The Work and Pensions Secretary appeared to suggest that people with disabilities were abnormal in a discussion about employment rates with other MPs.
SNP MP Eilidh Whiteford described the turn of phrase as a "shocking insight" into Mr Duncan Smith's mentality.
The minister used the term "normal, non-disabled people" whilst addressing the House of Commons on Monday afternoon. The words were recorded in Parliament's official record, Hansard.
"We have made huge strides in getting more people with disabilities back into work ... but the most important point is that we are looking to get that up to the level of normal, non-disabled people who are back in work," he told MPs.
The Department for Work and Pensions battled for months against campaigners in order to not to release the numbers, with Mr Duncan Smith at one point telling Parliament they did not exist.
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Other policies push by Mr Duncan Smith with regard to disabled people include the closure of Remploy factories, scrapping of the Independent Living Fund, cuts to payments on the disability Access To Work scheme and cuts to some groups on the Employment and Support Allowance benefit.
The Government's fitness to work tests have also been criticised, as has the disproportionate impact of the so-called 'bedroom tax' on disabled people.
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