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   III-A. The Usual Suspects: Common Logical Fallacies

None of these fallacies is set in stone, under some situations they may not even be fallacies. How much they weaken or strengthen an argument will depend on the situation.

1. Ad hominem

One of the most often employed fallacies, ad hominem means "to the man" and indicates an attack that is made upon a person rather than upon the statements that the person has made.

Your medical advice isn't worthy of consideration because you aren't a doctor.

Don't vote for him to be president, he is an incorrigible womanizer.

The converse of ad hominem is called an Appeal to Authority. In this case, an argument is valid based simply on the person's title or reputation:

Take my advice because I am a doctor.

Let me handle the operation, I am a surgeon.

You can bolster an Ad Hominem argument by showing that a personal characteristic is in fact relevant.

Don't vote for him to be president, he is an incorrigible womanizer.

Could be strengthened by: 1) numerous affairs could be a distraction from governance, or 2) numerous affairs may open a president to blackmail.

Show that personal traits aren't important at all.

Don't hire him as your landscaper, he is an incorrigible womanizer.

It is difficult to see how a landscaper's personal life could have any impact on his performance.

2. Straw Man

Here the speaker attributes an argument to an opponent that 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 who is not.

The Congressman wants to cut funding for the attack submarine program. I disagree entirely. I do not understand how he can be so irresponsible and leave us defenseless like that.

Perhaps the congressman does not believe that the attack submarine would have any defensive benefit. This is similar to an ad hominem attack because it attacks the person. An ad hominem is often a personal attack, which is simply creating a caricature of the person's beliefs.

Weaken: Show that this person really isn't the exaggerated caricature that the argument suggest.

The assault on the Congressman's character is far off base. The Congressman knows that the defense budget is finite and prefers spending scarce dollars on more effective defense systems unlike the already obsolete attack submarines.

Strengthen: Show that the person really is the straw man that he is portrayed to be.

The Congressman has stated numerous times that the Department of Defense should be shut down and that true peace can only be achieved by an open dialogue with dictators and terrorists where their desires are given respectful consideration.

3. The Fallacy of Faulty Analogy
 (very common on the LSAT)

Reasoning by analogy functions by comparing two similar things. Faulty Analogy arguments draw similarities between the things compared that are not relevant to the characteristic being inferred in the conclusion.

The logic behind analogies is this: All X does Y. This does Y. Therefore, this must also be an X. Here's an example of a Faulty Analogy fallacy:

Ted and Jim excel at both football and basketball. Since Ted is also a track star, it is likely that Jim 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. You can't "compare apples and oranges".

Strengthen: The assumption is that two things are parallel, so you can strengthen a faulty analogy argument by strengthening similarities.

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

Football and basketball emphasize speed, so it is like that Jim would excel at track.

Weaken: Emphasize the differences between the things being discussed in the analogy.

Ted plays point guard in basketball and wide receiver in football, two positions that emphasize speed. Jim plays center in basketball and nose tackle in football, two positions that emphasize size. Therefore, it is fallacious to suggest that Jim might excel in track, as Ted does.

Faulty Analogy Sample Question:

1. Long-distance runners sometimes get shin splints from over training.  Shin splints are also common among freestyle skiers.  Freestyle skiers are also guilty of over training.

Which of the following, if true, most weakens the conclusion drawn above?

    1. Sprinters are also prone to getting shin splints.
    2.  Freestyle skiers often exhibit other signs of over training such as dehydration.
    3. Long-distance runners are less prone to long-term stress injuries.
    4. Freestyle skiers get shin splints from landing jumps incorrectly.
    5. Freestyle skiers, on average, train fewer hours than do long distance runners.


The passage tells us two facts, one about long-distance runners (they sometimes get shin splints from over training) and one about freestyle skiers (they also get shin splints).  The conclusion, that freestyle skiers must also over train, depends on the faulty assumption that because runners’ shin splints are caused by over training, skiers’ must be as well.  Choice A is irrelevant.  Choice B strengthens the conclusion.  Choice C is irrelevant. 

Choice D attacks an assumption on which the conclusion depends. If skiers’ shin splints are not caused by over training, then it is not necessarily true that freestyle skiers are guilty of over training.  This weakens the conclusion considerably.  Choice E seems to weaken the conclusion by suggesting that freestyle skiers may be less likely to over train than are runners, but is in fact irrelevant.  Choice D is the best answer.  



4. The "After This, Therefore, Because of This" Fallacy (Post hoc ergo propter hoc)

This is a causal fallacy in which something is associated with something else because of mere proximity of time. This error is very common on the LSAT and it usually accompanies any chronological question (over years, days, etc). One often encounters people assuming that because one thing happened after another, the first must have caused it, as with:

I touched a toad last week; now I have a wart. The toad caused the wart.

The quarterback forgot to shave one morning and had the best game of his career. Since then he has stopped shaving to boost his performance.

You can claim that something is caused by something else that preceded it. The trick with this fallacy is that you need more evidence than just "this occurred after that".

The last thing I remember was a bus coming at me full speed. I am now in a hospital in a full body cast. The bus must have caused my injuries.

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.

You could strengthen the sickness argument by saying: the auditorium was later closed due to a gas leak, which resolves the assumption that something was in the air.

