Parties don't see election promises as "promises" in the plain English meaning of the word. Instead, promises are signals designed to express a deeper character of the political party, writes Chris Berg.

What are election promises for? Who on earth do they convince?

Tony Abbott walked into a trap during the 2013 campaign when he excluded a bunch of policy areas from budget reductions.

Those include the pension ("no change to pensions"), the public broadcasters ("no cuts to the ABC or SBS"), Commonwealth health spending ("no cuts to health"), and Commonwealth education spending ("no cuts to education"). Watch the video, Abbott was unequivocal.

Now, to bring the budget back to surplus, the Government is looking at changing the pension and cutting ABC and SBS.

No one can forget the enthusiasm with which the Coalition pursued Julia Gillard for her "no carbon tax" pledge.

As a consequence, one theme of Abbott's stint as opposition leader was his attempts to bind his future acts in government. But the Abbott obsession with promises predates Gillard's carbon tax backflip. In 2010 he even physically signed a "contract" that the Fair Work Act would not be amended.

I've put the word contract in quote marks because a contract that cannot be enforced is not a contract at all.

That contract would mean nothing if the Coalition announced tomorrow that the Fair Work Act was going to be abolished.

This is the promises dilemma. Elections are commitment games. We vote for the candidates and parties whose values and policies are most appealing. But how can we be sure they'll follow through? All we have is their word - their assurance they will act in a certain way under certain circumstances.

Of course, commitment problems are common across all spheres of human endeavour. In the marketplace we often pay up front for services to be rendered later. But we have courts to enforce commercial contracts that have not been fulfilled. In politics there is no institutional mechanism by which voters can enforce the pledges that their elected representatives have made.

H.L. Mencken famously defined an election as "an advance auction sale of stolen goods". He was not cynical enough. Voters bid without any guarantee that the auction will proceed to settlement.

So the mystery isn't why promises are broken, but why they are kept at all.

The most likely explanation is that politicians want to be re-elected, and a reputation for promise breaking is likely to damage their re-election chances.

But elections are weak discipline. They're only held every three years. And three years is a long time to wait to enforce a contract. Elections are an imperfect control. Sure, voters weigh up the honesty of candidates, but honesty is not the only factor that determines an election. Sometimes it's better to re-elect a liar than risk a potential incompetent.

There are other constraints on breaking election promises. A dissatisfied electorate, even in non-election years, can make it hard to pursue your agenda. Politicians may even be constrained by personal ethics ... who knows?

This 2009 survey of election promise studies across Europe and the United States found that political parties kept on average 67 per cent of their campaign commitments.

The inherent difficulties of measuring political promises aside, 67 per cent is surprisingly high. But how would voters react if they were plainly told that one out of three promises would be broken? How would consumers feel if one out of three products were lemons?

If politicians really wanted to demonstrate a credible commitment to the electorate, as the economist Robin Hanson writes, they would post personal bonds - say to their homes - that would be forfeited if a promise was broken. Then we'd know they had skin in the game. Of course no politicians do this.

One objection might be that in a representative democracy we do not vote for representatives as agents to do specific enumerated tasks, but instead install independent delegates who we trust to follow their own conscience.

And it's exactly what the Abbott Government is claiming now - that its general mandate to fix the Australian budget trumps any nit-picking over what was said or wasn't said in opposition.

But then why the elaborate, interminably detailed promises? The Abbott Government, like the Rudd government, released dozens of policy documents in the lead up to 2013 - full of specific itemised policies they planned to implement in government.

Here's one answer. Parties don't see election promises as promises in the plain English meaning of the word. Instead, promises are signals designed to express a deeper character of the political party. When Abbott promised not to change the pension and not to cut public broadcasters he was trying to signal that his would not be a radical government; that he was firmly targeting the median voter.

After all, why give the SBS promise? Did it win any marginal votes? Surely not. But it did suggest to the electorate he had no secret plan to burn through Australia's institutions. Promises like that increase the political cost of radical action.

This practice is of course deeply deceptive - election promises as signals rather than genuine commitments - but it's a deception we're used to.

Voters are rational. We know campaign nonsense when we see it. As this interesting 2004 paper points out, voters infer the true policy position of candidates for office despite the thicket of untruths.

Obviously Coalition failures deserve to be treated as harshly as Labor failures were. Perhaps more. The Coalition swore to be guided by higher ethical standards than its predecessors.

But let's not pretend to be surprised. Australia is one of the world's oldest democracies. We've been voting for broken promises for a very long time.

Chris Berg is a research fellow with the Institute of Public Affairs. Follow him at twitter.com/chrisberg and view his full profile here.