Human judgment relies on heuristics, not statistical calculation

Lewis explains that Kahneman and Tversky's foundational insight was that people facing uncertainty don't run mental probability calculations; they substitute simpler mental shortcuts, heuristics, that are fast and usually serviceable but predictably wrong in specific circumstances. Rather than treating human error as unpredictable noise around an otherwise rational core, as classical economics assumed, their research demonstrated that errors followed consistent, describable patterns that could be anticipated in advance once you knew which heuristic a person was likely using. This reframing turned irrationality itself into a subject that could be studied scientifically and modeled, rather than dismissed as a rounding error against an idealized rational actor. It became the founding insight of what would later be called behavioral economics.

Takeaway: People aren't randomly irrational, they're predictably irrational in specific, mappable ways.