The phrase “crisis of trust” had become a cliché long before voters learnt that their MPs expected them to pay for their pot plants and accountancy bills. Two years ago, when Gordon Brown became Prime Minister, building trust in government was on his list of promises, right after health, schools and housing.

His spectacular failure is not just his fault. It arises from the habit of treating trust as something to invoke, and to wring hands over, but not to examine. If we now want to respond to the drama, instead of just letting it unfold around us, we need to think about what trust is and what it is good for.

A useful first step would be to recognise that the question “Do you trust politicians?” - or anybody else - demands not an answer but a question in turn: “To do what?” People normally trust others to some extent, but only to a certain extent. You may trust a stranger to give you directions to the station; you may possibly trust another stranger to keep an eye on your bags; but you won't trust one with your PIN.

People have been saying they don't trust politicians for years. They have rarely been asked what they don't trust politicians to do. If they had, and if politicians had treated trust as an aspect of a relationship with the public they had to work on, rather than as a kind of civic mood music, we might have been spared the crisis.

Now we have learnt that whatever the voters didn't trust their MPs to do, they did trust them to devise a fair set of rules governing expenses and to operate them in a way that ordinary people would consider fair. That requires a considerable degree of trust - hence the outrage when it was revealed to have been so badly misplaced. Distrust in politicians had been a passively cynical stance. Now it is fired by real anger. The stark contrast between these moods reveals just how much people trusted politicians even when they said they didn't. Voters expected MPs to pursue their own interests, but they never expected MPs to take such liberties at the expense of the voters who put them in Parliament. It is a matter of the balance of interests - and so are all issues of trust.

One of the most useful ways of thinking about trust, which we owe to the American political scientist Russell Hardin, is as an expectation that somebody (or something) will incorporate your interests into their actions. We accept that others will pursue their own interests; we trust them to the extent that we expect our interests to be bound up with theirs. Building trust involves reducing conflicts between interests.

Before trust in British government can be rebuilt, though, it needs to be set in a frame of distrust. Democracy depends on it. The measure of the power of dictators and pseudo-democratic strongmen is the degree of unquestioning trust they command. The measure of a democratic system is how well it is organised to prevent the abuse of power - how well it structures distrust. Americans appreciate this particularly keenly, revering their Constitution and the checks and balances it applies on the assumption that unchecked power will be abused. The British have preferred to make their constitution up as they go along, with the result that a corrosive distrust is now out of control. Writing the constitution down makes distrust positive. If British children can be taught in school to be proud of it, as Americans are taught to be proud of theirs, so much the better.

Voting reform takes on a key significance when trust is seen as something that emerges from a balance of interests. People have a range of interests in mind when they vote. At one end are their basic ideas about what the country should be like and how it should be run. Generally they feel that what is in the national interest is in theirs too. At the other end are their own particular interests, many of which - jobs, hospitals, crime and schools - are bound up with where they live. They will be inclined to trust candidates whose view of the world is similar to theirs, and to trust representatives whose job it is to know about their local interests.

An electoral system designed to promote trust would give them both. It would be based upon constituencies, to build trust in individual MPs; and it would also allocate seats to parties in proportion to their votes, increasing voters' confidence that the assembly would represent their broader views fairly. In other words, it would work like the systems used for the Scottish Parliament and the Welsh Assembly, which combine constituency representatives with additional members chosen from party lists.

In the past, reforms like these were dismissed as hobby-horses for chatterers and anoraks. Now, in the light of popular anger, the point of them is becoming clearer. But the value of trust is still underappreciated - the more so for the incessant repetition of the word. Politicians still seem to believe that the reason they need to rebuild trust is because distrust gets in the way of their plans. Yet in most of the relationships we regard as healthy, trust is mutual. It allows people to do things together that they otherwise couldn't: raising families, building communities or doing business with each other.

Distrust tends to be mutual too, though. By proving themselves untrustworthy in the eyes of so many citizens, MPs have made themselves far less able to trust the public. For our part, we are right to be wary of people who want us to give them power over us, but suspicion is not constructive. If we are to avoid a relationship like a marriage mired in mutual resentment, we have to consider closely where our interests lie.

Marek Kohn is the author of Trust: Self-Interest and the Common Good (OUP)

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