In the US, if a person is working as an "independent contractor" (and they are properly classified as independent contractors according to state and IRS rules, see here) then they are normally responsible for paying all income, FICA, social security, and other employment taxes. However, if an employer wrongly classifies an employee as an "independent contractr" when they are held to be an "employee" then the employer may become liable for the portion of employment taxes they would normally have had to pay if the worker had been properly classified.
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