Mankiw’s 10 Principles of Economics

Economics is about decision making in situations of scarcity

ECONOMICS is the study of how individuals, firms and government make decisions to manage scarce resources.  What does this mean exactly?

Professor Greg Mankiw teaches economics at Harvard University and is the author of a popular economics text book called Principles of Economics which is used at many Ivy League schools. Mankiw’s status within the economics profession makes him uniquely well placed to help us understand the basic principles of economics.

Set out below are Mankiw’s 10 Principles of Economics:

How People Make Decisions

1. People face tradeoffs: To get one thing, you have to give up something else. You may have heard economists say “there is no such thing as a free lunch”. What they mean by this is that, for example, you might get a free bowl of soup at the student co-op, but the soup is not free because you have to give up 35-minutes waiting in line to be served.

2. The cost of something is what you give up to get it: Making a decision requires comparing the costs and benefits of alternative courses of action. The cost of one option is not how much it will cost in dollar terms, but rather the value of your second best alternative. For more explanation, see understanding the cost benefit analysis.

3. Rational people think at the margin: People make decisions by comparing the marginal benefit with the marginal cost. For example, you might buy one cup of coffee in the morning because it helps you start the day, but you might not buy a second cup because this gives you no extra benefit (and costs another $3).

4. People respond to incentives: Behaviour changes when costs or benefits change. For example, if your hourly wage increases then you are likely to work more (unless of course your income is already too high).

How People Interact

5. Trade can make everyone better off: Trade allows people to specialise in what they do best. By trading, each person can then buy a variety of goods or services. For example, you may be a skilled management consultant. Money you earn through your consulting work might be used to build a house even though you may not have the skills to build the house yourself.

6. Markets are usually a good way to organise economic activity: Individuals and firms that operate in a market economy respond to prices and thereby act as if guided by an “invisible hand” which leads the market to allocate resources efficiently. For example, if there is an oversupply of wheat on the world market then individual farmers will lower the price they charge until they can sell all of their wheat.  Lower wheat prices will also likely reduce the total quantity of wheat that farmers decide to produce. Market prices are able to adjust to equate supply and demand without the need for any central planning.

7. Governments can sometimes improve market outcomes: Sometimes a market may fail to allocate resources efficiently, and government regulation can be used to improve the outcome. Market failures can occur due to the existence of public goods, monopolies and externalities. For example, an electricity supplier might have a monopoly. Government regulation may be required to ensure that the supplier does not abuse its market power.

How the Economy Works

8. A country’s standard of living depends on its ability to produce goods and services: A country whose workers produce a large number of goods and services per unit of time will enjoy a high standard of living.

9. Prices rise when the government prints too much money: Printing money causes inflation. When a government prints money, the quantity of money increases and each unit of money therefore becomes less valuable. As a result, more money is required to buy goods and services. For more explanation, see quantitative easing.

10. Society faces a short-run tradeoff between inflation and unemployment: Reducing inflation often causes a temporary rise in unemployment. This tradeoff is the key to understanding the short-run effects of changes in taxes, government spending and monetary policy. For more explanation, see the Phillips curve.

NPV: Net Present Value

The NPV of an investment is the present value of the series of cash flows generated by the investment minus the cost of the initial investment

Net Present Value

(Source: Flickr)

1. Net Present Value (NPV) explained

THE net present value (NPV) of an investment is the present value of the series of cash flows generated by the investment minus the cost of the initial investment. Each cash inflow/outflow is discounted back to its present value and then summed together:


Where t is the time of the cash flow; r is the discount rate (see below for further explanation); Ct is the net cash flow (cash inflow minus cash outflow) at time t; and Co is the cost of the initial investment made at time zero.

NPV is used to analyse the profitability of an investment. As a general rule, assuming you have selected an appropriate discount rate, only those investments that yield a positive NPV should be considered for investment.

2. The discount rate

The rate used to discount future cash flows to their present value is an important variable in the net present value calculation. To some extent, the selection of the discount rate depends on the use to which the NPV calculation will be put.

2.1 Option 1 – cost of capital:

One option that is often used is to use a firm’s weighted average cost of capital.

There are two problems with using the cost of capital for the discount rate. Firstly, it may not be possible to know what the cost of capital will be in the future. One solution to this problem is to use a variable discount rate that increases over time to reflect the yield curve premium for long-term debt. A yield curve is the relation between the interest rate (or cost of borrowing) and the time to maturity and is usually upward sloping asymptotically. There are two common explanations for why the yield curve is upward sloping. Firstly, it might be that rising interest rates are expected in the future and investors who are willing to lock their money in now therefore need to receive a higher rate of interest. Secondly, even if interest rates are not expected to rise, longer maturities involve greater risks to an investor and so, to compensate for these inherent uncertainties about the future, a risk premium should be paid.

The second problem with using the cost of capital for the discount rate is that it does not take into account opportunity costs. A positive NPV calculation would tell us that the investment is profitable, but would not tell us whether the investment should be undertaken because there may be more profitable investment opportunities.

