Thinking, Fast and Slow - Daniel Kahneman

“How tall is John?” If John is 5' tall, your answer will depend on his age; he is very tall if he is
6 years old, very short if he is 16. Your System 1 automatically retrieves the relevant norm,
and the meaning of the scale of tallness is adjusted automatically. You are also able to match
intensities across categories and answer the question, “How expensive is a restaurant meal that
matches John’s height?” Your answer will depend on John’s age: a much less expensive meal
if he is 16 than if he is 6.
But now look at this:
John is 6. He is 5' tall.
Jim is 16. He is 5'1" tall.
In single evaluations, everyone will agree that John is very tall and Jim is not, because they are
compared to different norms. If you are asked a directly comparative question, “Is John as tall
as Jim?” you will answer that he is not. There is no surprise here and little ambiguity. In other
situations, however, the process by which objects and events recruit their own context of
comparison can lead to incoherent choices on serious matters.
You should not form the impression that single and joint evaluations are always
inconsistent, or that judgments are completely chaotic. Our world is broken into categories for
which we have norms, such as six-year-old boys or tables. Judgments and preferences are
coherent within categories but potentially incoherent when the objects that are evaluated
belong to different categories. For an example, answer the following three questions:
Which do you like more, apples or peaches?
Which do you like more, steak or stew?
Which do you like more, apples or steak?
The first and the second questions refer to items that belong to the same category, and you
know immediately which you like more. Furthermore, you would have recovered the same
ranking from single evaluation (“How much do you like apples?” and “How much do you like
peaches?”) because apples and peaches both evoke fruit. There will be no preference reversal
because different fruits are compared to the same norm and implicitly compared to each other
in single as well as in joint evaluation. In contrast to the within-category questions, there is no
stable answer for the comparison of apples and steak. Unlike apples and peaches, apples and
steak are not natural substitutes and they do not fill the same need. You sometimes want steak
and sometimes an apple, but you rarely say that either one will do just as well as the other.