Monthly Archives: April 2014

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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The Chicken or the Egg?

When I was in high school, I volunteered as a guest science teacher at a local middle school. One day a student raised his hand and asked me, “Which came first, the chicken or the egg?” I looked him in the eye and answered, “The egg.” The student who asked the question was trying to be funny, but my answer was completely serious. People think of the chicken-egg question as an unsolvable paradox: chickens all come from eggs, and those eggs are all laid by chickens. But this is not really how it is. There are a couple of different ways to look at this question.

All chickens hatch from eggs, but all eggs are laid by chickens. Image from

First is that the egg obviously came before the chicken. The egg evolved as an encapsulation and protection for a developing embryo long before chickens or other reptiles did (remember, chickens are reptiles). Amphibians, which are older than reptiles, lay eggs. Fish, which are older than amphibians, lay eggs. Even most invertebrates, which predate fish, lay eggs. Eggs were around during the Cambrian explosion half a billion years ago, whereas chickens have only been here for a few thousand years.

But this is not really the question he was asking. He was asking whether the chicken or the chicken egg came first. To answer this question, we have to go back in time a little bit but not as far back as the Cambrian. Chickens are descended from the Red Jungle Fowl, which is still a wild bird in Asia.

A small flock of red jungle fowl. The one on the left is a male, the other two are female. Image from

The process of domestication, which eventually resulted in the modern chicken, was in full swing at least 8,000 years ago.

Domestication is just a type of evolution called “artificial selection”: every generation the individuals with the most favorable characteristics are chosen for breeding. Thus, every generation during the domestication process brought the red jungle fowl closer to what we now call chickens, by however small an increment.

A fairly typical modern chicken. Image from

The point at which chickens became chickens is one that can be difficult to fully understand. Some people believe the following about the evolution of a new species: Species A continues for many generations until suddenly one individual of species A gives birth to the first individual of species B. A chimpanzee might give birth to a human, or a crocodile might give birth to a duck. This is not remotely what happens, or what any scientist thinks happens. During the evolution of modern chickens from their parent species (the red jungle fowl), there was a lot of grey area. During this time, scientists might have argued over whether what they were seeing was a new subspecies of jungle fowl, or it was just a population of the same subspecies of jungle fowl with slightly different characteristics, the same way dogs are all the same subspecies of wolf (Canis lupus familiaris) but different breeds have different characteristics. But just about everyone agrees that the chickens we have today are sufficiently different from their parent population of red jungle fowl to be called their own subspecies — the red jungle fowl are Gallus gallus, and the modern chicken is Gallus gallus domesticus.

The “silky bantam” is not a typical breed of modern chicken, but it was created through the same process of artificial selection. Image from

So when did the chicken egg evolve?

As domestication moved forward, generation by generation, early chickens became more and more chicken-like. And so did their eggs. There may not be a whole lot of differences between the eggs of the modern chicken and the eggs of the ancestral jungle fowl, but they were subject to some modifications — the size, for example. There was not a bright line that separated the chicken from their ancestor, but we can consider each generation of chicken to be more chicken-like than the last, and each generation of egg to be more chicken egg-like than the last. And each generation of more-chicken-like-than-the-last hatched from an egg that was more-chicken-egg-like-than-the-last. Thus, the egg always precedes the chicken, no matter how chicken-like it is at the time.




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The Truth About Zombies

As I sat down today to play The Last Of Us, I thought, as I sometimes do, about the actual possibility of zombies. I have always been a big fan of the zombie genre. In movies, TV and video games, zombies can be broken down into two types. The first are the ones that are raised from the dead through some sort of magic. (See: Dead and Breakfast, Dead Snow, The Evil Dead). The second type are the result of natural causes (that is, not supernatural). Sometimes the cause is a toxin, but it is much more typically an infectious disease.* (See: 28 Days Later, Quarantine, Resident Evil.) As a biologist, this second type has always appealed more to me.

There is a certain degree of biological plausibility to this premise. The primary “goal” of pathogens, like all other life forms, is to reproduce.** To this end, many, if not most, pathogens further their goal by controlling the behavior and physiology of their hosts in ways that increase the chance of infecting other hosts. This is broadly called “host manipulation.”

Take the common cold, for example. As a respiratory virus, the cold is spread through exposure to fluids from the mouth, nose and throat. The symptoms of this virus include runny nose, sneezing and coughing. The sneezing and coughing cause the virus to be spewed into the air and onto nearby people and objects. The runny nose makes you touch your face frequently and get the virus particles (which are called “virions”) on your hands, which are then left on anything or anybody you touch.

