Why is social security considered to be government revenue and government spending?

Social security in the form of such things as pensions, welfare and unemployment insurance is first of all an expenditure on the part of the government, ie spending based on the tax money taken in from both personal income taxes and sales tax including in Canada, the Goods and Service tax. It is also a form of potential revenue because the recipients have to spend it on basic things like clothes, gas for car usage ie some things which are taxed plus the recipient also has to pay income tax on some if not most of these payments so it becomes a source of revenue again for the government.

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