This is one of the largest groups in the ledger, and it has a clear centre: the ways human judgement departs from what the evidence warrants, predictably and in the same direction. The group spans two traditions that grew up two thousand years apart. One names errors in argument; the other measures errors in judgement.
Errors of argument
The oldest layer is the list of fallacies inherited from classical logic: the Straw Man, Ad Hominem, the False Dilemma, the Slippery Slope, circular reasoning and the confusion of sequence with cause. These describe arguments rather than minds, and they have been taught for centuries because the same moves recur in every age.
The modern additions show the tradition is still growing. Motte and Bailey names the move of retreating to a modest claim under challenge and advancing a bolder one when it is not. The Gish Gallop buries an opponent under more weak claims than can be answered. The Kafka Trap makes any denial count as confirmation.
Errors of judgement
The larger modern layer began with Tversky and Kahneman in the early 1970s. The Availability Heuristic judges frequency by how easily examples come to mind; the Representativeness Heuristic judges likelihood by resemblance; Anchoring lets an irrelevant number pull later estimates towards it. Around these sit the Framing Effect, Base Rate Fallacy, Hindsight Bias, Overconfidence and the Planning Fallacy.
Three sub-themes stand out. People misread randomness, seeing streaks in independent trials and patterns in noise. People misjudge time, overweighting the present, continuing failing projects because of what has already been spent, and underestimating how long things take. And people know less about their own minds than they think: the Introspection Illusion, Choice Blindness and the Bias Blind Spot show people explaining choices they did not make and seeing bias in everyone but themselves.
Where it applies
Few groups in the ledger have more direct uses. Forecasters use reference classes because the planning fallacy is so reliable, and the Iron Law of Megaprojects shows what happens without them. Negotiators know that an opening figure anchors the talks and that a concession loses value the moment an adversary offers it. Committees can use the Hidden-Profile Effect to design meetings that surface what only one member knows. Implementation Intentions and the Fresh-Start Effect turn the time findings into practical levers.
The most important application is structural. Because of the bias blind spot, knowing about a bias rarely removes it from one's own judgement. The remedies that work are procedures, such as checklists, base rates written down in advance and decisions separated from their outcomes, not resolutions to think more carefully.
Where it is argued over
The largest dispute is about the word bias itself. Gigerenzer and the ecological-rationality school argue that many of these "errors" are sensible shortcuts that look foolish only in artificial laboratory problems. The heuristics-and-biases school replies that the errors persist in real decisions with real money at stake.
The replication crisis has also sorted the group. Anchoring, framing and the planning fallacy have held up well. Decision Fatigue rests on a mechanism that has largely failed to replicate. The Hot-Hand Fallacy was reversed by a 2018 correction showing a small real hot hand, and the Endowment Effect turns out weaker among experienced traders.
Why it is worth knowing
This is the part of the ledger most people will use most often, because every decision passes through the same mind that these entries describe. Its lesson is double. It names the ways judgement goes wrong, which makes them easier to notice in others. And it warns, with evidence, that noticing them in oneself is much harder, which is why the most useful knowledge here is about building procedures that catch errors a person cannot catch alone.
The entries behind this essay: Evidence in Decision-making & Reasoning. Every subject has its essays on the essays page.