The post hoc fallacy (from the Latin post hoc ergo propter hoc, meaning “after this, therefore because of this”) is a logical fallacy that occurs when someone assumes that because one event occurred after another, then the second event must have been caused by the first.
For example, the post hoc fallacy occurs when someone assumes that orange juice can cure the flu, because they drank orange juice while they had the flu and then felt better a few days later.
The post hoc fallacy stands at the core of many fallacious arguments, so it’s important to understand it. As such, in the following article you will learn more about this fallacy, and see how you can respond to it effectively, as well as how you can avoid using it yourself.
Examples of the post hoc fallacy
An example of the post hoc ergo propter hoc fallacy is the claim that if the rooster crowed immediately before the sun rose, then that means that the rooster caused the sun to rise.
Additional examples of the post hoc fallacy appear in various fields, such as science, medicine, psychology, economics, business, law, and politics. For instance:
- In finance and investing, people sometimes assume that because a certain event occurred before a stock’s price rose or fell, then that event is necessarily the cause of the change in price.
- In marketing and advertising, companies sometimes use the post hoc fallacy to imply that their product is beneficial in some way. For example, a company might say “in the year after we released our new test-prep material to the market, the average score on the test increased by 5%”, even though this doesn’t mean that their test-prep material was responsible for the increase.
- In medicine, a notable example of the post hoc fallacy appears in some of the pseudoscientific arguments that suggest that there’s a link between vaccination and autism. As one study notes “[Dr. Andrew Wakefield] claimed that autism in the children he studied was associated with the MMR vaccination because the behavioural symptoms of the disorder first appeared after the children had been vaccinated. However, this observation overlooked the fact that autistic symptoms appear around 12 months of age which is when children are normally scheduled to receive the first dose of MMR vaccine.”
Furthermore, the post hoc fallacy can lead people to engage in superstitious thinking (also referred to as magical thinking), where they assume that coincidences and unrelated events are connected in a causal way, particularly as a result of supernatural influence. For example, a sports fan sitting at home might notice that their favorite team did well whenever the fan drank lemonade, which can cause the fan to believe that them drinking lemonade at home helps their team do well in the field.
Finally, the post hoc fallacy is sometimes combined with other fallacies. For instance, consider the following basic example of the post hoc fallacy (by itself) in the context of crime statistics:
“Crime rates increased after people started playing video games, so video games are responsible for the increased crime rates.”
Here, the post hoc fallacy can be further combined with a fallacy called denying the antecedent, which occurs when someone mistakenly assumes that if “if A, then B” is true, then “if not A, then not B” must also be true. For example:
“Crime rates increased after people started playing video games, so if people will stop playing video games, then crime rates won’t increase.”
Similarly, the post hoc fallacy can be combined with a fallacy called affirming the consequent, which occurs when someone mistakenly assumes that if “if A, then B” is true, then “if not B, then not A” must also be true. For example:
“Crime rates increased after people started playing video games, so if crime rates aren’t increasing, then that means that people aren’t playing video games.”
Explanation of the post hoc fallacy
The basic form of the post hoc ergo propter hoc fallacy can be described as follows:
A happened before B, therefore A caused B.
This is based on the false premise that if one event happens before another, then the first event must be the cause of the second. This premise is often implicit, meaning that people don’t state it outright, and are sometimes even unaware that they’re relying on it in their reasoning.
One reason why people use this kind of reasoning unintentionally is that, in general, event A needs to happen before event B in order to cause event B (though arguably there are some exceptions to this, such as when someone engages in event A in preparation of event B). However, just because this kind of chronological order (or temporal arrangement) is necessary for there to be a causal association between the two events, that doesn’t mean that there is necessarily such an association.
In addition, there are various cognitive phenomena that can make people prone to using this fallacy unintentionally, or to believing arguments that rely on it when they’re proposed by others. These include, for example, jumping to conclusions, in cases where people assume causality on the basis of insufficient information, apophenia, in cases where people see connections between unrelated events, and the confirmation bias, in cases where people search for, favor, interpret, and recall information in a way that confirms their preexisting beliefs.
However, people can also use this fallacy intentionally, even when they’re aware of the fallacious reasoning that it involves. This can happen, for example, when people want to convince the audience in a debate of something. In such cases, the person who’s using the fallacy might combine it with other rhetorical techniques in order to hide its use, including, for example, equivocation, circumlocution, and red herrings.
Note: the post hoc fallacy generally involves either events or types of events. For example, when it comes to people who claim that vaccines cause autism, they may make this argument about a case of a specific vaccine given to a specific individual (i.e., about a specific event), or they can make this argument about vaccines in general (i.e., about a type of event).
How to respond to the post hoc fallacy
To respond to the post hoc ergo propter hoc fallacy, you first need to recognize that it was used. To determine whether this is the case, you should consider whether someone stated or implied that just because one event occurred before another, then the first event caused the second. When doing this, it can also help to consider the following, which will help you assess the situation:
- Did they say that there is necessarily a causal connection between the events in question, or did they say that a connection is possible?
- Did they provide any evidence in support of the causal connection between the events, besides the order in which they took place?
