LSAT 118 | Section 1 | Logical Reasoning: Q24

LR Question 24 Explanation

QUESTION TEXT: In criminal proceedings, defense attorneys occasionally …

QUESTION TYPE: Flawed Reasoning

CONCLUSION: It’s a mistake to exonerate a suspect just because their DNA doesn’t match the DNA found on the crime scene.

REASONING: Every person’s DNA is unique. But sometimes, DNA tests fail to distinguish between samples from different people.

ANALYSIS: The stimulus says the issue with DNA tests is that they sometimes fail to distinguish between different people.

That means the test sometimes says: Samples A and B match, even though they’re actually from different people. Basically, the test sometimes produces false matches (false positives).

But the conclusion is about non-matches: If the test says the samples don’t match, we shouldn’t exonerate the suspect.

To reach that conclusion, we would need evidence that the test sometimes produces false nonmatches (false negatives). But we don’t have that.

So the author is confusing two kinds of testing errors. They have evidence about false positives, and use that to say the test might produce false negatives. But that’s a different type of error.

___________

1. Infallible means incapable of being wrong. If you know that, you can see this answer is silly. The whole argument is that DNA testing can be unreliable.
2. CORRECT. The argument confuses false positives with false negatives.
3. The argument is only about DNA testing, not all methods of identifying criminals. No overgeneralization here.
4. Nothing in the stimulus discusses experiments versus real-world conditions.
5. The conclusion is not about what evidence should be admitted in court. It’s about whether a DNA nonmatch should automatically exonerate a suspect.