LSAT Inference / Must Be True
Last updated: May 2, 2026
Inference / Must Be True questions are one of the highest-leverage areas to study for the LSAT. This guide breaks down the rule, the elements you need to recognize, the named traps that catch most students, and a memory aid that scales to test day. Read it once, then practice the same sub-topic adaptively in the app.
The rule
On a Must Be True question, the credited answer is a statement that is fully supported by the facts in the stimulus — you should be able to derive it without adding any outside assumption. Treat the stimulus as a small set of premises and ask: 'Does this answer have to be true if everything above is true?' If you can imagine even one consistent scenario where the stimulus is true but the answer is false, the answer is wrong. Do not evaluate whether the stimulus's argument is good; just take it as given and stay inside its boundaries.
Elements breakdown
Read the stimulus as fact
Treat every sentence as a stipulated premise, not a claim to evaluate.
- Suspend doubt about truth
- Note connectors: all, some, most, only
- Track conditional triggers and results
- Mark numerical or quantified language
Identify the question type from the stem
Confirm the stem asks what must, follows, or is most strongly supported.
- Spot 'must be true' / 'properly inferred'
- Spot 'most strongly supported'
- Distinguish from 'most weakens' or 'assumes'
- Note strength of language requested
Pre-phrase what you can derive
Before reading choices, combine premises to anticipate provable conclusions.
- Chain conditionals end-to-end
- Apply contrapositives where useful
- Combine quantifiers carefully
- Notice what is NOT said
Test each choice against the stimulus
For every choice, ask whether the stimulus forces it to be true.
- Try to construct a counterexample
- Check scope: people, time, place
- Check strength: must vs. might
- Reject choices needing outside knowledge
Calibrate to the question stem's strength
Match answer certainty to the stem's wording.
- 'Must be true' demands airtight support
- 'Most strongly supported' allows mild gap
- Eliminate stronger-than-stimulus claims
- Eliminate weaker-than-needed claims when 'must'
Common patterns and traps
The Plausible-But-Unproven Trap
A choice that sounds reasonable given real-world knowledge but is never actually established by the stimulus. Test-makers exploit your background assumptions, knowing you'll fill gaps the text never filled. The fix is to ask 'where exactly in the stimulus is this stated or entailed?' and reject the choice if you can't point to it.
A statement that any informed reader would nod along with — 'older buildings are usually expensive to maintain' — even though the stimulus said nothing about cost.
The Strength Inflation Trap
A choice that takes a hedged or partial claim from the stimulus and ratchets it up. 'Some' becomes 'most,' 'most' becomes 'all,' 'tends to' becomes 'always.' On 'must be true' questions, anything stronger than what the stimulus supports is wrong, even if the direction is right.
The stimulus says some surveyed voters favored the bill; the choice says a majority of voters favored the bill.
The Scope Shift Trap
A choice that swaps the stimulus's category for a related but broader or different one — moving from a specific group, time, or place to a generalized one. The stimulus may discuss 'first-year medical residents at urban hospitals' while the answer talks about 'medical residents' generally. The reasoning may look parallel, but the support evaporates.
A choice that draws a conclusion about an entire profession when the stimulus only discussed one subset of practitioners.
The Reversed-Conditional Trap
A choice that flips a conditional in the stimulus without contrapositive. If the stimulus says 'If X, then Y,' a wrong answer may assert 'If Y, then X.' These traps prey on students who treat conditionals as biconditional. The legitimate move is to take the contrapositive — 'If not Y, then not X' — not the converse.
Stimulus: 'All licensed contractors carry liability insurance.' Wrong choice: 'Anyone with liability insurance is a licensed contractor.'
The Out-of-Scope Comparison Trap
A choice that compares two things the stimulus never compared, or rank-orders categories the stimulus only described separately. Even if each side appears in the stimulus, drawing a comparative conclusion ('X is more common than Y') requires comparative evidence the stimulus rarely provides.
The stimulus describes two policies independently; the choice claims one policy was more effective than the other.
How it works
Imagine the stimulus says: 'Every architect at the Reyes firm has a graduate degree. Some architects at the Reyes firm specialize in historic preservation. No one specializing in historic preservation works on Mondays.' From these three sentences, you can chain: at least one architect at Reyes specializes in historic preservation, and that architect has a graduate degree, and that architect does not work on Mondays. So 'At least one Reyes architect with a graduate degree does not work on Mondays' must be true. But 'Most Reyes architects do not work on Mondays' is NOT supported — only the historic-preservation subset is covered. The trap is to drift from 'some' to 'most,' or from 'Reyes architects' to 'architects generally.' Stay inside the boundaries the stimulus actually drew.
Worked examples
If the statements above are true, which one of the following must also be true?
- A Most catalogers at the Linden Archive are trained conservators.
- B At least one Linden Archive employee with vault access is not permitted to handle vault items without a supervisor present. ✓ Correct
- C No administrator at the Linden Archive has access to the rare-manuscripts vault.
- D Every trained conservator at the Linden Archive is also an administrator.
- E Trained conservators at the Linden Archive are more common than untrained catalogers.
Why B is correct: From 'some catalogers are trained conservators' and 'every cataloger has vault access,' at least one trained conservator has vault access. From 'no trained conservator may handle vault items without a supervisor,' that same person cannot handle vault items unsupervised. So at least one Linden Archive employee with vault access is not permitted to handle vault items without a supervisor — exactly choice B.
