That notorious Newsnight interview is just one of many examples of the widespread tendency to stigmatize benefit recipients. Why does this happen?

It's not because of the numbers involved. DWP data show that in 2010-11, working age people received ''50.7bn in benefits; most of the welfare bill goes to pensioners. That's 3.4% of GDP and 9.6% of total taxes. Almost one-third of this (''15.5bn) was housing benefit, which - given that it tends to raise rents - might be more properly called landlords' benefit.This ''50.7bn was spread across 5.7 million people, implying an average benefit of less than ''170 per week.

Stories of households raking in thousands of pounds in benefit are, therefore, the exception. The government has estimated that its ''26,000 benefit cap will affect 67,000 households.Assuming two adults per household, this implies that only around one in 42 individuals on benefit get what the government deems to be a high income.

To callibrate this, the chances of winning a lottery prize are one in 54.The belief that benefit claimants are living it large is, therefore, only slightly more rational than the belief that the lottery is a good investment.

Given the paucity of the numbers, the question arises: why are people so quick to attack benefit recipients?

Part of the answer, I suspect is that humans have evolved a powerful norm of reciprocity. We therefore instinctively hate those who get something for nothing, and wish to see them punished.

This, though, can't be the whole story. There's another group that also gets public support - bankers. The Bank of England estimates that the subsidy to banks could be over ''100bn - twice the benefit bill.However, whilst there is public anger at bankers, this is not as well reflected in the media or mainstream politics as anger and benefit recipients.

Something else, then, is going on. I'd suggest three things.

One is the salience heuristic. The tiny minority of benefit recipients who do well get attention - either from the media or from being in the pub. The recipient who lies in hospital and has their benefit cut is less salient. In this way, the image of benefit recipients is systematically distorted.

Secondly, there's a managerialist belief in the fiction of a perfectible organization. The fact is that any feasible welfare system which supports the unlucky will also give handouts to a few scroungers, as the latter cannot be fully screened out. Attackers of benefit recipients forget Adam Smith's words, that "there is a great deal of ruin in a nation" - that some imperfections are inevitable.

Thirdly, there's a "blaming the victim" effect. The wishful thinking that causes us to believe (pdf) in a just world leads us to want to blame the victims of disability or recession for their plight. Such an instinct is as old as history. Job's friends think he is to blame for his sufferings; rape victims are sometimes blamed or even killed for misfortune; and Jon Elster says that in ancient Greece:

The irrational admiration of the beautiful or of those born rich was matched by an equally irrational contempt for the ugly or for those born poor (Alchemies of the Mind, p163)

Attacks on benefit recipients are, in this sense, an ancient and atavistic impulse - which just happens to serve a useful political function.

But let's not kid ourselves that a psychiatric disorder is part of legitimate political discourse.