It is a political divide-and-rule weapon designed to put working-class people at each other's throats and assist the Tory Party in its electoral fortunes by portraying Labour as the claimants' party.
As Liam Byrne has shown, this is untrue, as though there is nothing Labour would like less than to be described as compassionate towards people denied the right to work.
Duncan Smith claims that 12,000 people have taken up work since the benefit cap was on the way.
However, no-one can believe the figures that he bandies around, especially since the UK Statistics Authority hauled him over the coals for declaring that 8,000 people had taken up paid labour because of the benefit cap.
This shameless chancer has now adopted the Tony Blair method of argumentation, declaring that he "sincerely believes" what he is saying as though self-delusion somehow equates to fact-based conclusions.
On one issue the Work and Pensions Secretary is correct - his assertion that the "greatest effect" of the benefits cap will be in London and south-east England.
That's because of the cost of housing in this region, where housing benefit has proven a subsidy to private landlords who have been able to raise rents through the roof because of a shortage of council housing.
As the government's benefit cap bites, low-paid workers and the unemployed will be gradually forced out of affluent areas.
None of this will inconvenience Duncan Smith, whose comfortable situation derives more from marrying well than any professional efforts on his own behalf.
Nor will it particularly bother him since he has seen that opinion polls show 72 per cent public support for this calculated attack on people forced to exist on state benefits.
The Work and Pensions Secretary in common with many ministers constantly uses the term "welfare," as though to confuse the system in Britain, based on taxation and national insurance, with the various "handouts" payable in the US.
Our state benefits are not handouts. They are not charity. They form part of a comprehensive framework of provisions to help people cope with periods out of work.
The benefits system always coped well with the calls upon it until Margaret Thatcher's government 30 years ago attacked working-class living standards by trebling the jobless rate to three million and brought about a systematic transfer of wealth from poor to rich.
That process has continued unabated since then, with working people feeling their quality of life worsen while the cosseted rich minority see the value of their assets appreciate still further and the wealth gap yawn ever wider.
When Duncan Smith pretends to speak out for workers, blaming claimants for taking home more in benefits than they do for a week's work, he ignores the real problem.
Pay levels are too low, as is the national minimum wage, which was supposed to raise low-paid workers from poverty.
No-one in the labour movement can accept the assumptions of Duncan Smith and his ilk on social justice and supposed fairness.
A radical new approach, including a wealth tax and higher taxation of big business and the rich, is essential to promote economic growth and more jobs rather than workers squabbling among themselves over redistribution of poverty.