Self-study
Support Most people who become migraine sufferers as adults were prone to bouts of depression as children. █████ ██ ██████ ██ ██████ ████ █ █████ ███ ██ █████ ██ █████ ██ ██████████ ██ ██████ ██ ██████ █████████ ██████ ██████████
Method of Reasoning
Most members of a first group (adults who suffer from migraines) are also members of a second group (people who were prone to depression as a child). Because of this, the author concludes that most members of the second group (people who were prone to depression as a child) are also members of the first group (adults who suffer from migraines).
Identify and Describe Flaw
The flaw here is a misunderstanding of the quantifier “most.” This quantifier isn’t bidirectional—you can’t assume that because most a first group falls into a second, most of the second group falls into the first! Maybe there are 100 people in the world who became adult migraine sufferers, and 99 of them were prone to depression as a child (satisfying our premise). There could be 5,000 people in the world who were prone to depression as a child, in which case most of them can’t have become adult migraine sufferers, because there are only 100 adult migraine sufferers total!
21.
The flawed pattern of reasoning ██ ███ ████████ █████ ██ ████ ████████ ██ ████ ██ █████ ███ ██ ███ ██████████
a
Most good-tempered dogs ████ ██████████ ███████ ██████ ██ ████████ ██████████ █ █████ ████ ██ ██████████ ███████ ██████ ██ ██████ ██ ██████ █ █████████████ ████
Most members of a first group (good-tempered dogs) are also members of a second group (dogs that were vaccinated against rabies as puppies). Because of this, the author concludes that most members of the second group (dogs that were vaccinated against rabies as puppies) are also members of the first group (good-tempered dogs). Like the stimulus, this argument is flawed in that it misconstrues the quantifier “most” as bidirectional.
b
Most vicious dogs ████ ███████████ ████ ██████ █████ ██ ███ ██ █████████ ████ █ ███ █████ █████ ███ ██ ███████ ██ ██████ ██ ████ ███████ ███ ███ █████ ████ ██ ███ ██████
Wrong flaw. Based on the fact that most vicious dogs were ill-treated when young, we could say that any given vicious dog is likely to have been ill-treated when it was young. But that doesn’t mean its current owner is likely the one that mistreated it! This isn’t the flaw from the stimulus, though, which misconstrues the quantifier “most” as bidirectional.
c
Most well-behaved dogs ████ █████████ █████████ █████████ █████ ██ █ ███ ███ ███ █████████ █████████ █████████ ██ ████ ███ ██ ████ ████████
Wrong flaw. This argument takes a “most” premise and turns it into an “all” conclusion—a big flaw! Even if most well-behaved dogs have undergone obedience training, some dogs that haven’t undergone obedience training could still be well behaved. This isn’t the flaw from the stimulus, though, which misconstrues the quantifier “most” as bidirectional.
d
Most of the ████ █████ ██ █████████████ ███ █████ ██████████ ██ ██████ ██ ██████ ████ ████ ███ ████ █████ ██ ███████ ██ ████████ ████ ███ █████ █████
Wrong flaw. This is a cookie-cutter flaw of assuming causation. It could be true that most pets at the vet are dogs for some reason other than propensity for illness—maybe dog owners tend to be wealthier or more cautious than other pet owners! This isn’t the flaw from the stimulus, though, which misconstrues the quantifier “most” as bidirectional.
e
Most puppies are █████ ████ █████ ███████ ██ ███ ███ ██ █████ ██████ █████ █ █████ ████ ██ █████ ████ █████ █████ ██ ██████ ██ ████ ████ █████ ████ ███ ███████
No flaw. This is a valid argument. Since most puppies are taken from their mothers at eight weeks old, it’s likely that any given puppy that’s older than eight weeks has been taken from its mother.