The Usual Suspects: Common Logical Fallacies

(Much of this content is identical to the Critical Reasoning section).

There are seven logical errors that commonly appear in the essay questions. When writing your essay argument, explicitly identify the logical flaws. These flaws also tend to occur in the critical reasoning section of the Verbal LSAT, so your preparation here will benefit you when taking that section.

1. Circular Reasoning

Here, an unsubstantiated assertion is used to justify another unsubstantiated assertion, which is used to justify the first statement. For instance, Joe and Fred show up at an exclusive club. When asked if they are members, Joe says “I’ll vouch for Fred”. When Joe is asked for evidence that he’s a member, Fred says, “I’ll vouch for him”.

2. The Fallacy of the Biased Sample

The Fallacy of the Biased Sample is committed whenever the data for a statistical inference is drawn from a sample that is not representative of the population under consideration. The data used to make a generalization are drawn from a group that does not represent the whole.

The following argument commits a Fallacy of the Biased Sample:

In a recent survey conducted by Wall Street Weekly, 80% of the respondents indicated their strong disapproval of increased capital gains taxes. This survey clearly shows that increased capital gains taxes will meet with strong opposition from the electorate.

The data for the inference in this argument are drawn from a sample that is not representative of the entire electorate. Since the survey was conducted on people who invest, not all members of the electorate have an equal chance of being included in the sample. Moreover, people who read about investing are more likely to have an opinion on the topic of taxes on investments that differs from the opinion of the population at large.

3. Fallacy of the Insufficient Sample

The Fallacy of the Insufficient Sample is committed whenever an inadequate sample is used to justify the conclusion drawn.

The following argument commits a Fallacy of the Insufficient Sample:

I have worked with three people from New York City and found them to be obnoxious, pushy and rude. It is obvious that people from New York City have a bad attitude.

The data for the inference in this argument are insufficient to support the conclusion. Three observations of three people are not sufficient to support a conclusion about the entire population of a city.

4. Ad hominem

One of the most often-employed fallacies, ad hominem means “to the man” and indicates an attack made on a person rather than on the statements that person has made. An example is: “Don’t listen to my opponent, he’s a homosexual.”

5. Fallacy of Faulty Analogy

Reasoning by analogy makes an unsubstantiated assumption when comparing two similar things. The fallacy of faulty analogy assumes that since two things are alike in many ways, they will share other traits in common. Faulty Analogy arguments conclude that one similarity results in another, when in fact, there can be no way of inferring this extra similarity.

The following is an example of a Fallacy of Faulty Analogy:

Ted and Jim excel at both football and basketball. Since Ted is also a track star, Jim likely also excels at track.

In this example, numerous similarities between Ted and Jim are taken as the basis for the inference that they share additional traits.

6. Straw Man

Here, the speaker attributes an argument to an opponent, and that argument does not represent the opponent’s true position. For instance, a political candidate might charge that his opponent “wants to let all prisoners go free”, when in fact his opponent simply favors a highly limited furlough system. The person is portrayed as someone he is not.

7. The “After This, Therefore, Because of This” Fallacy (Post hoc ergo propter hoc)

This is a “false cause” fallacy in which something is associated with something else because of mere proximity of time. One often encounters, such as in news stories, people assuming that because one thing happened after another, the first caused it, as with, “I touched a toad; I have a wart; the toad caused the wart.” The error in the arguments that commit this fallacy is that their conclusions are simply claims and are insufficiently substantiated by the evidence.

Here are two examples of the After This, Therefore Because of This Fallacy:

Ten minutes after walking into the auditorium, I began to feel sick to
my stomach. There must have been something in the air in that building that caused my nausea.

The stock market declined shortly after the election of the president,
thus indicating the lack of confidence the business community has in the new administration.

In the first example, a causal connection is posited between two events simply on the basis of one occurring before the other. Without further evidence to support it, the causal claim based on the correlation is premature.

The second example is typical of modern news reporting. The only evidence offered in this argument to support the implicit causal claim that the decline in the stock market was caused by the election of the president is that the election preceded the decline. While this may have been a causal factor in the decline of the stock market, to argue that it is the main cause without additional information is to commit the After This, Therefore, Because of This Fallacy.

8. The Either-Or Thinking

This is the so-called black-or-white fallacy. Essentially it says, “Either you believe what I’m saying or you must believe exactly the opposite.” Here is an example of the black-or-white fallacy:

Since you don’t believe that the earth is teetering on the edge of destruction, you must believe that pollution and other adverse effects that man has on the environment are of no concern whatsoever.

The argument above assumes that only two possible alternatives are open to us. There is no room for a middle ground.

9. The “All Things are Equal” Fallacy

This fallacy is committed when background conditions are assumed, without justification, to have remained the same at different times/locations. In most instances, this is an unwarranted assumption for the simple reason that things rarely remain the same over extended periods, and things rarely remain the same from place to place.

The last Democratic Party winner of the New Hampshire primary won the general election. This year, the winner of the New Hampshire primary will win the general election.

The assumption operative in this argument is that nothing has changed since the last primary. No evidence or justification is offered for this assumption.

10. The Fallacy of Equivocation

The Fallacy of Equivocation occurs when a word or phrase with more than one meaning is employed in different meanings throughout the argument.

“Every society is, of course, repressive to some extent. As Sigmund Freud pointed out, repression is the price we pay for civilization.” (John P. Roche, political columnist)

In this example, the word repression is used in two completely different contexts. “Repression” in Freud’s mind meant restricting sexual and psychological desires. “Repression” in the second context does not mean repression of individual desires, but government restriction of individual liberties, such as in a totalitarian state.

11. Non Sequitur

This means “does not follow,” which is short for: the conclusion does not follow from the premise. To say, “The house is white; therefore, it must be big” is an example. The house may be big but there is no intrinsic connection with its being white.

12. Argument ad populum

A group of kindergartners is studying a frog, trying to determine its sex. “I wonder if it’s a boy frog or a girl frog,” says one student. “I know how we can tell!” pipes up another. “All right, how?” asks the teacher, resigned to the worst. Beams the child: “We can vote.”

This is argumentum ad populum, the belief that truth can be determined by more or less putting it to a vote. Democracy is a very nice thing, but it doesn’t determine truth. Polls are good for telling you what people think, not whether those thoughts are correct.

Common Student Errors

We’ve graded essays from thousands of students and we see recurring errors time and time again. The most common error on the Analysis of Argument essay is “Splitting Hairs“.

Splitting Hairs refers to trying to dissect errors that do not fall into the categories listed here. Remember that all Analysis of Argument questions have SERIOUS errors. The danger is that you could get distracted on a minor issue and miss the serious errors that the grader want to see.

Here is an example:
A company is cutting unneeded employees to cut costs and boost profitability. Is this a wise strategy?

Splitting Hairs: What if the employees refuse to go? What if the employees are actually volunteers? What if the employees are the company’s biggest customers? etc..

Next LSAT: March 30th