Tag Archives: fallacy

The Man Behind the Curtain Turns 1

The Man Behind the Curtain turns 1 today! In the past year, this blog has had over 4800 views from people in 104 nations. This surpassed what expectations I had, and I am looking forward to another good year.

The most popular posts this year were:

Dinosaurs are not Extinct

Hot or Not

Do genes skip generations?

Testing a Claim: Ceramic Knives

The least popular posts were:

Drug-Resistant Diseases

Skipping Generations Part 2

You’re Doing it Wrong, Part 2: Post Hoc Ergo Propter Hoc

A UFO (which was my first post)

And these are my personal favorite posts:

For All Mankind

Dinosaurs are not Extinct

10% of our Brains

The Evolution of Flight

Thanks for reading! I hope 2015 will be even better. (Tomorrow I will go through and fix all of the broken images. Sorry about that.)

Have a topic you want me to cover? Ask in the comments section or on Twitter @CGEppig

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You’re doing it wrong, part 2: Post hoc ergo propter hoc

(This post is the second installment in an ongoing series on logical fallacies. Check out my first one here.)


Let me tell you about a problem I’m having with my eye right now. For the past few days, I’ve had what appears to be some sort of allergic reaction — swelling and itching around both eyes but primarily my left one. I don’t know for a fact that it is an allergic reaction, but the symptoms seem to fit and it’s not bad enough to get it checked out. Whether or not it is one, I’m treating it like an allergic reaction by taking benadryl. After popping pills for four days, the swelling and itching have gone down almost entirely. So why am I writing about this on my science blog? Because it’s an excuse to talk about a common problem in assigning causation.

Post hoc, ergo propter hoc, or “after this, therefore because of this,” is a logical fallacy that deals with misattribution of causation. Here’s how this works:

1) event A occurs

2) event B occurs

3) event B is believed to be caused by event A

In my particular case, event A was taking antihistamine pills and event B was the swelling and itching around my eyes going down. These two events might be related, or they might not be. The swelling and itching may have gone down on their own even without taking the pills. How long do these things usually last if you don’t treat them?

This does not mean that events are never caused by other events that preceded them. On the contrary, events are ALWAYS caused by other, previous events (the quantum world notwithstanding). This is only a fallacy if you do not have good reason to believe that they are connected. If all you know is that two events happened and one happened first, then you cannot assume they are causally related.

If I knew for sure that my eye problem was an allergic reaction, it would be reasonable to assume that taking the benadryl — which is an experimentally-validated treatment for allergic reactions — was the cause of my eye problems subsiding. As it stands, though, all I can say is that my symptoms subsided over four days and also I was taking some pills. The two might be related but I cannot say with confidence whether or not they were. And what if I had taken something else? What if I had followed some bizarre old wives’ tale about burying a potato under a full moon?* Or used some bogus homeopathic remedy? If my symptoms eventually disappeared, I might conclude that, for example, sleeping with a pinecone under my pillow** had cured my ailment.

Avoiding this fallacy is exactly why scientists use a control group in their experiments. Let’s say you discover a new soil treatment that you believe will make plants grow faster. You plant 100 seedlings and give all of them your soil treatment. After 30 days the average height of the plants is 48cm. What have you learned from this? Absolutely nothing. Because you have nothing to compare your plants to, you can’t say anything about whether the height they reached had anything to do with your treatment. Maybe plants would have grown to 48cm in 30 days anyway.

As a more relatable example, lots of people will take a homeopathic remedy for a minor ailment. When their symptoms subside they will conclude that it was due to the pills they took. “It worked for me,” they will say. But to really know the effectiveness of the treatment, you need to do a study. Take a bunch of people people with the same problem, give half of them the homeopathic remedy, and half of them a placebo. If the people in both groups have the same level of symptoms after the same amount of time, then the treatment does not work. And this is what they have done for homeopathy, and they found that it doesn’t work.

The human mind is a remarkable thing. One of its abilities is to recognize patterns in the world. It is so good at finding patterns that it can find them even when they don’t exist. This is why we are susceptible to the post hoc, ergo propter hoc fallacy and why we develop superstitions if we’re not careful. If a bad thing happens to you for a random reason — let’s say your car got a flat tire, or you got struck by lightening — you may attribute it to the most recent thing that you did that was unusual, or to an event that you had already been primed to believe was the cause of bad luck. The same is true for positive events. As a result, people don’t like to break mirrors, walk under ladders or open umbrellas inside, they carry charms and trinkets for luck, throw salt over their shoulders, and cross their fingers.

* This is part of a fabled cure for warts.

