What it means
Modigliani and Miller published their first result in 1958. They argued that a company's value comes from the cash flows its assets and operations produce, not from the way those cash flows are split between lenders and shareholders.
Cutting a pie into different slices does not change the size of the pie. The theorem rests on strict assumptions.
It assumes no taxes, no bankruptcy costs, no transaction costs, equal access to borrowing for companies and individuals, and that everyone has the same information. In this idealised world, if a company borrows more, shareholders face higher risk and demand a higher return, which exactly cancels the benefit of cheaper debt.
The result is that the overall cost of capital is unchanged. More debt makes the cost of equity rise, but the weighted average of the two stays the same.
This is why financing choices cannot create value by themselves in the model. The practical value comes from relaxing the assumptions.
Taxes make interest deductible, which gives debt a benefit, while the risk of bankruptcy and the costs of financial distress give it a cost. Information differences and agency problems add further effects, so real companies weigh these trade-offs to find a sensible mix of debt and equity.
Managers can use the theorem as a reality check. If a financing proposal claims to add value, they should ask where the gain comes from, such as tax savings, lower distress costs or better incentives.
If the answer is only that debt is cheaper than equity, the claim ignores the higher risk that debt places on shareholders.
In practice
Real-world examples.
Example
A chief financial officer proposes borrowing to buy back shares, arguing that debt is cheaper than equity. The treasurer points out that shareholders will demand a higher return on their remaining shares, so the headline saving is partly an illusion.
Example
A professor uses the theorem in class to show that dividend policy and capital structure choices do not matter in a perfect market. Students then list the real-world frictions, such as taxes and bankruptcy costs, that make them matter.
Example
A private equity firm buying a company models a debt-heavy structure. It acknowledges that the benefit comes from the tax shield on interest and from management discipline, and not from a free reduction in the cost of capital.
Formula
Calculation
Value of the firm = operating cash flow / weighted average cost of capital, and this value is the same at every debt level in a perfect market
Cost of equity = unlevered cost of capital + (debt / equity) x (unlevered cost of capital - cost of debt)
A company produces operating cash flow of $1,000,000 a year indefinitely. With no debt and a 10% cost of capital, its value is 1,000,000 / 0.10 = $10,000,000. Now suppose it borrows $5,000,000 at 5% and has $5,000,000 of equity. The cost of equity rises to 10% + (5,000,000 / 5,000,000) x (10% - 5%) = 15%. The weighted cost of capital is 0.5 x 5% + 0.5 x 15% = 10%, so the value is still 1,000,000 / 0.10 = $10,000,000. As a check, equity holders receive 1,000,000 - 250,000 interest = $750,000, and 750,000 / 0.15 = $5,000,000.Case study
Seen in the real world.
Fairhaven Foods is an illustrative, fictional company with operating cash flow of $2,000,000 a year and no debt. Its board debated issuing $8,000,000 of bonds to buy back shares, hoping to raise the share price.
The finance director used the irrelevance proposition as a baseline. In a perfect market the value of the firm would not change, and shareholders would simply hold a riskier claim. She then added the real factors: interest of $480,000 a year would be deductible, saving tax of $120,000 at a 25% rate, while the extra debt would raise the chance of financial trouble in a downturn.
The board concluded that a moderate amount of debt made sense for the tax benefit, but not an extreme amount. The illustrative lesson is that the theorem does not say financing is unimportant in practice, only that any value must come from identifiable sources.
Watch out
Common mistakes.
- Reading the theorem as saying capital structure never matters in real life, when it only says so under perfect-market assumptions.
- Assuming debt is cheaper than equity and therefore lowers the overall cost of capital, when the cost of equity rises as debt increases.
- Ignoring taxes, when the interest tax shield is one of the main reasons real companies use debt.
Questions
People also ask.
Who developed the theorem?
Franco Modigliani and Merton Miller, who published their first paper on the subject in 1958 and both later received the Nobel Prize in economics.
Why study a theory built on unrealistic assumptions?
It shows exactly which real-world factors, such as taxes and bankruptcy costs, give financing choices their importance.
Does the theorem apply to dividends too?
A related result says that, in perfect markets, dividend policy does not change company value either.
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