Zombies are an example of a fictional parasite manipulating the host. Consider the properties of modern zombies in film, TV and video games: They are compelled to seek out uninfected humans and attempt to bite them. The bite transmits the infection and the bitten person becomes a zombie. Infected people only attack the uninfected. These behaviors all help to spread the infection. 28 Days Later (one of my favorites) went one further: the “rage virus” can be transmitted through any exposure to blood, not just a bite. The zombies in this film vomit blood everywhere so that an actual bite is not always necessary. (I couldn’t find a youtube clip of this. If you happen to find one, please post it in the comments.)

Here are some other real parasites that manipulate their host’s behavior:

Ophiocordyceps unilateralis: This fungus belongs to a larger group of fungal parasites that aggressively infect insects and arachnids, eventually killing the host. This particular species, which infects ants, controls the behavior of its hosts before it kills them. Ants infected with O. unilateralis will climb high into trees and bite into a leaf to anchor themselves against the wind. The ant will die, still clinging to the leaf, and the fungal spores will be released into the wind, where they can spread farther than they would have from an ant that died on the forest floor. In the video game The Last of Us, the zombies are caused by a fungal infection called “cordyceps,” which is presumably a relative of O. unilateralis.

Ants in the late stage of O. unilateralis infection. Image from

Rabies: The rabies virus can infect many species of mammal. The virus is concentrated in the saliva of the infected animal and is transmitted by bite. Infected animals become very aggressive and try to bite other animals including humans. The characteristic “foaming at the mouth” is just an excess production of saliva, which contains the virus. A rabies infection causes the host to avoid water — another name for rabies is “hydrophobia.” If the animal were to drink water as normal, the virions concentrated in the host’s mouth could be washed away and the virus’s ability to infect others would be diminished. There are videos on youtube that purport to show this, but I cannot vouch for their authenticity. **minor spoiler alert** In the movie Quarantine, it turns out that the zombies are the result of a mutated strain of rabies. This was a good call, since rabies already has most of the characteristics of a zombie infection.

Toxoplasma gondii: Toxoplasma is a parasitic protist that has a multi-stage life cycle. One stage is in rodents, such as mice. The next stage is in cats, who contract the parasite by eating infected mice. Cats eat plenty of mice on their own, so they are at risk of contracting toxoplasmosis*** without any help. But it is better for the virus if the cats do get a little bit of help. Mice are normally pretty good about avoiding cats. After all, mice who are better than others at not being eaten will be alive longer and therefore able to produce more offspring — natural selection works. But mice which are infected with Toxoplasma do not fear cats. On the contrary, they are attracted to the smell of cats. A hungry cat is happy to eat an easy meal, but will unknowingly contract toxoplasmosis in the process. The parasite is subsequently spread by cat feces, so it does not need to manipulate the cat’s behavior.

Mice infected with T. gondii do not fear cats. This mouse appears to be healthy. Image from

Mice infected with T. gondii do not fear cats. This mouse appears to be healthy. Image from

Dicrocoelium dendriticum: D. dendriticum is a parasitic flatworm that has a multi-stage life cycle a little bit like T. gondii. One stage is in an ant, and the next stage is in grazing herbivores like sheep or cattle. Being herbivores, sheep and cattle do not go out of their way to eat insects, but neither will they pick bugs out of their food if there are any in there. An ant infected with D. dendriticum will climb to the top of a blade of grass and hold on. By virtue of their location, they are more likely to be eaten by a herbivore. Unlike in T. gondii, the ant being eaten by the next host is incidental, not intentional — the cat wants to eat the mouse, but the sheep is indifferent to eating the ant. Once the ant has been eaten, the parasite will continue the next stage of its life cycle.

Crawling to the top of a blade of grass and holding on is not normal behavior for an ant. Image from

These are just a few interesting examples of host manipulation. There are many more.

All of this is not to suggest that the zombie apocalypse is imminent, but it is not particularly out of the question — there is precedent for diseases manipulating behavior in some fairly dramatic ways. The only primary aspect of zombies that is not plausible is for an infection to reanimate the dead, which some people consider to be a defining characteristic of zombies. If a disease is to control your behavior, it doesn’t need to kill you to do it. Death occurs because your body is no longer able to maintain homeostasis and fight entropy. No parasite has the energy or machinery to rebuild a dead body into working order. It is much more efficient to infect a living organism and to not kill it before the parasite has spread. People sometimes disregard 28 days later as not a real zombie movie because the people are just sick, not risen from the dead. I would argue that it is the most real zombie movie for exactly the same reason.

*There are also movies in which the cause of the zombies is not explained. Natural causes and supernatural causes are still the only possible options.

**Pathogens obviously lack the cognitive properties to have goals in the sense that we humans have goals. When I say that they have a goal, I mean that their physiology and behavior were designed by natural selection to accomplish a particular task.

*** Toxoplasma is the name of the disease-causing organism. Toxoplasmosis is the name of the condition caused by a Toxoplasma infection.

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Programming note

I’m moving to Chicago tomorrow. Unfortunately, I have been too busy packing to write a blog post this week. Stay tuned for next week, and thanks for reading!