Self-study
Psychologist: Some astrologers claim that our horoscopes completely determine our personalities, but Conclusion this claim is false. █ ███████ ████ █████████ ███████████ ████ ██ ███████ ████ ██ ███████████ ███ ████ ████████████ ██ ████ ███████ ██████████████ ████████ █████ ███████ ████ ████████ ██ ████ ███ ███████████ ███ ████ ████ ██ █████ ███ ██ ███ ████ ███ ███ ██ ███████ ███ ████ ██████████ ██ █ ████████ ██ ███████ ███ ███ ██ █ ████████ ██ ███ █████ ███████████ █████ ████████ ████ ███ █████████████ ██ █████ ███ ███████████ ███ ██ ████ ██████████
Summary
The psychologist concludes that horoscopes do not fully determine our personalities, as some astrologers claim. In support, the psychologist offers the example of two people born at the exact same time, on the same day, one in Toronto and one in New York. Despite their identical birth dates, these individuals still have different personalities.
The psychologist's argument relies on offering a counter-example in order to refute the astrologers' claim. If the astrologers say that your horoscope always determines your personality, all that's necessary to disprove them is to show one case where someone's horoscope certainly did not determine their personality.
Objective: Find a Necessary Assumption
The thing is, for the psychologist's counter-example to be effective, the two individuals in the argument need to actually have the same horoscopes—but the psychologist never says they do. All we know is that their day and time of birth were the same, but is that all that determines a horoscope? We don't know, meaning it's necessary to assume that no other factor makes their horoscopes different: for example, their different places of birth.
Although we've found one necessary assumption, we should still keep an open mind in the answer choices. We might find the answer we predicted, but there could also be a different, unexpected necessary assumption.
21.
Which one of the following ██ ██ ██████████ ██ █████ ███ ██████████████ ████████ ████████
a
Astrologers have not █████████ █████ ██████ ██ ████████ ████████████████
Whether or not astrologers have conducted experiments has no bearing on the psychologist’s reasoning. The question is just whether or not the astronomers are correct, which doesn't depend on the strength of their scientific methodology.
b
The personality differences ███████ ███ ███ ███████████ ██████ ██ █████████ ██ ███ ████████ ███████████ ███████ ███████ ███ ███ █████
To understand why (B) is not necessary, it's important to be clear on the psychologist's conclusion: it's simply that astrology does not determine personality. For that to be true, it's not necessary that personality is or isn't caused by any other particular factor, including cultural differences.
To demonstrate this, what happens if we negate (B)? Then, the personality differences can be explained by culture... which means they're still not because of astrology. Negating (B) actually strengthens the argument, whereas negating a necessary assumption would destroy it. So (B) can't be necessary.
c
The geographical difference ███████ ███████ ███ ███ ████ ███ ███ ██████ ██ ███ ███ ███████████ ██████ █████████ ███████████
This must be true in order for the conclusion to follow; in other words, it's necessary. We can test this with negation: if (C) is false, and the two individuals have different horoscopes, then this example no longer works as support for the conclusion that horoscopes do not determine personalities.
In other words, if we don't assume (C), then the psychologist no longer has a counter-example, just two individuals with different horoscopes and different personalities. Without (C), the psychologist can't prove that horoscopes don't determine personality, meaning (C) is necessary.
d
Complete birth records ███ ███ ████ ██ █████ ████ ████ ██ ████ ██████████
The psychologist establishes that the two individuals were born at the same time; whether the hospital has complete information about other people’s births is irrelevant.
If (D) were reworded, and instead said that the hospitals' records about these specific births were accurate, then it would be a much stronger candidate. But as it is, even if (D) were not the case, it wouldn't break the argument.
e
Identical twins have █████████ ███████ ██████████ ███ ███████ ████ ███████ ████ █████████████
The psychologist's specific claim is that horoscopes don’t determine personalities. (E) gives us some reasons why twins might have similar personalities even without astrological influences, but that's not necessary for the argument. The psychologist doesn't have to explain why twins have similar personalities, just prove that horoscopes don't always determine everyone's personality.