Logical reasoning PrepTest 121 · Section 1 · Question 1

Incorrect. If this subset of reactions is unique to lying, then the detector could still detect a lie. You don't need to detect all the "symptoms" of something to be able to determine that it's present.

Incorrect. People's awareness of these reactions isn't related to whether they're having them or whether they indicate that they're lying.

Incorrect. As long as the lying always produces the physiological reactions (and nothing else does), the lie detectors could still work, so this answer doesn't affect the argument.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Lying causes physiological changes, so lie detectors can measure these and are certain to detect lying.

Answer Anticipation:
The conclusion here is rather strong—by measuring certain physiological reactions, a lie detector is "a sure way" of determining if someone is lying. That doesn't leave any room for error!

Does the noted measurement allow for errors? It is established that lying causes certain physiological reactions. So wouldn't measuring these reactions tell you if someone is lying? Not if other things cause these reactions, as well! If the physiological reactions show up whenever someone is under stress, or in response to any number of other triggers. If the physiological reactions have any other cause, then measuring for them might not be "a sure way" to see if someone's lying.

Answer Explanation:
This answer establishes an alternative cause for the physiological reactions—taking the lie-detector test. If "identical" physiological reactions happen when someone tells the truth during a lie-detector test, then a lie detector might determine someone telling the truth is lying, and thus it wouldn't be a "sure" way to detect whether someone is lying.

Key Takeaway:
This argument uses one measurement as a proxy for another—physiological reactions as a proxy for lying. Whenever that happens, think about whether those two things are intrinsically linked in a way that means they each guarantee each other. If the proxy measurement doesn't guarantee the other, then the argument falls apart.

Incorrect. A motivation to lie is out of scope as to whether the person actually lies, and whether the lie detector can detect lies (and only lies) accurately.