The Enlightenment encouraged scientists to apply mathematics and physics to human nature and social dynamics, but these were of course blunt instruments for such complex work, requiring many simplifying assumptions. Over time, the caveat that these assumptions were simplifying fell away and what was left was a mechanical view that people are rational calculators of their own interest. Economists even today assume that an ordinary consumer can make complex instantaneous calculations about net present value and risk when making decisions in grocery stores between tomatoes and carrots. This homo economicus stands at the center of traditional economics, and his predilection for perfect rationality and selfishness permeates our politics and culture. By contrast, the behavioral science of our times is pulling us back to common sense and reminding us that people are often irrational or at least a-rational and emotional, and that we are at best approximators of interest who often don’t know what’s best for us and even when we do, often don’t do it. This accounts for the “animal spirits” of fear, longing, and greed that seem to drive markets in unpredictable and irrational ways.

Liu, Eric; Hanauer, Nick (2011-12-06). The Gardens of Democracy: A New American Story of Citizenship, the Economy, and the Role of Government (Kindle Locations 362-369). Perseus Books Group. Kindle Edition.