What is Prospect Theory?

Legal Definition
Prospect theory is a behavioral economic theory that describes the way people choose between probabilistic alternatives that involve risk, where the probabilities of outcomes are known. The theory states that people make decisions based on the potential value of losses and gains rather than the final outcome, and that people evaluate these losses and gains using certain heuristics. The model is descriptive: it tries to model real-life choices, rather than optimal decisions, as normative models do.

The theory was created in 1979 and developed in 1992 by Daniel Kahneman and Amos Tversky as a psychologically more accurate description of decision making, compared to the expected utility theory. In the original formulation, the term prospect referred to a lottery. The paper "Prospect Theory: An Analysis of Decision under Risk" (1979) has been called a "seminal paper in behavioral economics".
-- Wikipedia
Legal Definition
Developed in 1979 by Amos Tversky and Daniel Kahneman. Most individuals will make a decision that will lead to a gain rather than a loss. Risks are put into 2 categories depending whether the outcome is a loss or a gain. These 2 categories of risk are treated in different ways to get the required outcome.