Logical fallacies: Seven ways to spot a bad argument

When people are trying to persuade you, they sometimes reach for underhand tricks like the 'appeal to ignorance' or 'whataboutism' to seem more convincing. Amanda Ruggeri explains how to identify these logical fallacies.

Scroll social media, tune into the news, or simply raise a hot-button issue with an acquaintance, and within a matter of minutes, you're likely to encounter a trap. These traps are so time-worn that they date back to ancient Greece. 

They're called logical fallacies. Simply put, a logical fallacy is a flaw in reasoning that, despite having no bearing at all on a claim's actual merit, can – very confusingly – make that claim sound more convincing.

Using a logical fallacy doesn't necessarily mean someone is wrong. It can, however, indicate either faulty thinking and flawed logic, if used unintentionally, or an attempt to manipulate the truth to be more persuasive, if used deliberately. Either way, it's a red flag that should prompt further questioning and discussion. That includes, crucially, in your own thinking – and in arguments that you're inclined to agree with.

Once you know about logical fallacies, you'll see them everywhere. Why does this matter? Because the more practised you become at spotting them, the better you can be at identifying flaws in people's thinking, and refocusing dialogue back to an argument's merit. You'll also get better at thinking critically yourself.

Here are seven fallacies to look out for. Some are errors of logic (known as "formal" fallacies), while others are about the misuse of language and evidence ("informal" fallacies) – but the consequence is always a faulty argument.

This is when a lack of evidence is interpreted to mean a claim is real – rather than placing the burden of proof on the person making the claim. It's a fallacy that commonly underlines arguments for conspiracy theories. Ask one of the estimated 10 million-plus people who believe that lizards run the world about the evidence for their claim, for example, and they might counter, "Well, these lizards are too clever to leave any evidence – that's what makes this situation so dangerous! How can you be sure it's not true?" You might wind up scratching your head, but, hopefully, it's not because you've been persuaded; it's because they've set you the trap of the "appeal to ignorance" fallacy.

This is a fallacy in which a claim is rejected on the basis of an aspect of someone's character, identity, motivations, or even the relationships they have with others. Think of the health professional who is told that they are only recommending vaccines because they must be a shill for Big Pharma, or the research of climate scientists being dismissed on the basis that they must be ideologically motivated. The most obvious (and absurd) kind of ad hominem, though, is the sort that not only attacks a person instead of dealing with their argument, but goes after something completely irrelevant to the topic at hand – like a political candidate in a TV debate saying their opponent's clothing choices, golf prowess or hairstyle mean they can't possibly be a good leader.

This is the argument that taking one step, or putting into place one measure, will inevitably lead to more and more drastic measures – like an object sliding down a slippery slope. It's particularly common in debates over policy. Think of the argument that some opponents of same-sex marriage made against legalising it in places like the US or Europe. In 2016, researchers at the University of California, Los Angeles found that many people who were against the policy were persuaded by the argument that it would lead to greater sexual promiscuity across society, and threaten their own way of life. This particular argument is fallacious because, rather than debating the policy change itself (whether same-sex marriage should be legalised), the policy was dismissed because of the fear of its predicted outcome (the breakdown of traditional society).

I see this one on social media, especially, all the time. It's misrepresenting the argument of the other side to make it seem more ridiculous, and therefore easy to defeat. Think of someone who puts forth a nuanced argument that excessive sugar intake may raise the risk of health issues like heart disease. A strawman response would be, "Oh, so what, sugar is killing everyone and should be outlawed? That's absurd!" This distorts the original argument, making it easier to defeat – a strawman. One intriguing way to not only fight this tendency, but sharpen your own thinking, is to try a "steelman": you present your opponent's argument in the best possible way (perhaps even better than they have themselves) before you state why you disagree.

This pernicious argument holds that someone's credentials, fame or reputation alone prove that they must be right. If people perceive someone as an authority, they have an innate cognitive bias to assume they have expertise in all things (even subjects they have no background in). Like many logical fallacies, it feels like it could, or should, be relevant: if someone has credentials and expertise in a certain area, shouldn't their opinion on that area be more trustworthy? To be clear, it should. What makes this a fallacy is when someone buys into an argument solely because of who the person is, rather than because of the argument's evidence or reasoning.

More problematic still is the version known as "appeal to irrelevant authority". Our tendency to believe something because, say, a celebrity states it, even if they have no expertise at all in the topic at hand – a classic tendency in today's influencer-obsessed world. But "irrelevant authorities" aren't always so obvious. Take arguments about climate change, for example, when sceptics quote someone like a theoretical physicist as an expert – despite the fact that theoretical physics generally has very little to do with climate science.

Presenting a complex scenario as if there are only two either-or, often opposing options, rather than multiple options. Think of that famous, often-recycled and even ancient phrase, famously used by President George W Bush shortly after 9/11: "You're either with us or against us." It implied to the international community that they had only two options – back the United States completely, including in its invasion of Afghanistan, or consider themselves enemies. In reality, of course, there were a spectrum of other options nations could take, and kinds of allies (or enemies) they could be.

Sometimes considered a type of red herring – a logical fallacy that uses unrelated information to redirect away from the argument's flaws – whataboutism is intended to distract attention. It describes when, normally in response to an accusation or a question, someone responds with their own accusation. In an argument with a partner, for example, you might say, "It hurt my feelings when you did X." A whataboutism response might be "Well, you never take out the trash!"

In politics, one of the most infamous examples has been when Russia is accused of human rights violations, and its leaders respond "Well, what about the West?". While a whataboutism can serve to illustrate hypocrisy, it deflects from the original argument. Two wrongs don't make a right, but a whataboutism can make it seem like they do.

Understanding and spotting logical fallacies like this these can be a really useful way to think critically about what you read or watch, and steer (and keep) conversations back on track.

However, because we started this with talk of nuance, let's underscore: if someone uses a logical fallacy, it doesn't necessarily mean their conclusion is incorrect. That, in fact, would be its own fallacy, and perhaps my favourite one of all: the "fallacy fallacy".

Amanda Ruggeri is an award-winning science and features journalist. She posts about expertise, media literacy and more on Instagram at @mandyruggeri.

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