Economics: Principles, Applications, and Tools

Authors: Arthur O'Sullivan, Steven M. Sheffrin, and Stephen J. Perez | Source: 769 pages, 32 chapters | Edition: Ninth Edition, Global Edition (2018) | Generated: 2026-08-20

Core Frameworks & Mental Models

Use the economic way of thinking before reaching for a policy answer: identify scarcity, state the relevant assumptions, hold other causes fixed, compare marginal benefits and costs, and predict how incentives change behavior. Separate positive questions (what is or will be) from normative questions (what ought to be). A model is a purposeful simplification; reject an assumption only if it changes the conclusion for the question being asked.

The five key principles

1. The Principle of Opportunity Cost: The opportunity cost of something is what you sacrifice to get it. Count explicit payments and implicit costs such as forgone wages, interest, time, and the best alternative use of an owned asset.
2. The Marginal Principle: Increase the level of an activity as long as its marginal benefit exceeds its marginal cost. Choose the level at which the marginal benefit equals the marginal cost. Ignore sunk costs when they cannot change the current decision.
3. The Principle of Voluntary Exchange: A voluntary exchange between two people makes both people better off. For specialization, calculate comparative advantage from opportunity cost, not absolute productivity.
4. The Principle of Diminishing Returns: If we increase one input while holding the other inputs fixed, output will increase, but at a decreasing rate. Always name the fixed input and the time horizon before applying the rule.
5. The Real-Nominal Principle: What matters to people is the real value of money or income - its purchasing power - not the face value of money or income. Deflate wages, benefits, debt, interest, and GDP before comparing welfare or output across time.