For the self-employed it is 12.4% (the so-called "self-employment tax"), and a great many economists give credence to the idea that how it's broken down for payroll employees. The general idea is that if the employer's half were shifted to the employee, then wages/salaries would increase to compensate and about all that would change would be the bookkeeping since the employeers would have the same total outlay and the employee would take home the same amount.

This is similar to the myth that business pay any sort of taxes assesd upon them. With rare exceptions, the taxes merely increase prices charged to customers (who are the primary revenue stream for businesses) or force cost-cutting measures in other parts of the operation (skimpimg on materials, cutting corners in quality, reducing wages paid). One way or another, we all eventually end up paying "corporate" taxes either in the form of higher prices, lower quality of goods, or some comination of both.