Why have our MPs changed so much in 40 years?
There is a well attested thesis that anyone who wants to be elected as a Member of Parliament has to be just a little mad.
It is a difficult way of life and a highly inconvenient one.
The hours are dreadful, the rewards (relatively) insubstantial.
Getting selected as a candidate in the first place is a profoundly unpleasant process involving many hours in the company of others with whom one might not naturally choose to spend one's evenings.
If elected after this gruelling exercise, the outcome is likely to be a great deal of travel between Westminster and the constituency, a serious decline in any sort of private life and no thanks from anybody, particularly the little strangers who call one "Daddy" - or, less frequently, "Mummy".
It is, therefore, quite curious that so many seek such a career and that is the important point to bear in mind here: politicians are self-selecting.
None of them had to do this, no-one imposed these hardships upon them. They chose this way of life for themselves.
I raise this interesting question about the character of politicians because a programme I have made for BBC Radio 4, looking at the changes at Westminster in the last 40 years, suggests that today we probably have a more unrepresentative parliament than at any time since before women were given the vote.
Money is key
And the reason for that, which has once more been thrown into sharp focus by the latest allegations about cash for access involving Jack Straw and Malcolm Rifkind, is all to do with money.
The nature of the people who want to be professional politicians has not changed.
They are still the somewhat unusual characters I have described, but what has happened, however, since the 1970s is that the make-up of the membership of the House of Commons has changed profoundly.
This is because the background from which today's MPs are elected is almost exclusively that of the comfortable middle classes.
This might have been expected, of course, given the rise in living standards and the growth of university education over the period of years in question, but it is still remarkable that 40 years ago there were more than 30 miners, more than 20 engineering workers, a host of other manual workers and a total of over 150 teachers sitting in the House of Commons.
As former Chancellor Ken Clarke observed, when we spoke of his election in 1970, one of the differences between then and now was that his intake was the first that wanted a political career.
It was the dawn of meritocracy, yet he was struck by the fact that most trade unionists on the Labour side had no interest in becoming ministers.
The contrast with today in his view is that social distinctions have disappeared, exactly the same type of person represents each party and everyone wants to be chancellor.
Many MPs in the 1970s, notably the lawyers - and there were more than 150 of them as well - had other jobs, paid employment which enabled them to afford a relatively affluent lifestyle.
That was one reason why parliament did not sit until the afternoons - to enable those who wished to do so to earn their money in the mornings.
There were also those who had family money or the sort of business interests which made the size of the parliamentary salary immaterial.
But by far the vast majority of other MPs relied on that salary for their main family income. It wasn't generous.
Although expenses were paid for travel to and from the constituency, for stationery and postage, and while food and drink in the Houses of Parliament was subsidised (as it remains today, but considerably less generously) the subsistence allowances paid were less than munificent.
That meant MPs shared dingy rooms and flats on the cheaper side of the Thames and could scarcely afford a secretary.
Members from the same areas would share a car home at weekends to save money from the mileage allowance.
And they employed other family members to help with their office and constituency work.
One interviewee for the programme, a former railway guard, Peter Snape - now a member of the House of Lords - recalled that the hardships were such that representations were made to Harold Wilson, prime minister after the 1974 election, for an increase in parliamentary pay.
Wilson was not new to the job. His six years as prime minister in the 60s had taught him the wily wisdom on which political survival depends and his instinct was that public opinion would not endorse paying more for politicians.
Instead he offered to do something about expenses and allowances and thus the seeds of today's troubles were sown.
The parliamentary expenses scandal, with all its attendant horrors of publicly funded duck houses and moat-cleaning, did much to damage the reputation of MPs.
The subsequent reforms are meant to have restored a degree of political propriety to the way we pay for our politics, but it brings the issue dodged by Harold Wilson back into focus and no government has yet had the courage significantly to increase MPs' pay.
As Jack Straw himself tells me before the latest allegations came to light, it is highly regrettable that today's party leaders apparently intend to abandon their previously declared promises to uphold the official recommendations made by the new independent body (Ipsa) established for precisely this purpose.
The situation thus remains unresolved. But there are other aspects besides the continuing potential for what Straw described to me as the "egregious abuse" of the exposed expenses.
MPs are paid a salary of £67,060 per annum. There are 800 head teachers in the state system paid over £100,000 and a further 800 earning between £90,000 and £100,000. General practitioners can easily earn more than MPs and some can reportedly clock up as much as £300,000.
What incentive is there for clever university graduates to choose politics as a career when they can earn more in the City, at the Bar or in journalism?
Less desirable choice
As Shirley Williams observes, bemoaning as she does the unrepresentative parliament we have today, politics is a much less desirable choice for the brightest and the best.
The result of this is that those who seek to stand for parliament in 2015 are drawn from a smaller section of society than once was the case.
There are few candidates today who are not university educated and few who have much experience of working in industry or in the wider world beyond Westminster.
It is rare for those from outside the "bubble" to secure selection in competition with those who know the parliamentary score.
The salary may not be competitive for high earners but it is nevertheless between two and three times the national average salary and it has a commensurate appeal for those who seek to breathe the unadulterated oxygen of neat politics.
Thus it is that today's House of Commons has about 90 Members who have never had a "real" job outside politics including, as it happens, the Labour leader, Ed Miliband, and the Liberal Democrats' Nick Clegg, who both briefly flirted with careers in journalism before entering politics full-time at an early age.
As for David Cameron: he was canny enough to get a job working in public relations for a TV company, when he had already worked as a Conservative adviser for six years and perhaps spotted that this lack of outside experience could have been a career handicap.
And as for the rest of them, the miners and the plumbers and the telephone engineers are almost all long gone and at the last count there were just 24 teachers.
Julia Dear Boy - Welcome to Westminster is broadcast on BBC Radio 4 on 28 February at 10:30 GMT.