LSAT 159 | Section 1 | Logical Reasoning: Q5

LR Question 5 Explanation

QUESTION TEXT: Researchers gave each of eighteen subjects a playing …

QUESTION TYPE: Weaken

CONCLUSION: fMRI will eventually let us detect lies.

REASONING: When subjects lied in a study, their brains showed increased activity in stress-related areas.

ANALYSIS: The key here is stress-related activity. The argument assumes that this is a good indicator of lying.

To weaken that assumption, we can show that stress-related brain activity is not unique to lying.

If people who are telling the truth show this same stress activity, then the fMRI signal is useless. It doesn’t help distinguish lies from truth.

This question is an example of a false comparison. The author really gives the impression they compared telling lies to telling the truth. But they only told us what happens in an fMRI when you lie. Maybe all talking in an fMRI lights up stress areas.

___________

1. The fact that other lie detectors are unreliable doesn’t mean this one will be.
2. Nothing in the stimulus relies on the study or its participants being ethical. Evil studies can still be truthful.
3. Too weak. This shows that there’s variation in stress levels, but not that truth and lying could look the same.
    
   In general, you can’t take a vague word such as “variation” or “affected” and interpret it only in a way that weakens an argument. This answer is completely consistent with lying showing much more stress activity in all cases than telling the truth.
    
   A weaken answer needs to actually weaken the argument without multiple added assumptions.
4. Not clear how this applies. Were the lies in the study carefully planned? Off the cuff? We don’t know.
    
   The key point is that this answer is completely consistent with all lies being more stressful than telling the truth.
5. CORRECT. If truthful subjects show the same stress-related brain activity as liars, fMRI cannot reliably distinguish lies from truth. So it can’t be an effective lie detector.