidence. For the Coalition, it’s because the polls are just so demoralising – being the underdog is fine up to a point, but many supporters might just give up if they think they’re facing annihilation. So both sides will try to play down the swing and paint the situation as close, regardless of the evidence.

I first noticed this phenomenon in the Victorian election of 2002, when all the evidence pointed to a crushing Labor victory (which in due course is what happened). Right up to the last week, both sides pretended the result was still in doubt. They were eagerly abetted by the media, hoping to attract more viewers by the illusory prospect of a close contest.

The same pattern has been repeated since in other states, and now we are seeing it federally. That means getting your privileged information from both sides won’t make it more likely to be true, because they’re both singing from the same song-sheet.

No alternative, then, but for journalists in search of the truth to actually go out and look at the evidence themselves. But don’t hold your breath.