What it means
Most basic models treat investment as a fixed amount decided by interest rates and business confidence. A more realistic view says that when incomes and sales rise, firms need more capacity and invest more.
The marginal propensity to invest (MPI) measures that induced investment in a single figure. It is calculated from changes, not totals.
If national income grows by a given amount and total investment spending grows by a certain portion of it, that portion is the MPI. A higher MPI means that investment is more sensitive to economic growth.
The measure matters because it strengthens the multiplier. When induced investment is added to induced consumption, each round of spending creates further rounds, so a first injection has a bigger effect on income.
This is why booms can feed on themselves, and why downturns can deepen when firms cut back investment as sales fall. It is closely related to the accelerator effect, the idea that investment depends on the rate of change of output.
When growth is strong, firms invest to expand, and when growth slows, investment can fall sharply. For businesses, this explains why capital goods suppliers often see more volatile sales than consumer goods makers.
The MPI is an estimate and not a fixed rule. It depends on spare capacity, interest rates, expectations, access to credit and technology.
Economists measure it using past data, so forecasts that rely on it should be treated with caution. For a company planning capacity, the idea is a reminder to look at where the economy is in the cycle.
Investing heavily at the peak of a boom, when everyone else is also expanding, can leave the business with spare capacity when growth slows.
In practice
Real-world examples.
Example
A manufacturing sector enjoys a surge in orders, and firms respond by ordering new machines and expanding factories. Economists note that investment rose by 12 cents for every extra dollar of national income. They expect the effect to fade once spare capacity is used up.
Example
A central bank economist builds a model of the economy and includes an MPI of 0.1. The model shows that a rise in government spending leads to higher private investment, amplifying the effect. The bank uses the result in its growth forecast.
Example
A construction equipment maker examines how its sales move with the economic cycle. By looking at the MPI of its customers' economies, it plans for sharp swings in demand as growth speeds up or slows down. It keeps its own capacity flexible by using leased equipment.
Formula
Calculation
MPI = Change in investment / Change in national income
Multiplier with induced investment = 1 / (1 - MPC - MPI)
Suppose national income rises by $200,000,000 and investment spending rises by $30,000,000. The MPI is $30,000,000 / $200,000,000 = 0.15. If the marginal propensity to consume is 0.6 and there are no taxes or imports, the multiplier is 1 / (1 - 0.6 - 0.15) = 1 / 0.25 = 4. Without induced investment the multiplier would be 1 / (1 - 0.6) = 2.5, so the extra investment response raises the effect of a spending injection by 60%.Case study
Seen in the real world.
Brightmoor Economy is an illustrative, fictional country whose planners were forecasting the effect of a new infrastructure programme. They had assumed a multiplier of 2.5, based on a marginal propensity to consume of 0.6.
An economist at the central bank argued that firms in the country expanded investment by about 10 cents for each extra dollar of income. With an MPI of 0.1, the multiplier was 1 / (1 - 0.6 - 0.1) = 3.33, so the programme could have a bigger effect.
In this illustrative story, the planners staged the programme to avoid overheating and watched capacity use in industry. The example shows how investment behaviour can amplify, and also destabilise, the response to policy.
Watch out
Common mistakes.
- Confusing the marginal propensity to invest with the investment rate, when the first is about changes and the second is about total investment.
- Assuming the MPI is stable, when it varies with confidence, credit conditions and spare capacity, so an estimate from one period may not hold in the next.
- Forgetting that investment can fall as well as rise, which makes the multiplier work in reverse during downturns and can turn a slowdown into a slump.
Questions
People also ask.
What is the MPI in simple terms?
It is the extra investment spending that follows each extra dollar of national income.
How does it affect the multiplier?
A higher MPI increases the multiplier, because induced investment adds to the rounds of spending, as long as saving, taxes and imports do not offset it.
Is it the same as the accelerator?
They are related ideas, but the accelerator links investment to the change in output, while the MPI links investment to the change in income.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%Related