Heuristics are mental shortcuts the brain uses so it does not have to weigh every option from scratch. They save time, but they also skip a full check of the facts. Cognitive psychologists call the fast, automatic version of this System 1 thinking, as opposed to slower System 2 reasoning.

Cialdini (1984) showed that persuasion principles such as scarcity, social proof, and authority work because they trigger those shortcuts. Marketers design prices, labels, and product pages to keep you in that fast mode long enough to click buy.

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