#20 - Normal full-term babies are all born with certain

[Administrator]

Complete Question Explanation

Parallel Reasoning-SN. The correct answer choice is (D)

The stimulus states that all normal full-term babies are born with some reflexes that disappear by two months. It concludes that since a particular three-month old baby still has those reflexes, it is not a normal full term baby:

• Normal full term baby Instinctive reflexes that end at two months
  
  Instinctive reflexes that end at two months Normal full term baby

The argument consists of a premise and its contra positive, and is formally correct.

Answer choice (A): This is basically a Mistaken Negation, and is incorrect.

Answer choice (B): This reasoning is formally correct. However, the conditionals are as follows:

• Ape Talk,
  
  Suzy Ape,

Therefore

• Suzy Talk.

You will notice that this does not involve the use of the contra positive, and uses a "none" concept that the stimulus did not include.

Answer choice (C): This response requires the linking assumption that Henry is normal because he shares a characteristic with humans, but the stimulus requires no linking assumption, so this choice is incorrect. Furthermore, the response might be read as:

• Human Social
  
  Henry Social
  
  Henry Normal (because he is human, and it is Normal for humans to be social)

That is basically a Mistaken Reversal.

Answer choice (D): This is the correct answer choice. The premise states that opossums have abdominal pouches, and the conclusion that this animal with no abdominal pouch is not an opossum is an application of the contra positive.

Answer choice (E): Because of the use of the word "some," this choice is immediately wrong. Since "some" could indicate a very small number, it is not a good judge of what is normal, and the reasoning in this response is unjustified.

[niketown3000]

Hello,

I do not understand how the stimulus statements are conditional statements. I can see how the first sentence is conditional, but the second sentence does seem to have conditional indicators, and is just like a conclusion statement. How does the facts of the second sentence play into the conditional reasoning to get to the AC?

[Nikki Siclunov]

The stimulus has the following structure:

Premise 1: All normal babies are born with certain instinctive reflexes that disappear by the age of 2 months.

Premise 2: This 3-month old baby exhibits these reflexes.

Conclusion: This is not a normal baby.

The conditional reasoning in the first premise can be diagrammed as follows:

Normal Baby --> Reflexes Disappear by 2 mo

The second premise shows that the baby in question (let's call it baby A) failed to satisfy the necessary condition in the first premise. By the contrapositive, baby A cannot be a normal baby.

When paralleling the argument, look for an answer choice in which one of the premises establishes a conditional relationship, and the other premise describes an entity that does not satisfy the necessary condition in that relationship. The conclusion must be the contrapositive of that conditional statement:

Premise 1: A --> B

Premise 2: No B

Conclusion: no A

[nyc431]

Could you explain the "none" concept referenced in the explanation for answer choice B?

[Beatrice Brown]

Hi NYC! Happy to explain the "none" concept involved in answer choice (B)

In answer choice (B), the first conditional statement is formed with a "none" statement, which, in general terms, can be diagrammed as follows:

A NOT B,
or
A B

This diagram means that if something has the sufficient condition (in this case, if something is an ape), then it cannot have the necessary condition (in this case, it cannot talk). In answer choice (B), we then know that something cannot both be an ape and also talk.

In other words, "none" statements mean that nothing can possess both the sufficient and necessary conditions. Another example of a "none" statement would be the following:

No student that receives an F can receive a perfect GPA.

This can be diagrammed as:

Student receives an F perfect GPA

Since this is a parallel reasoning question, for answer choice (B) to be correct, the stimulus would also need to make use of a "none" statement. The stimulus does not do this but instead relies on a contrapositive.

I hope this helps!

[anureet]

Hello, I just have a quick question.

The first sentence in the stimulus states: "Normal full-term babies are all born with certain instinctive reflexes that disappear by the age of two months".

My approach to drawing this conditional relationship is that ALL is a sufficient condition indicator and anything that follows it should be on the sufficient side. Therefore, I drew the relationship as follows

Normal Full-term Babies (N)
all born with certain instinctive reflexes that disappear by the age of two months : (B)

B------>N

However, this is wrong. Can you please explain to me the correct interpretation of the first sentence in the stimulus?

[Beatrice Brown]

Hi Anureet! Happy to help you with this

You're correct that the word "all" indicates a sufficient condition! To figure out what the sufficient condition is, though, we need to determine what the word "all" is referring to. In the first sentence of the stimulus, the word "all" is referring to "normal full-term babies." Therefore, when we diagram this sentence, we want to put "normal full-term babies" in the sufficient condition, and "instinctive reflexes that end at two months" in the necessary condition.

Another way to look at this is thinking what the first sentence really means without diagramming it formally, which is the following: if a baby is a normal full-term baby, then it is born with instinctive reflexes that end at two months. The way that you diagrammed the sentence suggests a different meaning, which is: if a baby is born with instinctive reflexes that end at two months, then it is a normal full term-baby. But what the word "all" is telling us in the first sentence is that all normal full-term babies have these instinctive reflexes, not that all babies that have these reflexes are normal full-term babies.

In general, with conditional indicators, make sure to evaluate what part of the sentence that indicator is referring to. Here, the "all" is referring to the part of the sentence that precedes the word "all," not the remainder of the sentence. One way to see this is that the sentence could have instead said "All normal full-term babies are born with..." and had the same meaning. In this phrasing, the word "all" precedes the sufficient condition that it is referring to, which makes it clearer to see what the sufficient condition is. But remember that sufficient condition indicators don't always have to physically come before the phrase that they are referring to.

I hope this helps, and let me know if you have any further questions!