2.2 Option 2 – opportunity cost:

A second option is to use a discount rate that reflects the opportunity cost of capital. The opportunity cost of capital is the rate which the capital needed for the project could return if invested in an alternative venture. Obviously, where there is more than one alternative investment opportunity, the opportunity cost of capital is the expected rate of return of the most profitable alternative.

For example, assume that a firm has two investment options, investing in Project A (its existing line of business) or Project B (a new line of business). Based on past experience, the firm knows that it can obtain a 15% return from investing in Project A. This means that the opportunity cost of capital for investing in Project B is 15%. Thus, an NPV calculation for Project B will use a discount rate of 15%.

3. Common pitfalls

3.1 Negative future cash flows:

One potential problem with NPV is that if, for example, the future cash flows are negative (for example, a mining project might have large clean-up costs towards the end of a project) then a high discount rate is not cautious but too optimistic. A way to avoid this problem is to explicitly calculate the cost of financing any losses after the initial investment.

3.2 Adjusting for risk:

Another common pitfall is to adjust for risk by adding a premium to the discount rate. Whilst a bank might charge a higher rate of interest for a risky project, that does not necessarily mean that this is a valid way to adjust a net present value calculation. One reason for this is that where a risky investment results in losses, a higher discount rate in the NPV calculation will reduce the impact of such losses below their true financial cost.

3.3 Dealing with negative NPV:

The general rule is that only those investments that yield a positive NPV should be considered for investment. However, this will only be true if we have selected an appropriate discount rate. For example, in the example in section 2.2, if we used a discount rate higher than 15% in the NPV calculation for Project B then obtaining a negative NPV calculation does not necessarily mean that we should reject Project B. Unless we have intellionally chosen a higher discount rate to adjust for the risk of the project, the negative NPV result does give us any useful information.

[For more information on consulting concepts and frameworks, please download “The Little Blue Consulting Handbook“.]

Cost Benefit Analysis

The cost benefit analysis is a basic analysis framework that involves weighing up the costs and benefits of one course of action against another

Cost Benefit Analysis

IN YOUR consulting case interview you will most likely be required to make a recommendation on a hypothetical business problem. Understanding how to use the cost benefit analysis could come in handy.

The cost benefit analysis

One of the most basic analysis frameworks you can use to solve a business problem is the “cost-benefit analysis”. This method is fairly self-explanatory. It involves weighing up the total expected costs and benefits of one course of action against another. Having done this, you will be able to formulate a more well-thought-out solution to the business problem.

For example, consider the following business problem (Project Gold Mine). Your client says to you, “Currently we run a single gold mine (Mine A) and are trying to decide whether we should expand Mine A or build a second mine (Mine B). Which project should we undertake?” In this problem there are three possible recommendations: (1) expand Mine A, (2) build Mine B, or (3) do nothing (i.e. maintain the status quo). To make a recommendation, you will need to consider the benefits and costs of each course of action.

Evaluating the benefits

In considering the benefits, you will mainly want to think about revenue (Revenue=quantity x price); more on this in a later post.

Counting the cost

As far as I’m aware, there are four types of costs that you need to pay attention to: sunk costs, fixed costs, variable costs, and opportunity costs. I will consider them in turn.

1. Sunk costs

Sunk costs are expenditures that have already been made and cannot be recovered. As such, sunk costs should not be factored into your decision-making process. For example, in Project Gold Mine the original cost of building Mine A is a sunk cost. This money has already been spent and cannot be recovered, it is therefore a sunk cost, and should not be factored into the decision-making process.

2. Fixed costs

Fixed costs are costs that do not vary with the quantity of output produced. In the Project Gold Mine example, fixed costs might include things like rent, land taxes, utilities and other overheads.

It is important to understand that fixed costs are fixed only in the short term. In the short term, the cost of labour may be a fixed cost if the mining company cannot vary the number of employees due to contract obligations. In the long run, these contracts can be renegotiated. In the long run, nearly all costs are variable, even things like rent, because the mining company could move its operations overseas to a country where operating costs are lower.

3. Variable costs

Variable costs are costs that vary with the quantity of output produced. In the Project Gold Mine example, the main variable costs would be the cost of extracting the ore from the ground, and the cost of transportation.

When making decisions in the short run, variable costs are the only costs that should be considered because, in the short term, a company cannot change its fixed costs.

4. Opportunity costs

The opportunity cost of an item is what must be given up to obtain that item.

In the Project Gold Mine example, failing to consider opportunity costs could lead to the wrong decision being made. Consider the following hypothetical:

If the mining company expands Mine A the profit is $1 million, and if it builds Mine B the profit is $2 million. Which project should it undertake? Building Mine B is the more profitable of the two projects; however, the company also needs to consider the opportunity cost of building Mine B. In this hypothetical example, the profit obtained from not undertaking either project is $3 million. So, although building Mine B is the most profitable project, doing nothing is even more profitable.

[For more information on consulting concepts and frameworks, please download “The Little Blue Consulting Handbook“.]