Irrelevance Proposition Theorem
Definition of 'Irrelevance Proposition Theorem'
A theory of corporate capital structure that posits financial leverage has no effect on the value of a company if income tax and distress costs are not present in the business environment. The irrelevance proposition theorem was developed by Merton Miller and Franco Modigliani, and was a premise to their Nobel Prize winning work, “The Cost of Capital, Corporation Finance, and Theory of Investment.”


Investopedia explains 'Irrelevance Proposition Theorem'
In developing their theory, Miller and Modigliani first assumed that firms have two primary ways of obtaining funding: equity and debt. While each type of funding has its own benefits and drawbacks, the ultimate outcome is a firm dividing up its cash flows to investors, regardless of the funding source chosen. If all investors have access to the same financial markets, then investors can buy into or sell out of a firm’s cash flows at any point.
Criticisms of the irrelevance proposition theorem focus on the lack of realism in removing the effects of income tax and distress costs from a firm’s capital structure. Because many factors influence a firm’s value, including profits, assets and market opportunities, testing the theorem becomes difficult. For economists, the theory instead outlines the importance of financing decisions more than providing a description of how financing operations work. Miller and Modigliani used the irrelevance proposition theorem as a starting point in their tradeoff theory. 