Flaw questions are the most common question type in LSAT® Logical Reasoning - roughly four per section by 7Sage's count. Two of your three scored sections are Logical Reasoning. That makes "can you name what is wrong with this argument" close to the highest-value single skill on the test.
The good news is that the list is finite and it repeats. The same shapes come back administration after administration in different clothes.
Do not memorize the names
The LSAT never says "post hoc ergo propter hoc." Answer choices describe flaws in plain English - "takes for granted that a correlation demonstrates a causal relationship." So the useful skill is recognizing the shape, and the Latin is only a filing system.
Which is also why a definition list does not get you there. You learn a flaw by seeing it worn by five different arguments about five different topics until the shape stops depending on the subject matter.
The 31, grouped
These are the 31 types Come Flaw With Me drills, with at least five distinct arguments each.
Conditional logic errors - learn these first
Confuses Necessary for Sufficient - treats a requirement as a guarantee.
Confuses Sufficient for Necessary - treats a guarantee as the only route.
Thinking LSAT names this pair "hands-down the most common flaw on the LSAT." If you drill one thing on this page, drill these. Full walkthrough here.
Causal errors
Post Hoc (False Cause) - B followed A, so A caused B.
Slippery Slope - the first step must lead all the way to the last.
Sunk Cost - we have already spent so much, so we must continue.
Time Shift - what was true then is assumed true now.
Sampling and numbers
Hasty Generalization - a sweeping claim from too few cases.
Sampling Error - the sample cannot represent the group it is drawn from.
Anecdotal Evidence - one vivid story outweighing the data.
Numbers vs Percentage - a rising percentage of a shrinking total, and the reverse.
Relative vs Absolute - "twice as likely" concealing "still almost never."
That last pair is worth extra attention. They are quiet, they look like arithmetic rather than logic, and the LSAT likes them precisely because a hurried reader accepts the number without asking what it is a number of.
Attacking the person instead of the argument
Ad Hominem - the speaker is bad, so the claim is false.
Tu Quoque - you do it too, so your point does not count.
Genetic Fallacy - the idea came from a bad source, so it is wrong.
Straw Man - refuting a weaker argument than the one made.
Appeals to the wrong thing
Appeal to Inappropriate Authority - an expert, but not in this field.
Appeal to Emotion - feeling substituted for evidence.
Appeal to Ignorance - unproven false, therefore true.
Appeal to Tradition - we have always done it this way.
Bandwagon - lots of people believe it, so it is right.
Structural and definitional errors
Circular Reasoning - the conclusion smuggled into the premises.
Equivocation - one word doing two different jobs in one argument.
Fallacy of Composition - true of the parts, so true of the whole.
Fallacy of Division - true of the whole, so true of each part.
False Analogy - a comparison that breaks where it matters.
No True Scotsman - redefining the category to save the claim.
Equivocation deserves a note: it is the most frequent flaw in some published counts, and it is the hardest to see, because the argument reads perfectly smoothly. The trick is to notice a key word appearing twice and ask whether it means precisely the same thing both times.
Dodges and false framings
False Dilemma - two options presented where more exist.
Middle Ground - the truth must be halfway between two positions.
Red Herring - a change of subject dressed as a response.
Loaded Question - a question that presumes its own answer.
Moving the Goalpost - the standard of proof rises whenever it is met.
How to actually learn them
Three things separate students who can name flaws from students who can spot them at speed.
- See each flaw in several different arguments. One example teaches you that example. Five teach you the shape.
- Read why the right answer is right, on that specific question. A generic definition of Straw Man does not tell you why this stimulus was a Straw Man.
- Get faster on purpose. Recognizing a flaw in forty seconds is not the same skill as recognizing it in ten, and only the second one is worth points.
Come Flaw With Me is built around all three. 156 questions across the 31 types, at least five per type. Every answer comes with an explanation written for that particular argument rather than a definition pasted in. And the game speeds up as you improve: three difficulty levels, and the questions come at you noticeably faster at Expert than at Beginner - because the goal is not "can you find it" but "can you find it in time."
The part most study tools do not do
Reading about logic is not the skill. Doing it fast, under pressure, on a sentence you have never seen, is the skill - and the only thing that builds it is a lot of short reps with instant correction.
That is what the Lawgic Lab games are for, and they do not treat every student the same way:
- Come Flaw With Me adjusts to you. Three difficulty levels - Beginner, Intermediate and Expert. Score above 79% over a minimum ten-question sample and you move up. Slip below 51% at Expert and you drop back for a refresh. The questions get faster as you climb.
- It remembers what you have already mastered. Answer a question correctly twice in a row and it retires from your rotation, so you spend your time on the fallacies you actually keep missing rather than the ones you already own.
- Your Student Home shows you where you stand - fallacy recognition across all 31 types, argument-part accuracy in Mines of Argumentia, and per-module results in Diagram Cracker including exactly which questions you got wrong.
- Progress lives on your account, not in your browser, so it follows you from your laptop to any other computer.
This is the opposite of a fixed syllabus that treats a student who has never seen a conditional the same as one who has done four hundred of them.
Where flaws meet the rest of the test
Flaw questions do not sit on their own. The same structural errors drive Weaken questions, Necessary Assumption questions and Parallel Flaw questions - once you can name what is broken, half those question types answer themselves.
And a large share of flaws are conditional-logic errors wearing a costume. If the arrow direction is not automatic for you yet, that is the bottleneck, not the flaw list - start with formal logic and the LSAT and the guided tutorial in Aristotle's Booty, then come back here.
Every tool named in this post is part of a Lawgic Lab subscription. Aristotle's Booty for the guided tutorial, Diagram Cracker for open diagramming practice and the nine built-in module sets, Come Flaw With Me for all 31 fallacy types, Mines of Argumentia for argument structure.
Common questions
What is the most common flaw on the LSAT?
Confusing a sufficient condition with a necessary one. Thinking LSAT calls it "hands-down the most common flaw on the LSAT" and the one that "tends to trip up novices the most." Correlation mistaken for causation is second.
How many flaw types do I need to know?
Around thirty covers essentially everything the test uses. Lawgic Lab drills 31, with at least five separate arguments per type, because the point is to learn the shape of a flaw rather than memorize one example of it.
Do I have to learn the Latin names?
No. The LSAT never uses them. Answer choices describe flaws in plain English, so recognizing the structure matters and knowing the label does not. The names are a filing system, useful for talking about flaws and useless for spotting them.
Read next: Sufficient versus necessary, in depth, how to diagram "unless" with Swap-Flip-Keep, or how much formal logic the LSAT actually needs.