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Evaluating Online Information: Logical Fallacies

Logical Fallacies in Social Media

Although social media have many wonderful uses, often, when people share information online, they do not always employ much rigor in the way they share that information. Broadcast media, like television and radio news, usually apply a rigorous examination of information before presenting it to the public. They employ reporters who have some background in reporting on a particular subject, fact-checkers to make sure that the news that's being reported is accurate, and editors to make sure that the news will make sense to the reader.

Because posts on social media are typically very brief, especially on sites like Twitter where brevity is enforced, there is often little room for detailed argumentation, and therefore social media users sometimes resort to types of arguments that are simplistic, lack supporting evidence, and are based on faulty reasoning. When reading social media posts, you may intuitively know that an argument is wrong, but you may not be sure why. 

Like lateral reading and identifying fake news, identifying logical fallacies is another method we can use to determine whether online information is valid. For many decades, we have relied on broadcast news organizations to filter, edit, and fact-check the information they share with us. In terms of information shared via social media, we are the ones who must do the very difficult and challenging work that used to be done by editors and fact-checkers. Learning how to identify fallacies of logic can help you know, and explain, why someone’s argument does not prove their point.

Since the time of Ancient Greece, philosophers, logicians, and regular people have developed ways to identify types of illogical arguments. These logical fallacies are errors in reasoning. In a logical fallacy, the arguer does not provide enough evidence to support their claim. It is important to note that just because someone uses a logical fallacy, their claim may not necessarily be wrong, it simply means that the arguer has not provided either enough, or the right kind, of evidence, and therefore has not proven their point.

There are literally hundreds of logical fallacies. In this guide we'll look closely at five. If you are interested in learning more of them, Stanford, Purdue, the University of North Carolina, and the University of Texas at El Paso have put together some great logical fallacy lists.  Below are definitions of our five logical fallacies from these institutions, and we have also included links to some YouTube videos that explain each of these fallacies in more detail.

Ad Hominem

The ad hominem fallacy involves bringing negative aspects of an arguer, or their situation, to bear on the view they are advancing.

Example: Thompson’s proposal for the wetlands may safely be rejected because last year she was arrested for hunting without a license.

The hunter, Thompson, although she broke the law, may nevertheless have a very good plan for the wetlands. (Stanford)

False Dichotomy

Definition: In false dichotomy, the arguer sets up the situation so it looks like there are only two choices. The arguer then eliminates one of the choices, so it seems that we are left with only one option: the one the arguer wanted us to pick in the first place. But often there are really many different options, not just two—and if we thought about them all, we might not be so quick to pick the one the arguer recommends.

Example: “Caldwell Hall is in bad shape. Either we tear it down and put up a new building, or we continue to risk students’ safety. Obviously we shouldn’t risk anyone’s safety, so we must tear the building down.” The argument neglects to mention the possibility that we might repair the building or find some way to protect students from the risks in question—for example, if only a few rooms are in bad shape, perhaps we shouldn’t hold classes in those rooms.

Tip: Examine your own arguments: if you’re saying that we have to choose between just two options, is that really so? Or are there other alternatives you haven’t mentioned? If there are other alternatives, don’t just ignore them—explain why they, too, should be ruled out. Although there’s no formal name for it, assuming that there are only three options, four options, etc. when really there are more is similar to false dichotomy and should also be avoided. (UNC)

(The name used to describe the fallacy in this video is the "Black-and-White" fallacy, but this is just a different name for the False Dichotomy fallacy.)

Slippery Slope

Definition: The arguer claims that a sort of chain reaction, usually ending in some dire consequence, will take place, but there’s really not enough evidence for that assumption. The arguer asserts that if we take even one step onto the “slippery slope,” we will end up sliding all the way to the bottom; he or she assumes we can’t stop partway down the hill.

Example: “Animal experimentation reduces our respect for life. If we don’t respect life, we are likely to be more and more tolerant of violent acts like war and murder. Soon our society will become a battlefield in which everyone constantly fears for their lives. It will be the end of civilization. To prevent this terrible consequence, we should make animal experimentation illegal right now.” Since animal experimentation has been legal for some time and civilization has not yet ended, it seems particularly clear that this chain of events won’t necessarily take place. Even if we believe that experimenting on animals reduces respect for life, and loss of respect for life makes us more tolerant of violence, that may be the spot on the hillside at which things stop—we may not slide all the way down to the end of civilization. And so we have not yet been given sufficient reason to accept the arguer’s conclusion that we must make animal experimentation illegal right now.

Like post hoc, slippery slope can be a tricky fallacy to identify, since sometimes a chain of events really can be predicted to follow from a certain action. Here’s an example that doesn’t seem fallacious: “If I fail English 101, I won’t be able to graduate. If I don’t graduate, I probably won’t be able to get a good job, and I may very well end up doing temp work or flipping burgers for the next year.”

Tip: Check your argument for chains of consequences, where you say “if A, then B, and if B, then C,” and so forth. Make sure these chains are reasonable. (UNC)

Post Hoc, Ergo Propter Hoc

This is a conclusion that assumes that if 'A' occurred after 'B' then 'B' must have caused 'A.'

Example: I drank bottled water and now I am sick, so the water must have made me sick.

In this example, the author assumes that if one event chronologically follows another the first event must have caused the second. But the illness could have been caused by the burrito the night before, a flu bug that had been working on the body for days, or a chemical spill across campus. There is no reason, without more evidence, to assume the water caused the person to be sick. (Purdue)

Straw Man

Definition: One way of making our own arguments stronger is to anticipate and respond in advance to the arguments that an opponent might make. In the straw man fallacy, the arguer sets up a weak version of the opponent’s position and tries to score points by knocking it down. But just as being able to knock down a straw man (like a scarecrow) isn’t very impressive, defeating a watered-down version of your opponent’s argument isn’t very impressive either.

Example: “Feminists want to ban all pornography and punish everyone who looks at it! But such harsh measures are surely inappropriate, so the feminists are wrong: porn and its fans should be left in peace.” The feminist argument is made weak by being overstated. In fact, most feminists do not propose an outright “ban” on porn or any punishment for those who merely view it or approve of it; often, they propose some restrictions on particular things like child porn, or propose to allow people who are hurt by porn to sue publishers and producers—not viewers—for damages. So the arguer hasn’t really scored any points; he or she has just committed a fallacy.

Tip: Be charitable to your opponents. State their arguments as strongly, accurately, and sympathetically as possible. If you can knock down even the best version of an opponent’s argument, then you’ve really accomplished something. (UNC)