Rationality
The standard of reasoning and choosing well — and the benchmark biases are measured against.
What it means
Rationality is the standard by which beliefs and choices are judged to be coherent, well-reasoned, and conducive to one's goals, and it serves as the normative benchmark against which behavioral science measures human departures. Economists usually mean instrumental or means-ends rationality, formalized as choosing consistently to maximize expected utility while obeying axioms such as transitivity and independence; epistemic rationality concerns holding beliefs that are well-calibrated to evidence, ideally updated by Bayes' rule. A crucial distinction separates this normative rationality — what an ideal agent should do — from descriptive accounts of what people actually do, and the heuristics-and-biases program documented systematic gaps between them. Yet 'irrational' is contested: bounded rationality argues people reason well given limited time, information, and computation, and ecological rationality holds that simple heuristics can be optimal when matched to the structure of the environment, so apparent errors may be adaptive. It matters because what we treat as the standard of good thinking determines which behaviors get labeled biased and which interventions count as improvements.
Examples
Preferring A to B, B to C, yet C to A is irrational by the transitivity axiom — a coherence failure that no consistent set of values could justify.
A fielder who could not write one equation of ballistics catches the ball by running so the angle of gaze stays fixed — ecologically rational, and quicker than the sums.
Abandoning a well-supported belief after one alarming headline is an epistemic failure: evidence should move your confidence in proportion to its weight, not replace the prior wholesale.