Editing General equilibrium theory (section)

==Modern concept of general equilibrium in economics==
{{see also|Financial economics#State prices|Financial economics #Arbitrage-free pricing and equilibrium|Asset pricing#General Equilibrium Asset Pricing}}
The modern conception of general equilibrium is provided by the [[Arrow–Debreu model|Arrow–Debreu–]][[Lionel W. McKenzie|McKenzie]] model, developed jointly by [[Kenneth Arrow]], [[Gérard Debreu]], and [[Lionel W. McKenzie]] in the 1950s.<ref>{{cite journal |author-link=Kenneth Arrow |last1=Arrow |first1=K. J. |first2=G. |last2=Debreu |year=1954 |title=The Existence of an Equilibrium for a Competitive Economy |journal=[[Econometrica]] |volume=22 |issue=3 |pages=265–290 |jstor=1907353 |doi=10.2307/1907353}}</ref><ref>{{cite journal |last=McKenzie|first=Lionel W. |title=On the Existence of General Equilibrium for a Competitive Economy |journal=[[Econometrica]]|year=1959 |volume=27 |issue=1 |pages=54–71 |jstor=1907777 |doi=10.2307/1907777}}</ref> Debreu presents this model in ''Theory of Value'' (1959) as an axiomatic model, following the style of mathematics promoted by [[Nicolas Bourbaki]]. In such an approach, the interpretation of the terms in the theory (e.g., goods, prices) are not fixed by the axioms.

Three important interpretations of the terms of the theory have been often cited. First, suppose commodities are distinguished by the location where they are delivered. Then the Arrow-Debreu model is a spatial model of, for example, international trade.

Second, suppose commodities are distinguished by when they are delivered. That is, suppose all markets equilibrate at some initial instant of time. Agents in the model purchase and sell contracts, where a contract specifies, for example, a good to be delivered and the date at which it is to be delivered. The [[Arrow–Debreu model]] of [[intertemporal equilibrium]] contains [[future market|forward markets]] for all goods at all dates. No markets exist at any future dates.

Third, suppose contracts specify states of nature which affect whether a commodity is to be delivered: "A contract for the transfer of a commodity now specifies, in addition to its physical properties, its location and its date, an event on the occurrence of which the transfer is conditional. This new definition of a commodity allows one to obtain a theory of [risk] free from any probability concept..."<ref>{{cite book|url=http://cowles.econ.yale.edu/P/cm/m17/index.htm|title=Theory of Value|last=Debreu|first=G.|publisher=Wiley|year=1959|location=New York|pages=98|author-link=Gérard Debreu}}</ref>

These interpretations can be combined. So the complete Arrow–Debreu model can be said to apply when goods are identified by when they are to be delivered, where they are to be delivered and under what circumstances they are to be delivered, as well as their intrinsic nature. So there would be a complete set of prices for contracts such as "1 ton of Winter red wheat, delivered on 3rd of January in Minneapolis, if there is a hurricane in Florida during December". A general equilibrium model with complete markets of this sort seems to be a long way from describing the workings of real economies, however, its proponents argue that it is still useful as a simplified guide as to how real economies function.

Some of the recent work in general equilibrium has in fact explored the implications of [[incomplete markets]], which is to say an intertemporal economy with uncertainty, where there do not exist sufficiently detailed contracts that would allow agents to fully allocate their consumption and resources through time. While it has been shown that such economies will generally still have an equilibrium, the outcome may no longer be [[Pareto optimal]]. The basic intuition for this result is that if consumers lack adequate means to transfer their wealth from one time period to another and the future is risky, there is nothing to necessarily tie any price ratio down to the relevant [[marginal rate of substitution]], which is the standard requirement for Pareto optimality. Under some conditions the economy may still be [[constrained Pareto optimal]], meaning that a central authority limited to the same type and number of contracts as the individual agents may not be able to improve upon the outcome, what is needed is the introduction of a full set of possible contracts. Hence, one implication of the theory of [[incomplete markets]] is that inefficiency may be a result of underdeveloped financial institutions or credit constraints faced by some members of the public. Research still continues in this area.