Logical ReasoningDifficulty: Easy

PT159 S1 Q5 ExplanationResearchers gave each of eighteen subjects

A free, expert breakdown of this official LSAT Logical Reasoning question.

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Full official LSAT questions are available through LawHub. This page provides LSAT Lab's explanation, strategy, and review tools without republishing the full official question.

Stimulus

Researchers gave each of eighteen subjects a playing card, then offered them money to lie to a computer about the identity of the card while undergoing a scan by functional magnetic resonance imaging (fMRI). When subjects lied, the scans revealed increased activity in several regions of the brain known to be can eventually form the basis of an effective lie detector.

What this question is testing

Weaken

Conclusion

The researchers think fMRI could become a working lie detector.

Evidence

Why? Because when subjects lied, scans lit up in stress-related brain regions.

Evaluate

Here is the move to watch: a lie detector is useful only if it can distinguish liars from truth-tellers. The evidence shows liars light up — but it doesn't tell us whether truth-tellers also light up.

Imagine a "lie detector" that beeps every time anyone speaks. Liars would always set it off — and so would everyone else. That would not be a lie detector at all.

Goal

Find an answer showing truth-tellers also produce a similar fMRI pattern, breaking the ability of fMRI to distinguish lies from truth.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
5.

Which one of the following would, if true, most weaken the

Answer choices, explained

  1. No Impact2% picked this

    Existing methods of lie detection, including the polygraph, have not been shown

    The fact that other lie-detection methods are unreliable does not bear on whether fMRI in particular can become an effective lie detector. If anything, an unreliable status quo would make a working fMRI more useful, not less. This does not weaken the argument.

  2. No Impact0% picked this

    The majority of people regard lying in an experimental situation

    What people think about the ethics of lying in experiments is irrelevant to whether fMRI can detect lies. The question is technical (can the scan distinguish liars?) not ethical (was the experiment ethical?). This does not weaken the argument.

  3. Weaker Objection8% picked this

    The amount of stress that accompanies lying varies widely from person to person and is also affected by the circumstances in

    Variation in how much stress people feel when lying is a real concern, but it does not break the argument as cleanly. Even if the amount varies, the test would still pick up some stress-region activity from any liar — meaning fMRI could still indicate lying, just with calibration challenges. The much sharper attack is to show truth-tellers also produce stress-region activity, which removes the test's ability to distinguish lies from truth at all.

  4. Weaker Objection2% picked this

    Telling a carefully planned lie causes less stress than telling a

    A difference in stress between planned and spontaneous lies is a complication for fMRI's reliability, but it does not refute the basic claim — both kinds of liars would still show some stress activity. Choice E is much more devastating because it removes any difference between liars and truth-tellers, making lie detection impossible in principle, not just imprecise.

  5. Correct87% picked this

    Stress reactions in the brains of subjects who are being truthful are similar to those that occur in the brains

    Why this is right

    This is the killer weakener. The argument relies on the idea that increased activity in stress regions distinguishes liars. But if truth-tellers' brains show the same kind of stress reactions as liars' brains, then fMRI cannot reliably tell the two apart. A lie detector that lights up for both liars and truth-tellers is not a working lie detector. The conclusion that fMRI could form an effective basis for lie detection collapses.

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

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