You could weaken the argument by targeting the assumption: Before going to the auditorium, he ate lunch at a restaurant that just reported a high level of food poisoning.

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.

The second example is typical of modern news reporting. The only evidence offered in this argument to support the claim that the decline in the stock market was caused by the election of the president is the fact that election preceded the decline. Another factor could have been a collapse of a bank in Asia that had no relation to the election. The underlying assumption behind After this therefore before this is to assume that there are no other factors at work that could be the real cause.


Over the past three years, the crime rate in the city has steadily declined.  Four years ago, a new mayor took office on a third party ticket whose platform included a tougher stance on crime and improved funding for after-school and other youth programs.  Without this mayor’s leadership, it is certain that this positive change in the crime rate would never have occurred.

Which of the following statements, if true, would most weaken the argument above?

  1. In the first year the mayor was in office, the crime rate rose by 1.5%.
  2. Due to budget cuts, the mayor’s proposed funding for after-school and other youth programs was never implemented.
  3. Three years ago, a new chief of police was appointed who instituted foot patrols in high crime areas.
  4. The crime rate in neighboring cities has been on the rise for the past three years.
  5. The after-school programs had an even higher rate of attendance than was expected.


This question asks you to weaken the argument.  The author writes that without the new mayor’s leadership, the recent decline in the crime rate would not have occurred.  However, the only evidence we’re given is that the mayor’s platform included anti-crime programs preceded the drop in crime.  There are doznes of possible reasons for a decline in crime, therefore this After This Therefore Because of This argument isn't very persuasive. To weaken the argument, find a statement that shows that the decline in the crime rate may have been caused by something other than the mayor’s taking office. 

Choice C suggests that it could have been caused at least in part by the new chief of police, whose increase in officers patrolling by foot could very well have made a positive impact on the crime rate.  Choice A is irrelevant; we’re only concerned with the past three years.  Choice B fails to weaken the conclusion because it’s possible that while the youth programs were never implemented, other anti-crime programs were.  Choice D is irrelevant.  Choice E strengthens the argument.  Choice C weakens the argument, and is the best answer.

5. The "All Things are Equal" Fallacy (very common on the LSAT)

Plus ça change, plus c'est la même chose
(The more things change, the more they remain the same.)
- Jean-Baptiste Alphonse Karr

This fallacy is committed when it is assumed, without justification, that background conditions 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 of time, and things rarely remain the same from place to place.

These questions can be easily spotted because they talk about "last year", or some past time and try to create an analogy to predict future events. Over time, however, all sorts of dynamic factors change, so it is difficult to draw direct inferences from past events.

Ten years ago I got a 750 in the LSAT, so I expect to get the same score again.

The last Democrat 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.

6. 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:

Either you are with me or you are against me.

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

You can strenghten these arguments by showing that their generally isn't a middle ground. The problem is that most arguments do have a middle ground, meaning that this argument doesn't work and is very often a fallacy.

7. Argument ad populum

This is argumentum ad populum, the belief that truth can be determined by, more or less, putting it to a vote.

A group of kindergartners are 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."

Democracy is a very nice thing, but it doesn't determine truth. Polls are good for telling you what people think, not whether those arguments are valid or not.

8. Slippery Slope

This argument assumes that just because things go badly, they will automatically get much, much worse. All graphs use by slippery slope look like hockey sticks.

The anti-terrorist laws that monitor international currency transfers, phone calls and emails are the first step to turning our fragile democracy into a new Nazi regime.

Although infringements of civil liberties are troublesome, it is another argument to suggest infringements of civil liberties will lead to a fascist dictatorship.

10. The Fallacy of Equivocation

    The Fallacy of Equivocation occurs when a word or phrase that has 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 that 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. It may be a big house but there is no intrinsic connection with its being white.

2. Mayor of town T decided to lower sales tax in order to boost sales volume. He believes that lowering the tax will increase the sales tax generated since there will be much more total sales volume. The mayor wants to follow the example of town J, where such an experiment helped increase the budget twice in a three year term.

Which of the following statements is the best proof the opponents of the mayor's proposal can use in order to persuade the population of town T not to support this decision?

a) Town J is located very close to the borders of other three states. The sales taxes in those other states are higher than in Town J's state. This causes residents of other states to shop in town J, to save money. Town T is located far from any state border.

b) Town T relies recieves only a small portion of its tax receipts from sales taxes. Most taxes come from property taxes and this policy would have no impact on property tax returens.

c) Town J has many more industrial plants that purchase raw materials from the town's mines.

d) This kind of experiment did not work in any other of the six towns that lowered sales tax.

e) The mayor is corrupted by several groups of residents of town T. These groups are highly interested in lowering the sales tax because it would make them much richer.


This is a faulty comparison question where two towns are compared.

a. This is a non sequitor trap choice. The issue of a city's tax level has nothing to do with what states you are in. You could just travel from city to city (not state to state) since this is a city tax issue, not a state tax issue.
b. Although the tax benefit may be small, this does not counter the Mayor's argument tha it would increase tax revenues.
c. Out of scope.
d. Correct. Six other examples where this idea didn't work is a reasonable counterpoint.
e. Out of scope.

12. Argument ad populum

     A group of kindergartners are 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.

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