Walking into the LSAT without learning formal reasoning is like walking into a used car lot without knowing anything about cars. You need to know the common flaws or you won’t be able to spot them.

Note that some of these logical error types won’t appear on the LSAT, but learning how to think logically and pull out assumptions will help you get a higher score and help you on the AWA analysis of argument section.

You don’t have to perfectly memorize every logical fallacy with its formal Latin name, but you do need to get a general idea about what fallacies are and change the way you think so that it’s easier to spot them.

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.

Strengthen: 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.

Weaken: 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 likely 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:

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?

  • Sprinters are also prone to getting shin splints.
  • Freestyle skiers often exhibit other signs of over training such as dehydration.
  • Long-distance runners are less prone to long-term stress injuries.
  • Freestyle skiers get shin splints from landing jumps incorrectly.
  • 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 runners, but it 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 the 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 assumes that there are no other factors at work that could be the real cause.

Example

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 that preceded the drop in crime. There are dozens of possible reasons for a decline in crime, therefore 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 strengthen 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.

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?

  1. 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.
  2. Town T relies receives 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 returns.
  3. Town J has many more industrial plants that purchase raw materials from the town’s mines.
  4. This kind of experiment did not work in any other of the six towns that lowered sales tax.
  5. 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.

  1. This is a non sequitur 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.
  2. Although the tax benefit may be small, this does not counter the Mayor’s argument that it would increase tax revenues.
  3. Out of scope.
  4. Correct. Six other examples where this idea didn’t work is a reasonable counterpoint.
  5. Out of scope.

 Statistical Reasoning

There are lies, damn lies – and statistics.
-Mark Twain

To help you prepare for the many statistical reasoning questions that you will likely encounter on test day, we provide a primer on statistical reasoning similar to what you would get in a college-level introductory statistics class.

1. The Biased Sample Fallacy

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 drawn and used to make a generalization is drawn from a group that does not represent the whole. Here is an argument that commits the fallacy of the biased sample:

A recent study showed that over 60% of Oregon residents watched cartoons. Based on this study, executives at Cartoon Channel spent $10 million to expand their access to Oregonians, who appear to be avid fans of cartoons.

Note that this survey doesn’t say anything about the specific Oregon residents polled. Are they school children? The results would seem to indicate this. A sample must be representative of the overall population that wants to be studied in order to make a general conclusion.

Here is another example:

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

The data for the inference in this argument is drawn from a sample that is not representative of the entire electorate. The survey was conducted of just people who invest, and not random members of the electorate. People who read about investing are more likely to have an opinion on the topic of taxes on investments different than the population at large.

2. The Insufficient Sample Fallacy (Hasty Generalization/Sweeping Generalization)

The Fallacy of the Insufficient Sample is committed whenever an inadequate sample is used to justify the conclusion drawn. In a Biased Sample, people are pulled from a non-representative group, in an Insufficient Sample, not enough people are polled to make a statistically significant result.

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.

Observations of three people are not sufficient to support a conclusion about 10 million. Bad luck could account for meeting three bad people. Try this one:

After living and working in New York City for 12 years, I have met thousands of people and with very rare exception, I have found them to be obnoxious, pushy and rude. It is obvious that people from New York City have a bad attitude.

This latter argument is something to take more seriously given the larger pool from which the observation is drawn.

3. Correlation does not prove causality

A correlation is a statistical linking between two items that seems to be parallel. One of the LSAT’s “Greatest Hits” you see time and time again is the attempt to link up two separate items that seem to statistically correlate and then establish one of the two as the “cause”.

The relation between an association and a cause is difficult.

  1. Heavier people tend to be taller.
  2. Weight is correlated with height.
  3. Gaining weight will make you taller.

This assumes a relationship between correlated data where if you change one element you can change the other one.

Another obvious one:

  1. More fire trucks tend to be at more serious fires
  2. We can reduce the severity of fires by reducing the fire trucks.

Here is a more challenging example:

  1. Young people who watch more TV violence are more likely to engage in violence.
  2. The recent increase in TV violence is associated with an increase in violence society-wide.
  3. If children would watch less TV, they would be less violent.

This one seems intuitive enough and it’s the “sentimental favorite”, but the reality is that (3) can’t be proven from 1 and/or 2. You can’t assume that just because things correlate you can change one factor and it will automatically change the other. Children who watch large amounts of TV may have inattentive parents, and this may be the underlying hidden causal factor (not watching too much TV violence in itself). This argument could use more evidence, like a study showing that violent children are more successfully rehabilitated by cutting off violent shows.

Example

Studies have shown that men aged 18-27 who have owned a pet for at least 2 years before marrying are 35% less likely to divorce. Researchers conclude that caring for a pet prepares men for long-term, healthy relationships in marriage.

Which of the following, if true, most strengthens the conclusion that men who have owned pets are prepared for healthy marriages?

  1. Studies have shown that pet ownership drastically reduces daily stress levels.
  2. Many successful marriages are based on emotional investment in a common interest, such as a pet.
  3. Many men who have been married for 25 years or more continue to own pets.
  4. Men who have not owned pets for at least two years before marrying are more likely to divorce.
  5. Men whose wives who owned a pet for at least two years are equally as unlikely to divorce.

