included, the proportion of this part shrinks even more (to one-sixth to one-eighth of the total capital). Human capital and labour markets. Marx did not analyze the effects of human capital—that is, the skills and knowledge lodged in the physical bodies of workers, and the very large sums of money invested into education systems. He was aware of the idea of human capital, but he regarded it as a fictitious, reified notion that, he argued, would imply that workers were really capitalists. Nevertheless, the stock of skills and knowledge in the economy can represent a very large tradable value, which can profoundly influence economic relationships, insofar as workers can increase their income because they possess specialized skills and knowledge. Considerable money is spent on education and research and development in developed capitalist countries. Marx intended to write a separate study about the labour market, considering the different forms that wages could take, but he never did. Rent seeking. Marx did not analyze the effects of "rent-seeking" for economic reproduction. In conventional economics, the term "rent-seeking" was coined in 1974, and refers mainly to a form of parasitism or economic opportunism by government bureaucrats in a privileged position (though sometimes it is applied to firms and business managers). Definitions of "economic rent" are much disputed in economic theory, but in radical theory they refer to an unearned income (which may be conceptualized as being not a true "factor-income" but rather an income in excess of factor-income) that derives from a favorable trading position or from the monopolization of a resource. Marx only considered the special case of land rent ("ground rent"), but modern theorists argue that the scope of economic rents has become much larger. If investors can earn rents simply by capitalizing on the ownership or use of a resource (and make money simply by trading in the ownership of assets, which may themselves be borrowed assets), then, if this sort of activity makes more profit faster, with less risk and lower tax than investing in production, capital will shift more and more to trade in already existing assets. If there exist plenty of such assets, a very large trade in such assets can develop, causing speculative bubbles. The result is that output growth is curbed and may even turn negative. Thus, the relative proportions of industry profits, interest, and rent income in total surplus value have a decisive effect