Principles of Economics (Mankiw)

Notes from Mankiw’s Principles of Economics. The book’s framing is deceptively simple — ten principles that supposedly explain most of what economics studies. What I found useful is how each principle directly contradicts a naive intuition, which is exactly why the subject exists.

Chapter 1 — Ten principles of economics

• Principle 1: People face trade-offs. To get one thing, you give up another. Two goals that constantly trade off:
  • Efficiency — getting the maximum possible benefit from scarce resources.
  • Equality — distributing those benefits evenly across society.
• Principle 2: The cost of something is what you give up to get it. That “opportunity cost” is everything you forgo. Good decision-makers weigh the opportunity cost of every option, not just its price tag.
• Principle 3: Rational people think at the margin. A rational person acts purposefully to achieve a goal. A marginal change is a small incremental adjustment to a plan — most real decisions are “a little more or a little less,” not all-or-nothing.
• Principle 4: People respond to incentives. An incentive is something that induces a person to act. Change the incentive and behavior follows.
• Principle 5: Trade can make everyone better off. By trading with others, people obtain a wider variety of goods and services at lower cost than they could produce alone.
• Principle 6: Markets are usually a good way to organize economic activity. Prices and self-interest guide decisions better than a central planner in most cases.
• Principle 7: Governments can sometimes improve market outcomes. Markets need rules (property rights, antitrust) and can fail (externalities, inequality) — that’s where government has a role.
• Principle 8: A country’s standard of living depends on its ability to produce goods and services. Productivity, not money supply, is the root cause of living-standard differences.

Chapter 4 — The forces of supply and demand

Supply and demand are the forces that make a market economy work; they determine both the quantity of each good produced and the price at which it sells.

4.1 Markets and competition

• A market is a group of buyers and sellers of some good or service. Buyers determine demand; sellers determine supply.
• A competitive market requires two features:
  • the goods on offer are identical, and
  • there are so many buyers and sellers that none can influence the market price.

Because they must accept the market price, competitive buyers and sellers are called price takers.

4.2 Demand

• Quantity demanded is the amount a buyer is willing and able to purchase.
• The law of demand: when the price of a good rises (all else equal), the quantity demanded falls, and vice versa.
• Market demand is the sum of individual demands.

Two ways to reduce demand (using cigarettes as the example):

1. Public-health ads, warning labels, and ad bans shift the demand curve left (at any given price, people want less).
2. A tax raises the effective price consumers pay, moving along the curve to a higher price / lower quantity point.

4.4 Supply meets demand

• Equilibrium is the point where the supply and demand curves cross — the market-clearing price and quantity.
• To analyze any shock, follow three steps:
  1. Is it the supply curve or the demand curve that shifts (or both)?
  2. In which direction does it shift?
  3. Use the supply-demand diagram to see how the shift changes equilibrium price and quantity.

My take

The principle I keep coming back to — both in economics and in engineering — is thinking at the margin (Principle 3). Almost every “should we?” question is really “should we do a little more of this?” Marginal cost vs. marginal benefit is the actual decision rule, yet people instinctively reason in absolutes. And the three-step equilibrium analysis is a genuinely useful template: whenever something in a system changes, ask what shifts, which direction, and what’s the new steady state — it works for queues, caches, and markets alike.