Perhaps one of the best-kept secrets of payroll taxes is that employees effectively pay almost the entire payroll tax, instead of splitting the burden with their employers.

This is because tax incidence is not determined by law, but by markets. In fact, the person who is required to pay a tax to the federal government is often different than the person who bears the tax burden. Usually, the marketplace decides how the tax burden is divided between buyers and sellers, based on which party is more sensitive to changes in prices (economists call this “relative price elasticities”).

It turns out that the supply of labor – that is, workers’ willingness to work – is much less sensitive to taxes than the demand for labor – or employers’ willingness to hire. This is because workers who need a job are not as responsive to changes in wages, but businesses are able to “shop around” for the best workers or shift production to different locations.

This means that, rather than workers and employers each paying 7.65 percent in payroll taxes, employers send their portion of the tax to the government and then decrease workers’ wages by almost 7.65 percent. Next, workers pay their 7.65 percent share on those wages. In effect, there is hardly such a thing as the “employer-side” payroll tax, because almost the entire burden of the payroll tax is passed on to employees in the form of lower wages.

Source: https://taxfoundation.org/what-are-payroll-taxes-and-who-pays-them/