Weaken vs. Flaw Questions on the LSAT: The Critical Difference

Both weaken questions and flaw questions feel like they're asking you to criticize an argument. You're picking an answer that hurts the argument in both cases. That shared intuition is exactly what makes these two question types so easy to conflate — and so expensive to confuse on test day.

But they operate on opposite sides of the argument. A flaw question asks you to describe something wrong that's already inside the argument. A weaken question asks you to bring in outside information that damages the argument's conclusion. Once that distinction clicks, you'll read the two question types very differently.

Here it is: a flaw question asks you to *diagnose* the argument; a weaken question asks you to *damage* it.

Flaw: the answer must describe a logical error the argument already contains — circular reasoning, an unrepresentative sample, confusing correlation with causation, or any other internal problem built into the reasoning.

Weaken: the answer is a new fact, outside the argument, that makes the conclusion less likely to be true. The argument isn't necessarily flawed — the new information just works against it.

Flaw question stems describe the task in abstract, analytical terms. Common phrasings include: 'The reasoning in the argument is most vulnerable to criticism on the grounds that it...', 'Which one of the following indicates a flaw in the argument?', and 'The argument is flawed because it...'

Notice that flaw stems often end with a partial sentence ('it fails to...', 'it assumes...', 'it overlooks...') — because the answer will complete the description of the internal error.

Weaken question stems are phrased in terms of information or evidence: 'Which of the following, if true, most seriously weakens the argument?', 'Which of the following, if true, would most undermine the conclusion above?'

The phrase 'if true' is a signature of weaken (and strengthen) questions. It signals that you're being asked to treat the answer choices as external facts and evaluate what they'd do to the argument.

Flaw answers are written in abstract, diagnostic language. They describe *how* the reasoning goes wrong — 'treats a necessary condition as sufficient,' 'assumes without justification that correlation implies causation,' 'uses the term X in two different senses.' The correct answer doesn't introduce new facts; it characterizes an error in the existing reasoning.

Weaken answers are written as factual statements — things that are (or would be) true about the world. 'The study participants were not representative of the target population.' 'The same effect has been observed in the absence of the supposed cause.' These are new pieces of information, not descriptions of logical errors.

This is the clearest test: read the correct answer and ask yourself whether it's describing something about the argument's internal logic, or whether it's introducing new information. If it's describing internal logic, you're on a flaw question. If it's a new fact, you're on a weaken question.

Argument: 'Every time it has rained in our city, traffic accidents have increased. Therefore, rain causes traffic accidents to increase.'

Flaw question correct answer: 'The argument concludes that one event caused another based solely on the fact that the two events occurred together.' — This describes the internal logical error (confusing correlation with causation) without adding any new facts.

Weaken question correct answer: 'In cities with similar rainfall patterns, accident rates do not increase on rainy days.' — This is a new fact from outside the argument that works against the conclusion.

Both answers criticize the argument, but in completely different ways. The flaw answer names the error; the weaken answer brings evidence against the conclusion.

The LSAT regularly places 'flaw-style' answer choices on weaken questions and 'weaken-style' answer choices on flaw questions. These are the most seductive wrong answers on both question types.

On a weaken question, you might see an answer phrased like 'The argument assumes that X, but X is not established.' That sounds critical, but it's describing the argument's structure — it's a flaw-style answer, not a weaken answer. On a weaken question, you want a new fact, not a structural critique.

On a flaw question, you might see an answer phrased as a factual statement: 'Experts in this field have reached a different conclusion.' That's new information from outside the argument — it's a weaken-style answer. A flaw question doesn't ask whether outside facts contradict the conclusion; it asks whether the reasoning itself is logically defective.

When you're uncertain, return to the question stem. 'If true' = weaken (or strengthen). No 'if true' = flaw. That single read will save you repeatedly.

Reread the stem first. This sounds obvious, but many students skip back to the stimulus without re-reading the stem when they get confused on a hard question. The stem contains the entire answer about what kind of task you have.

Once you've confirmed the type, apply the right filter to the answer choices. For flaw: which answer describes something wrong inside this argument's logic? For weaken: which answer, treated as true, would make the conclusion less convincing?

If two answers both seem to 'attack' the argument, ask: is each one describing internal logic, or bringing in new information? Only one of those belongs to the question type you're on.