Fully weighing every relevant piece of evidence before making a judgement would be exhausting, and often impossible given real time constraints. Instead, people rely heavily on heuristics — mental shortcuts that substitute a simpler, faster judgement for a more complete one. Behavioural economists and psychologists, most notably Daniel Kahneman and Amos Tversky, have shown these shortcuts aren't random or haphazard: they work well often enough to be genuinely useful in everyday life, and they fail in specific, predictable circumstances that researchers can identify and demonstrate in advance.
Availability: judging likelihood by how easily examples come to mind
The availability heuristic estimates how common or likely something is based on how easily relevant examples come to mind, rather than on actual statistical frequency. This works reasonably well in many everyday situations, since things that genuinely happen more often generally are easier to recall. It fails predictably, though, when memorability and actual frequency come apart — vivid, dramatic, heavily reported events (like plane crashes or shark attacks) come to mind far more easily than statistically far more common but less memorable risks (like heart disease or car accidents), leading people to systematically overestimate the likelihood of dramatic, rare events relative to mundane, common ones, a bias that shows up consistently across many different populations and contexts precisely because it stems from a reliable feature of how human memory retrieval actually works.
Anchoring: letting an irrelevant first number shape the estimate
The anchoring heuristic causes an initial number encountered, even one arbitrary and clearly irrelevant to the actual judgement at hand, to noticeably shape subsequent numerical estimates. In classic experiments, participants asked to estimate an unrelated quantity after being shown a random starting number gave systematically higher estimates when the random starting number was high, and systematically lower estimates when it was low, even though the starting number had no genuine connection to the correct answer and participants were often explicitly told it was random. This kind of anchoring effect has been replicated across a wide range of judgement tasks, from price negotiations to legal sentencing decisions, and its predictability is exactly what makes it a genuine cognitive bias rather than simply random error — the direction and rough size of the effect can be anticipated in advance, given the anchor value used.
What we're still unsure about
The existence and general behaviour of heuristics like availability and anchoring are extremely well replicated across decades of psychological and behavioural economic research, and are not seriously disputed as real phenomena. What remains more actively debated is the underlying theoretical explanation for why these particular shortcuts evolved or developed as reliable features of human cognition, and how to best categorise the full range of documented biases into a coherent overall theory — some researchers argue certain "biases" reflect genuinely adaptive strategies well suited to particular environments rather than simple cognitive errors, and reconciling that adaptive framing with the error-prone framing more common in behavioural economics remains an active area of theoretical debate.
This sits inside Heuristics & Cognitive Biases, one of seven topics in Behavioral Economics, one of five domains in Economics, one of seventeen subjects the app can quiz you on.