of the law of value and the formation of prices of production was modified in capitalist agriculture, because prices for farm output were co-determined by land yields and land ownership-rents quite independently of labor-productivity. For example, a poor harvest in a major agricultural region due to adverse weather conditions, or the monopolization of the supply of farmland, could have a big effect on world market prices for farm products. Marx extends his theory of agricultural rents to building rents and mine rents, and considers the effect of rent income on land prices. Theoretical significance This theory is the least known part of Marx's economic writings, and among the more difficult ones, because earnings from farm work can be affected by many different variables, even at a highly abstract level of analysis. However the theory became very important to neo-Marxists such as Ernest Mandel and Cyrus Bina who interpreted late capitalism as a form of increasingly parasitic rentier capitalism in which surplus profits are obtained by capitalists from monopolising the access to resources, assets and technologies under conditions of imperfect competition. Marxist writers such as Cyrus Bina have extended the concept of rents to oil rents. Sources Marx's main texts on rent theory can be found in the second (edited) volume of Theories of Surplus Value and in Part 6 of Capital, Volume III. Gibson & Esfahani (1983) comment that: Rent in macro-economics Another possible reason for the relative obscurity of the theory is that in modern macro-economic statistics and national accounts, no separate and comprehensive data are provided on the amounts of land rents and subsoil rents charged and earned, because they are not officially regarded as part of value-added, and consequently are not included in the calculation of GDP (except for the value of productive lease contracts). The tax data on land transactions are unreliable because of valuation inconsistencies. The underlying conceptual argument in national accounts is, simply put, that such rents do not reflect earnings generated by production and are unrelated to production, and consequently that such earnings do not make a net addition to the value of new output. Implicitly, therefore, many land rents are treated as if they are a transfer of income. Typically only the annual value of expenditure on land improvements and the value of leases of productive equipment are recorded as "productive", value-adding earnings. In Marx's theory, however, land rents do not simply