The assumption of quasi-linear preferences is a critical factor in determining the number of Pareto-efficient outcomes. Under quasi-linearity, a single, unique level of output is Pareto efficient. In the absence of this assumption, multiple Pareto-efficient output levels can exist, with the specific outcome being dependent on the incomes of the involved parties.

Effect of Quasi-Linearity on the Number of Pareto-Efficient Outcomes

A key analytical benefit of assuming quasi-linear preferences in externality models is the resulting stability of the Marginal Social Cost (MSC) curve. Because the Marginal External Cost (MEC) is independent of wealth, compensatory payments between parties do not alter the MEC. Consequently, the MSC curve, which is the sum of Marginal Private Cost and MEC, does not shift with income redistribution. This stability is essential for identifying a single, unambiguous Pareto-efficient level of output.

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The assumption of quasi-linear preferences implies that individuals in an economic model focus exclusively on maximizing their total monetary payoff. A direct consequence of this is that the marginal costs associated with production are not affected by an individual's level of income.

Effect of Quasi-Linearity on Payoff Maximization and Marginal Costs

Stability of the Marginal Social Cost Curve under Quasi-Linear Preferences

In the Weevokil model, the assumption of quasi-linear preferences is justified because the utility for both plantation owners and fishermen is based entirely on their net monetary profits. Since their well-being is tied directly to financial outcomes from banana cultivation and fishing, their payoffs can be represented using a quasi-linear function, which includes components that are dependent on banana output (Q) and components that are not. [1]

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