This book is an expanded and revised edition of the author’s critically acclaimed volume Nuts and Bolts for the Social Sciences. Preface; Introduction; Part I. Explanation and Mechanisms: 1. Explanation; 2. Mechanisms; 3. Interpretation; Part II. The Mind: 4. Motivations; 5. Self-interest and altruism; 6. Myopia and foresight; 7. Beliefs; 8. Emotions; Part III. Action: 9. Desires and opportunities; 10. Persons and situations; 11. Rational choice; 12. Rationality and behavior; 13. Responding to irrationality; 14. Some implications for textual interpretation; Part IV. Lessons from the Natural Sciences: 15. Physiology and neuroscience; 16. Explanation by consequences and natural selection; 17. Selection and human behavior; Part V. Interaction: 18. Unintended consequences; 19. Strategic interaction; 20. Games and behavior; 21. Trust; 22. Social norms; 23. Collective belief formation; 24. Collective action; 25. Collective decision-making; 26. Organizations and institutions; Conclusion.