Marxist economics refers instead to "an individual's social capital—individuals are sources neither of creativity and innovation, nor management skill. A problem with that analysis is that it simply cannot explain the substitution problem and lack of demand that occurs when, for instance, an understudy takes on a leading role, or a second author takes over writing a popular book series. At the very least there must be some conditional, if not firm-specific then "class specific", special ability to command premiums for outstanding personal performance.