Informal Reasoning

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Thinking Strategies
Formal reasoning follows the rules of
logic, but there are no foolproof rules for
informal reasoning, as this fool demonstrates.
© Scott Adams/Dist. By United Feature Syndicate, Inc.
Psychologists have discovered that both kinds of pitfalls we just described can lead
people to make errors in logical reasoning. This finding is one reason that misleading
advertisements or speeches can still attract sales and votes (Cialdini, 2001).
Informal Reasoning
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Heuristics
Using the rules of formal logic to deduce answers about specific cases is an important
kind of reasoning, but it is not the only kind. A second kind, informal reasoning,
comes into play when we are trying to assess the believability of a conclusion based on
the evidence available to support it. Informal reasoning is also known as inductive reasoning, because its goal is to induce a general conclusion to appear on the basis of specific facts or examples. Psychologists use informal reasoning when they design experiments and other research methods whose results will provide evidence for (or against)
their theories. Jurors use informal reasoning when weighing evidence for the guilt or
innocence of a defendant. And air crash investigators use it in their efforts to discover
and eliminate the causes of commercial aviation accidents.
Formal reasoning is guided by algorithms, or formulas, but there are no foolproof
methods for informal reasoning. For instance, how many white swans would you have to
see before concluding that all swans are white? Fifty? A hundred? A million? Formal logic
would require that you observe every swan in existence. A more practical approach is to
base your conclusion on the number of observations that some mental rule of thumb
leads you to believe is “enough.” In other words, you would take a mental “shortcut” to
reach a conclusion that is probably, but not necessarily, correct (there are, in fact, black
swans). Such mental shortcuts are called heuristics (pronounced “hyoor-IST-ix”).
Heuristics can be helpful, but they can also bias our thinking and cause errors.
Suppose your rule of thumb is to vote for all the candidates in a particular political
party instead of researching the views of each individual. You might end up voting
for someone with whom you strongly disagree on some issues. The extent to which
heuristics are responsible for important errors in judgment and decision making is
still being studied and debated by cognitive psychologists (e.g., Hilton, 2002; Medin
& Bazerman, 1999). Nevertheless, Amos Tversky and Daniel Kahneman (1974;
1993) have described three potentially problematic heuristics that often affect people’s
judgments. These are the anchoring heuristic, the representativeness heuristic, and the
availability heuristic.
People use the anchoring heuristic when they estimate the probability of some event by adjusting their existing estimate rather than
starting from scratch on the basis of new information (Rottenstreich & Tversky, 1997).
This strategy sounds reasonable, but their existing estimate biases their final judgment.
So even if new information suggests that their first estimate is way off, people may
not adjust that estimate enough. It is as if they have dropped a “mental anchor” that
keeps them from drifting too far from their original judgment. Suppose you think that
the chance of being mugged in Los Angeles is 90 percent, but then you see evidence
that the figure is closer to 1 percent. You might reduce your estimate, but only to
80 percent, so your new estimate is still quite inaccurate. The anchoring heuristic
The Anchoring Heuristic
informal reasoning The process of
evaluating a conclusion based on the
evidence available to support it.
heuristics
thumb.
Mental shortcuts or rules of
anchoring heuristic A shortcut in the
thought process that involves adding
new information to existing information to reach a judgment.
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Chapter 7 Thought, Language, and Intelligence
presents a challenge for defense attorneys in U.S. criminal courts, because the prosecution presents its evidence first. Once this evidence has created the impression that
a defendant is guilty, some jurors mentally anchor to that impression and may not be
swayed much by defense evidence to the contrary (Hogarth & Einhorn, 1992). In a
similar way, first impressions of people are not easily shifted by later evidence (see the
chapter on social psychology).
The Representativeness Heuristic People use the representativeness heuristic
when they conclude that something belongs in a certain class based on how similar it is
to other items in that class. For example, consider this personality sketch:
Tom W. is of high intelligence, although lacking in true creativity. He has a need for
order and clarity and for neat and tidy systems in which every detail finds its appropriate place. His writing is rather dull and mechanical, occasionally enlivened by
somewhat corny puns and by flashes of imagination of the sci-fi type. He has a strong
drive for competence. He seems to have little feeling and little sympathy for other
people and does not enjoy interacting with others. Self-centered, he nonetheless has
a deep moral sense.
Do you think it is more likely that Tom is majoring in computer science or psychology? Research by Kahneman and Tversky (1973; Tversky & Kahneman, 1974)
showed that most people would choose computer science. But this answer would probably be wrong. True, the description given is more similar to the prototypical computer
science major than to the prototypical psychology major. However, there are many more
psychology majors than computer science majors in the world. So there are probably
more psychology majors than computer science majors who match this description. In
fact, almost any personality sketch is more likely to describe a psychology major than
a computer science major.
The representativeness heuristic affects many real-life judgments and decisions. For
example, jury decisions depend partly on the degree to which a defendant’s actions are
representative of a particular crime category. So someone who abducts a child and asks
for ransom is more likely to be convicted of kidnapping than someone who abducts
an adult and demands no ransom (Smith, 1991). Both crimes constitute kidnapping,
but the first is a more representative example.
People use the availability heuristic when they judge
the likelihood of an event or the correctness of a hypothesis by how easy it is to think
of that event or hypothesis (Tversky & Kahneman, 1974). In other words, they tend
to choose the hypothesis or predict the event that is most mentally “available,” much
as they might select the box of cereal that happens to be at the front of the supermarket shelf. Although the availability heuristic tends to work well, it, too, can lead to
biased judgments—especially when the mental availability of events doesn’t match
their actual frequency (Morewedge, Gilbert, & Wilson, 2005). For example, news
reports about shark attacks and urban shootings lead many people to overestimate
how often these memorable, but relatively rare, events actually occur. As a result, people may suffer undue anxiety over swimming in the ocean or being in certain cities
(Bellaby, 2003). Similarly, many students stick with their first responses to multiplechoice test questions because it is especially easy to recall those galling occasions on
which they changed a right answer to a wrong one. Research shows, though, that
changing an answer in light of further reflection is more likely to be correct than
incorrect (Kruger, Wirtz, & Miller, 2005).
The three heuristics we have presented represent only a few of the many mental
shortcuts that people use more or less automatically in making judgments, and they
describe only some of the biases and limitations that operate in human reasoning
(Hogarth & Einhorn, 1992). Other biases and limitations are described in the following sections, as we consider two important goals of thinking: problem solving and
decision making.
The Availability Heuristic
representativeness heuristic A mental
shortcut that involves judging whether
something belongs in a given class on
the basis of its similarity to other members of that class.
availability heuristic A mental shortcut through which judgments are based
on information that is most easily
brought to mind.