Q1 - The effort involved in lying

[ginsburgb]

Can you please explain why D is a better answer choice than A? I was very torn and answered this incorrectly. Thanks.

[ManhattanPrepLSAT1]

It sounds like you have no problem seeing why answer choice (D) is correct, but you need help seeing why answer choice (A) is wrong.

Pay careful attention to what the conclusion is saying. It says that "lie-detector tests that can detect these reactions..." The conclusion is strictly about lie-detector tests that can detect measurable physiological reactions. So if there are some physiological reactions that occur when someone is lying that lie-detector tests cannot measure, the conclusion would remain unaffected.

The argument does not rely on the assumption that lie-detector tests can measure every physiological reaction that occurs when someone is lying. If it did, then answer choice (A) would be a good answer.

Does that answer your question?

[yoon1song]

I also have a question regarding (A) and (D).

At first, I chose (A) because detecting only some of the reactions would miss the other reactions and felt that lie-detector wouldn't be a sure way of determining when someone is lying.
But for (A) to be incorrect, are we assuming all physiological reactions to occur at the same time?

For example, if there are 2 physiological reaction that accompany lying, (1) speedup of the heartbeat & (2) sweaty hands and the lie detector can detect only the heartbeat rate and not sweaty hands, then only if the liar is showing both symptoms, can the detector reliably detect that the person is lying. But, if the liar is getting only sweaty hands and steady heartbeat, then the detector wouldn't be a sure way of determining someone is lying because it can only tell someone is lying if the person shows signs of increased heartbeat.

So, are we assuming that all physiological reactions to lying in a simultaneous manner?


I am also not sure why (D) is correct. The choice tells us that we will have some false positives, but the stimulus is focusing on detecting when someone is lying. If the detector can determine both the true liar AND false liars, it is still effectively determining when someone is lying. I felt, these false positives are out of scope.


Thank you in advance!

[nflamel69]

so the core is that

lying causes (produces) some physiological effects -> lie detectors that can detect these effects can determine whether someone is lying.

the flaw is that we know one cause for some physiological effects, but do we know it is the only cause? it could be that not lying could also have these effects. Thus, by only having these effects as evidence, we cannot be certain whether someone is lying or not lying, which is exactly what D is suggesting. they simply gave a reason why someone not lying can have these effects, which is irrelevant to evaluation of the answer itself.

As for A, we really dont' have to worry about those effects that cannot be measured, since the premise explicitly restricted to detectors that can measure THESE effects.

Hope that helps.

[mayahford]

D is correct because if the physiological reactions to the tensions of taking a lie detector test are identical to the reactions accompanying lying then the lie detector test will not be able to distinguish which were the cause. Therefore would NOT be a "sure way of determining when someone is lying".

A is incorrect because regardless if if the test can only measure some PR those reactions that it can measure may be adequate to determine if some one is lying.