Must Be True—Formal Logic. The correct answer choice is (A)
Begin by diagramming each of the statements in the stimulus and combining them via any common terms.
The first statement is very straightforward:
- P = parrots
LS = learn to speak a few words and phrases
The contrapositive of this statement is probably not useful, since the entire stimulus is about “parrots” rather than “not parrots.”
The second sentence, “Not all parrots have equally pleasant dispositions,” can be diagrammed as:
- EPD = equally pleasant dispositions
“Some” statements do not yield contrapositives, but may lead to other inferences. Because the first and second sentences have a common term, P, they can be combined:
- LS P EPD
Since “some” statements are reversible (they can be read from left to right or right to left), the inference from these two sentences is that some parrots that can learn to speak a few words or phrases do not have equally pleasant dispositions. Diagrammatically:
- LS EPD
The statement “Some of those native to Australia can be counted on for a sweet temper” should be diagrammed thus:
- PA = parrots native to Australia
ST = can be counted on for a sweet temper
Since all parrots (native to Australia or elsewhere) can also learn to speak a few words and phrases (from the first sentence), combining this statement with the above “some” statement yields the following:
- LS PA ST
So it can be concluded:
- LS ST
That is, some parrots that can learn to speak a few words and phrases can be counted on for a sweet temper (and vice versa). This is the strongest inference made thus far. Some test takers will recognize this inference and immediately scan the answer choices to find it, and by doing so can find the correct answer without working through the rest of the stimulus. Recognizing powerful inferences like this one can help to save valuable time in Logical Reasoning.
The third statement says, “Almost any parrot will show tremendous affection for an owner who raised the bird from a chick by hand-feeding it.” This statement is a little bit more difficult to diagram than the others. “Almost any” is logically equivalent to “most” and “show tremendous affection for an owner” is easy to represent. Keep in mind the group of parrots to which this statement applies. It is only valid for parrots that were raised from a chick by hand-feeding, so this must be represented this in the diagram.
This is an effective representation:
- PHF = parrots that were raised by hand-feeding
AO = show tremendous affection for owner
Remember that “most” also implies “some,” meaning that “Some parrots which show tremendous affection for an owner were raised from a chick by hand-feeding.” Since this is true, the following can be concluded:
- LS AO
Please note that “affection for an owner,” “sweet temper,” and “pleasant disposition” are closely related, but are not similar enough to allow for inferences to be made (that is, they cannot be considered common terms).
Answer choice (A): This is the correct answer choice. From the above discussion it should now be clear that some parrots that can learn to speak can also be counted on for a sweet temper. This is the inference made from combining the first sentence and the second half of the second sentence.
Answer choice (B): This statement means that all sweet tempered parrots are either native to Australia or hand-fed as chicks. It also means that if a parrot is sweet tempered and not native to Australia, it must have been hand-fed as a chick. Any of these statements will be a variation of this diagram:
There are several problems with this answer choice. Even if “sweet-tempered” is assumed to be equivalent to “shows affection for owner,” this conclusion would still contain a Mistaken Reversal. Furthermore, all that can be known about parrots not native to Australia is that all of them can learn to speak and not all of them have an equally pleasant disposition. Thus this answer choice cannot be inferred from the statements in the stimulus.
Answer choice (C): Most test takers have little difficulty eliminating answer choice (C). Nothing in the stimulus implies that the sweetest tempered parrots are those native to Australia. The author states that some parrots that are native to Australia have a sweet temper, but it is never made clear how sweet these parrots are.
Answer choice (D): This answer choice says that if a bird can learn to speak and be relied on for a sweet temper, it must be from Australia. Here is a diagram:
Answer choice (E): Again, very few test takers selected this answer choice. The stimulus deals exclusively with different groups of parrots and one cannot use information about parrots in Australia to make claims about all species of pet birds in Australia.