The proposition that limited natural resources provide a limit to growth and to the sustainable size of population is an old one. The natural resource that was the centre of the discussion in Malthus' day was land; more recently, some concern has been expressed over the limitations imposed by the supplies of oil, or more generally, energy sources, of phosphorus, and of other materials required for production. Those who predicted imminent doom in the nineteenth century were obviously wrong. Were they simply wrong about the immediacy of catastrophe, or did they leave out something fundamental from their calculations? There are at least three economic forces offsetting the limitations imposed by natural resources: technical change, the substitution of man-made factors of production (capital) for natural resources, and returns to scale. This study is an attempt to determine more precisely under what conditions a sustainable level of per capita consumption is feasible, to characterize steady state paths in economies with natural resources, and to describe the optimal growth path of the economy, in particular to derive the optimal rate of extraction and the optimal savings rate in the presence of exhaustible natural resources.

This paper is concerned with situations where firms not only recognize the dependence of quality on price (of productivity on wages, of default probability on the interest rate charged), but also attempt to use what control they have over price (wages, interest rates) to increase their profits. The recognition of this possibility has important implications for economic theory, which have recently been explored in a large number of papers in several disparate fields. The objective of this paper is to survey these papers and to draw out the central themes of this literature. This paper is divided into four parts, In Part I, we discuss the most important implications of the dependence of quality on price for competitive equilibrium theory--the repeal of the law of supply and demand (Part I.1), the repeal of the law of the single price (Part I.2), the existence of discriminatory equilibria (Part I.3), the comparative static consequences (Part I.4), and the inefficiency of market equilibria (Part I.5). Part II discusses alternative explanations for the dependence of quality on price in labor, capital, and product markets.

We examine a situation in which insurance is characterized by moral hazard. When market insurance is provided, supplementary mutual assistance between family and friends (unobservable to market insurers) will occur. When nonmarket insurers have no better information than market insurers, the mutual assistance not only crowds out market insurance but is also harmful and therefore dysfunctional. Alternatively, when nonmarket insurers can observe each other's effort perfectly, mutual assistance is beneficial. These results point to the potential importance of peer-monitoring mechanisms in mitigating moral hazard.