Why each wrong choice fails:
- A: The stimulus only says 'some' catalogers are trained conservators. 'Some' does not entail 'most'; it could be one cataloger out of fifty. (The Strength Inflation Trap)
- C: The stimulus says every cataloger has vault access; it says nothing about whether administrators do or do not. The claim about administrators is unsupported. (The Plausible-But-Unproven Trap)
- D: The stimulus tells us some catalogers are trained conservators, which means those conservators are catalogers — not administrators. This choice contradicts the stimulus rather than following from it. (The Reversed-Conditional Trap)
- E: The stimulus never compares the size of the trained-conservator group to the untrained-cataloger group. No comparative claim is supported. (The Out-of-Scope Comparison Trap)
Which one of the following is most strongly supported by the information above?
- A Establishing a weekly farmers' market in any neighborhood will increase residents' trust in local government.
- B In at least one of the neighborhoods Reyes surveyed, the presence of a long-running weekly farmers' market preceded a measurable difference in residents' trust in local government. ✓ Correct
- C Residents who attend farmers' markets trust local government more than residents who do not attend them.
- D Local governments that sponsor farmers' markets are generally more trustworthy than those that do not.
- E No factor other than the farmers' markets contributed to the differences in trust Reyes observed.
Why B is correct: The stimulus tells us, for every surveyed neighborhood with a five-year-plus market, trust was higher than in comparable neighborhoods without one, and the markets came first. From that, in at least one such neighborhood, the market preceded a measurable trust difference — which is exactly what B states, hedged appropriately.
Why each wrong choice fails:
- A: The stimulus describes an observed correlation in neighborhoods Reyes surveyed; it does not license a universal causal prediction about 'any neighborhood.' (The Strength Inflation Trap)
- C: The stimulus discusses neighborhood-level trust differences, not differences between attendees and non-attendees of the markets. That comparison is never made. (The Scope Shift Trap)
- D: The stimulus reports residents' reported trust, not the actual trustworthiness of governments. These are different concepts and the stimulus says nothing about real trustworthiness. (The Plausible-But-Unproven Trap)
- E: The stimulus only confirms timing; it never rules out other contributing factors. 'No other factor' is far stronger than the evidence supports. (The Strength Inflation Trap)
If the statements above are true, which one of the following must be true?
- A Every manuscript accepted for publication at Heliconia Press is reviewed by Fei Liu.
- B If a manuscript at Heliconia Press has not been recommended by the acquisitions committee, then it has not been reviewed by Fei Liu. ✓ Correct
- C Every manuscript recommended by the acquisitions committee is eventually accepted for publication.
- D Tomas Aldecoa reviews more manuscripts than Fei Liu does.
- E No manuscript reviewed by only one senior editor is recommended by the acquisitions committee.
Why B is correct: Chain the conditionals: reviewed by Fei Liu → reviewed by Aldecoa → recommended by the acquisitions committee. Taking the contrapositive of the full chain: not recommended by the acquisitions committee → not reviewed by Fei Liu. That is precisely what B asserts.
Why each wrong choice fails:
- A: The stimulus says accepted manuscripts get at least two senior-editor reviews, but never specifies that Fei Liu is one of those editors. Fei Liu may review only a subset. (The Plausible-But-Unproven Trap)
- C: This reverses the relationship. The stimulus says Aldecoa only reviews recommended manuscripts; it does not say recommended manuscripts must be accepted, or even reviewed by him. (The Reversed-Conditional Trap)
- D: The stimulus establishes that every Fei Liu review is also reviewed by Aldecoa, but Aldecoa could review the same number — for example, exactly the same manuscripts. No comparison of totals is forced. (The Out-of-Scope Comparison Trap)
- E: The stimulus says accepted manuscripts get at least two reviews, but it never restricts which manuscripts the acquisitions committee may recommend. A recommended manuscript could in principle be reviewed by only one senior editor and not accepted. (The Scope Shift Trap)
Memory aid
S.A.F.E.: Stimulus-only, Airtight derivation, Force the counterexample, Exact-match scope and strength.
Key distinction
'Must be true' means the stimulus forces the answer; 'could be true' or 'is consistent with' is a much weaker bar that wrong answers exploit by sounding plausible without being guaranteed.
Summary
On Must Be True, the right answer is the one statement you can prove using only what's in the stimulus — nothing stronger, nothing broader, nothing imported.
Practice inference / must be true adaptively
Reading the rule is the start. Working LSAT-format questions on this sub-topic with adaptive selection, watching your mastery score climb in real time, and seeing the items you missed return on a spaced-repetition schedule — that's where score lift actually happens. Free for seven days. No credit card required.
Start your free 7-day trialFrequently asked questions
What is inference / must be true on the LSAT?
On a Must Be True question, the credited answer is a statement that is fully supported by the facts in the stimulus — you should be able to derive it without adding any outside assumption. Treat the stimulus as a small set of premises and ask: 'Does this answer have to be true if everything above is true?' If you can imagine even one consistent scenario where the stimulus is true but the answer is false, the answer is wrong. Do not evaluate whether the stimulus's argument is good; just take it as given and stay inside its boundaries.
How do I practice inference / must be true questions?
The fastest way to improve on inference / must be true is targeted, adaptive practice — working questions that focus on your specific weak spots within this sub-topic, getting immediate feedback, and revisiting items you missed on a spaced-repetition schedule. Neureto's adaptive engine does this automatically across the LSAT; start a free 7-day trial to see your sub-topic mastery climb in real time.
What's the most important distinction to remember for inference / must be true?
'Must be true' means the stimulus forces the answer; 'could be true' or 'is consistent with' is a much weaker bar that wrong answers exploit by sounding plausible without being guaranteed.
Is there a memory aid for inference / must be true questions?
S.A.F.E.: Stimulus-only, Airtight derivation, Force the counterexample, Exact-match scope and strength.
What's a common trap on inference / must be true questions?
Importing real-world knowledge
What's a common trap on inference / must be true questions?
Strengthening 'some' to 'most' or 'all'
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