** This is not a cure for anything I’m aware of.



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You’re Doing it Wrong, part 1: The Naturalistic Fallacy

This is the first installment of what I hope will be a series of posts (probably not consecutive) on logical fallacies. Logical fallacies are a category of arguments that are never valid — they are incorrect ways of thinking. Today’s logical fallacy is the naturalistic fallacy (also called the “appeal to nature” or “Hume’s Guillotine”), which incorrectly equates what is true in the natural world with what is morally correct. 

Consider the following statements:

Eating meat is okay because humans evolved to eat meat.

Warfare is wrong because other animals don’t do it. (Some animals actually do engage in warfare, but people make this argument nonetheless.)

Homosexuality is okay because homosexuality is present in other animals.

Each of these arguments attempts to justify or condemn a moral claim based on what is or is not observed in nature. The first part of each statement (before the “because”) is an ideological claim and the second part of each statement (after the “because) is an empirical claim. Each part of each argument is valid as it is, and the opposite moral claims would also be equally valid. They become fallacious, however, when the two sides are associated. Science can only tell us what exists or what is true about the natural world — it absolutely cannot give us moral guidance.

No matter what your ideology is, you can find facts about nature that are consistent with your morals and facts about nature that are inconsistent with your morals. Some people believe that eating meat is wrong and almost everyone agrees that rape, murder and slavery are wrong. But these moral stances cannot be gained from nature, and neither can the opposite stances (that eating meat, slavery, rape and murder are morally sanctioned). Many animals eat other animals (e.g. dolphins, sharks, lions, wolves) and many don’t (e.g. cows, giraffes, horses, snails). Many animals rape (e.g. chimpanzees, chickens, orangutans, sea turtles). Some ants take slaves. Chimpanzees, langur monkeys and many other animals kill members of their own species. Nature is, as Alfred, Lord Tennyson said, “red in tooth and claw.”

There are a variety of ways the naturalistic fallacy can manifest:

1) People refuse to accept empirical findings because they are ideologically opposed to the outcome.

One time, I was explaining to someone the phenomenon that animals may kill some of their offspring when they cannot feed them all. She refused to believe it on the grounds that animals are more moral than humans.

2) People assign moral beliefs to a researcher based on the researcher’s findings.

For example, people sometimes claim that scientists studying sex- or race-differences are sexist or racist. For ideological reasons, some people like to believe that there are no differences between the sexes or among the races. When research threatens this, some people assume that the researchers making the discoveries are morally endorsing the findings. Interestingly, people don’t make this type of claim about medical researchers. When someone discovers a new form of childhood cancer, he or she is not accused of endorsing childhood cancer. Rather, it is seen as a previously unknown problem that we can choose to fix if our ideology drives us to do so. All scientific discoveries are morally neutral, and cannot threaten anyone’s ideology. Science tells us what is, not what we should do about anything, and the same goes for the scientists making discoveries. Things that are objectively true are true no matter who discovers it or what their own ideology is.

3) People claim that something is or is not moral because it does or does not exist in nature.

Many people who oppose homosexuality or gay marriage argue for their side by claiming that other animals are not homosexual. Many people who accept homosexuality and gay marriage argue for their side by pointing out that other animals ARE homosexual. Only the latter claim about non-human homosexuality is true, but both statements make the naturalistic fallacy. Science can tell us that homosexuality exists and why it exists and in what species, but it cannot tell us how we should feel about it.

There are two ways that science can interact with morality. First, science can study morality as a natural phenomenon, as some of my colleagues do. What parts of the brain are involved in morality? How is morality transmitted as an aspect of culture? Why is morality different in different areas? When these topics are studied by scientists, scientists are still not endorsing any given morality. They are not making a claim about which moral system is superior to another. They are just reporting on it as they would any other aspect of nature. (Note: humans and our morality are a part of nature.)

The second area is to determine how best to achieve one’s moral goals. For example, many people are morally opposed to war and are interested in reducing or stopping it. Science cannot justify this belief, but it can help you to understand the problem you are trying to solve and help you develop action plans for solving it. Some colleagues of mine have found that exposure to pathogens increase warfare. This finding in itself does not, of course, advocate a particular action. But if you are interested in reducing war, you could use this information to help. Conversely, if you are interested in increasing war, you could also use this information.

None of this is to say that you cannot have opinions or argue about morality — just that science cannot justify any particular moral or ideological stance.


See also:

You’re doing it wrong, part 2: Post Hoc Ergo Propter Hoc


Have a topic that you want me to cover? Let me know in the comments section.

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