Argument Evaluation

Situation: Researchers have concluded that men who have owned a pet for at least 2 years are prepared for healthy marriages.

Reasoning: Which option most strengthens the conclusion? Researchers base their conclusion on an assumed connection between sustained care for a pet and care for a spouse. Men who care for pets before marriage, the argument runs, are also statistically more likely to sustain marriage relationships. The problem is that correlation doesn’t prove causality, so that link alone is not enough.

  1. While this may be true, it does not introduce additional evidence to support the conclusion.
  2. This option does not address the question of why men who own pets are less likely to divorce.
  3. The question concerns men who have owned pets before marrying, not after.
  4. Correct. This option provides additional evidence of a causal correlation between pet ownership and the likelihood of divorce.
  5. The question concerns men, not their wives.

The correct answer is D.

4. Confounding Factors (aka “Lurking Variables”)

A confounding factor is an additional factor that may be responsible for a correlation. “Con” is a latin root for “with”, so confounding means literally to found with.

Example 1: The Miracle Hospital

A sports injury treatment center in New York has the lowest rate of recovery for sports injuries. A treatment center in rural Pennsylvania has the highest and quickest recovery rate. If you have just been severely injured while playing softball, should you go to Pennsylvania?

In this example it appears pretty obvious that this hospital in New York is bad for your health. So you follow the statistics and go to Pennsylvania, right? The treatment center in New York is an option of last resort for serious sports injury patients like you. The Pennsylvania hospital is so poor that no one with a serious injury ever goes there. The hospital’s patients consist of those with minor injuries who recover quickly. The hidden confounding factor in this argument is that people with more severe injuries are choosing to go to New York, meaning that looking at their injuries is a biased sample.

Example 2: The Secret Conspiracy Against Men

At the University of California at Berkeley, the school had a much lower acceptance rate for men than for women and administrators could not determine why since the male applicants had higher SAT scores and grades.

Are the lower admissions rates of men a result of systematic bias?

Looking at the information, it appears that someone in the admissions department doesn’t like men and has been secretly rejecting their applications.

When looking more carefully at the data, men were much more likely to apply to the highly-competitive engineering program. The result was that men had lower rates of admission overall at the Berkeley. In non-engineering programs, however, the acceptance rates were identical. So gender played no direct role in admissions rates, the factor was the major chosen by the applicants.

Summary Case Study

Let’s review some common arguments supporting or refuting the following statement:

Man-made carbon dioxide output increases will cause catastrophic global warming in the next century.

There is a correlation between carbon dioxide swings and temperature based on ice core data going back thousands of years.

Correlation does not prove causality because of confounding factors. The correlation between carbon dioxide and global temperature does not mean that higher carbon dioxide levels cause higher temperature. The hidden confounding factor may be that higher temperatures cause the oceans to release large amounts of carbon dioxide. It is possible that carbon dioxide doesn’t increase temperature, but that higher temperatures increase carbon dioxide, causing the apparent correlation.

Temperatures in Nome, Alaska have increased more than 5 degrees in the last 20 years.

Insufficient sample. It is unlikely that a single weather event could be evidence for global warming since chance could play a role in that individual event.

There is universal consensus in the academic community that global warming is occurring and man-made pollution is to blame.

Appeal to authority (academics)

Argument ad populum (validity isn’t a popularity contest).

The “skeptics” of global warming often have their research paid for by large oil companies

Ad hominem- it is irrelevant who pays, what matters is the accuracy of the research.

Venus is the hottest planet in the solar system because its atmosphere is 90% carbon dioxide, which traps heat.

False analogy – you can’t compare the Earth, which has a 0.5% concentration carbon dioxide concentration, to Venus, with a 90% concentration.

Even though temperatures have only increased by about one degree in one hundred years, the future will be catastrophic.

Slippery slope.

How could the carbon-dioxide based global warming argument be strengthened?

Show very direct and clear historical evidence that an increase in carbon levels precedes global temperature increases without any other factors.

This would strengthen the argument that carbon dioxide increases cause global temperature increases and not the other way around.

Eliminate the role of any other factors such as solar activity, heat islands, other greenhouse gases, weather patterns or other factors as playing any major part in the recent temperature increase.

This would weaken confounding factors and make carbon dioxide the only possible culprit.

Notice the use of confounding factors. Why the global warming exercise? As discussed above, one of the best ways to attack a correlation-based causal argument like carbon dioxide causes global warming is to suggest confounding factors. You can strengthen the argument by ruling out confounding factors. Since Strengthen/Weaken questions comprise over a quarter of critical reasoning questions, using confounding factors is valuable skill.

LSAT Classes

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Prep Guide

Table of Contents
Chapter 1
Chapter 2
Chapter 3
Critical Reasoning
Argument Structure
Reasoning Skills
Common Questions
Advanced Questions
Sample Questions
Chapter 4
Chapter 5

Read a free excerpt of the PowerScore LSAT Logical Reasoning Bible™, the most comprehensive book available for the Logic Reasoning section of the LSAT. This book will provide you with an advanced system for attacking any Logical Reasoning question that you may encounter on the LSAT.