Tony Travers: What are you really getting when you hire an MP?

Honourable members must be wondering when the public's taste for day-by-day exposure of their finances will finally be satisfied. With the ink barely dry on the expenses scandal, the press have now turned their attention to Members' outside earnings.

What each earns in addition to their annual £64,000 salary is not by any means the same issue as mortgage "flipping" or claims for domestic luxuries. However the public, in its current febrile mood, is unlikely to see MPs' outside earnings in a happy light.

It is worth looking back at the origins of modern parliamentary practice. Since 1945, and indeed before, it was commonplace for MPs to supplement their modest earnings with their private wealth or outside employment. Trades unions supported some Labour MPs. The desire to see Parliament representing people from different backgrounds informed the debate about the need for "professional" politicians.

One risk of the current debate is that it may end up accidentally making Parliament a place for the very rich and for those content to be relatively poor. Even in the recent past, most MPs had "proper" jobs for a number of years and then, later on, went into Parliament. Farmers, miners, teachers, businesspeople and, famously, lawyers looked for a seat once they had established themselves in a career. Michael Heseltine and Geoffrey Robinson are prime examples of people who have had successful business and parliamentary careers. It is now almost a cliché to complain that new MPs tend to be drawn from a pool of think-tankers, ministerial advisers, union officers, NGO officials and local councillors. These new jobs provide brilliant contacts in politics.

Being an MP has also changed. To some extent, MPs have become local "citizens advice" agencies, usurping the role of councillors. Government whips are often content to keep their potentially errant brood of backbenchers away from Westminster. Statistics suggest the number of sitting days may be slowly declining, as is the length of the average House of Commons day, though in fairness, more of the House's work now goes on in committees. When they are voting, heavy whipping is intended to keep MPs in line.

If the Commons chamber has, as is widely believed, become less of a forum for debate on matters of moment, it is possible some MPs may feel they have more time to devote to potential outside earnings. MPs' basic pay is not high. For those with marginal constituencies, political extinction may be only a general election away. Arguably, they need to maintain outside interests and contacts in case their seat is lost. There is never a massive demand for ex-MPs after a Westminster clear-out.

But, as the employment backgrounds of MPs have changed, so has the argument about their outside interests. In the past, an ex-businessperson who retained a part-time role in a company might have brought experience to a debate about industrial policy. Today, the link will often be of a different kind.

An MP may be appointed to the board or as an adviser by a company seeking contacts and access. They may also be employed to provide "strategic advice" about the public sector, particularly as the state has become muddled with the private sector because of the contracting-out of services. Although there is nothing wrong with such arrangements, they are often different kinds of outside link to those based on a previous career.

In the light of the rumpus over MPs' expenses, it is bound to be seen as problematic when parliamentarians have two or more non-executive directorships or advisory roles, each of which brings in a sum close to the average national earnings. Either people are earning what most people will see as a substantial amount of money for little effort or, potentially worse, members must be spending a good deal of time away from their parliamentary duties. No wonder many MPs are announcing they will leave the House of Commons or cut back on their external earnings.

The apparently endless drip-drip of stories about MPs' expenses has created massive public aggravation. Now the details about outside earnings have taken another thin layer off the historic veneer of our system of government.

In the brief period between now and the general election, MPs need, in effect, to agree new working arrangements with the electorate that are acceptable both to Parliament and to voters. The election itself needs to allow us all to start again, producing a set of legislators that we respect. If it proves impossible to achieve this kind of outcome, we risk heading off into a nasty and dangerous democratic vacuum. The stakes are much higher than just MPs' earnings.

Tony Travers is a professor of government at the LSE