It was recognized as an intangible quality of persons in economics back to at least Adam Smith. He distinguished it (as "enterprise") from labour (economics) which can be coerced and is usually seen as strictly imitative (learned or transmitted, via such means as apprenticeship).
Marxist economics refers instead to "an individual'ssocial 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.
Fusions of terminology are common. Sociological analysts refer to "individual-level elements of social capital" or "an individual's social capital" or just "individual social capital" while economic analysts often use the phrase firm-specific human capital. In either case the clearly includes individual capital but also some "activity-", "community-" or "firm-specific" social capital (community trust) and instructional capital (shareable knowledge or skills). This is easy to measure: its yield is your salary in your current job.
To the degree this is consistent if you take other work nearby, this opens the questions of what is not "firm-specific" and whether a nation is just a bigger "firm": Some analyses see political capital, or just "influence" or "trust of professionals" as a full style of capital of its own. Some ethicists, most clearly Jane Jacobs, see this as simple corruption. Nonetheless, corruption clearly has a cash value, involves some creativity to arrange, and is a decision factor. It is a skill like any other.
Perhaps because of this, not all theorists recognize individual capital as being as essential as labour, or distinct from social or political influence, or from instructional capacity. These theorists often refer to "intellectual capital", which more properly describes a debate or locus of complexity that arises when individuals take key instructional roles. Some refer to celebrity as another fusion, when individuals take key social roles.
Those who differentiate individual capital tend to see it as something that one can invest in, directly, and see grow, directly. For individual skill, even skill at a highly imitative enterprise, like sports or mastery of a musical instrument, this is very often quite measurable. Many enterprises, for instance, a music conservatory or circus school or creative writing coach, are clearly making a living on the identification and (somewhat) measurable enhancement of the individual.