Then, you can ask the other person to explain their reasoning, and potentially ask about the specific considerations that are outlined above. Doing this has several benefits. First, it helps you ensure that you’ve understood their argument properly, and consequently determine whether the argument was indeed fallacious. Second, if the original argument was fallacious but could be made logically sound if they modify it, this can prompt them to add relevant information or re-phrase their argument accordingly. Third, if the argument is indeed fallacious and cannot be made logically sound, asking the person who made it to explain their reasoning could help them see and internalize their error. Finally, asking the other person about their reasoning shows that you’re truly trying to understand them, which can lead to more productive dialogue and make them more willing to change their mind.
You can also directly point out the issue with their reasoning. To do this, you can explicitly outline the flawed premise that underlies their argument (“if A happened before B then A caused B”), and explain why it’s fallacious, potentially by using relevant examples to illustrate the issues with it.
When doing this, it may also be beneficial to explore other potential causal associations. For example, you could consider the possibility that the second event caused the third, that the two events were caused by a joint third event, or that the two events are unrelated.
Finally, when responding to the post hoc fallacy, it’s important to keep in mind that just because an argument is fallacious doesn’t mean that its conclusion is necessarily wrong, and assuming otherwise is fallacious in itself. This means that even if someone’s argument about the association between two events is fallacious, that doesn’t necessarily mean that there isn’t a connection between the events in question.
Overall, to respond to the post hoc fallacy, you can ask the person who used it to elaborate on their reasoning, or you can explain to them why this type of reasoning is fallacious (potentially using relevant examples). When doing this, it helps to first assess their reasoning, and remember that even if an argument uses the post hoc fallacy, that doesn’t necessarily mean that the associated causal relation doesn’t exist.
How to avoid the post hoc fallacy
To avoid using the post hoc ergo propter hoc fallacy, you should identify situations where you suggest that there’s a causal link between two events (i.e., that one caused the other), ask yourself what evidence you have for this, and make sure that you’re not arguing (even implicitly) that just because one event followed another, then the first event must have caused the second.
To help yourself do this, you can clearly outline your reasoning, in terms of the premises and conclusion of your argument, to determine whether you’ve argued that causality exists, and if so, then what evidence you’ve presented to support it. You can also explore the possibility that other causal relations exist, for example in the sense that two events in question were caused by a shared third event.
This can be useful both for arguments that you’re currently making or are about to make, as well as for arguments that you’ve made in the past, since past arguments might still be influencing your thinking, and since analyzing them can help you identify common flaws in your reasoning.
If you find that you’ve demonstrated the post hoc fallacy or that you’re about to do so, you can retract your argument, or you can modify it accordingly, for example by changing its phrasing to convey the uncertainty involved.
Overall, to avoid using the post hoc fallacy yourself, you should identify situations where you’re suggesting that there’s a causal link between two events, outline your reasoning while asking yourself what supporting evidence you have, and make sure that you’re not arguing (even implicitly) that just because one event followed another, then the first event must have caused the second.
Additional information
Causal fallacies
A causal fallacy is a logical fallacy that occurs when someone incorrectly assumes the existence of causality (i.e., that one thing causes another), on the basis of flawed reasoning. This type of fallacy is sometimes referred to as the fallacy of false cause, faulty causation, questionable cause, and non causa pro causa (non-cause for the cause), though in some cases these names are used to refer to specific types of causal fallacies.
The post hoc ergo propter hoc fallacy is one type of causal fallacy. Other types of causal fallacies include the following:
- Cum hoc ergo propter hoc (a Latin phrase meaning “with this, therefore because of this”). This fallacy occurs when someone assumes that because two events occurred at the same time, then one must have caused the other.
- Ignoring common cause (also known as the third cause fallacy). This fallacy occurs when someone assumes that the correlation between two (or more) variables must imply causation, while ignoring that an additional variable (or variables) may be the cause.
- Fallacy of the single cause (also known as complex cause, causal oversimplification, causal reductionism, and oversimplified cause). This fallacy occurs when someone assumes that there’s only a single cause for something, in a situation where it’s possible that there are additional causes.
- Inflated causality. This fallacy occurs when someone overestimates the degree to which one thing causes another, often while ignoring the influence of other important causes.
- Last straw fallacy. This fallacy occurs when someone misattributes the cause of an event, by claiming that the last or least known cause of it is more important than all the other factors that contributed to it.
- Confusing causation and resemblance. This fallacy occurs when someone assumes that because two things resemble one another, then one must cause the other.
- Reverse causality (also known as reverse causation or wrong direction). This fallacy occurs when someone reverses the cause and the effect in a certain situation (i.e., when someone assumes that B caused A, when the opposite is true).
Some of these fallacies can be viewed as subtypes of other fallacies. For example, the last straw fallacy can be viewed as a subtype of inflated causality, since it represents a specific case of it.
Furthermore, these fallacies sometimes occur in conjunction. For example, someone who uses the post hoc fallacy might simultaneously ignore a common cause between two events.
In addition, there are other fallacies that can be used to argue for causality in a fallacious manner. For example, people sometimes engage in fallacious cherry-picking, by ignoring or hiding information that contradicts the causal link that they proposed. Similarly, people sometimes focus on a cause that, while true, tells only a very limited part of the story. As one paper notes:
“… causal ‘talk’ is strongly linked to practical production and prevention. Thus, it can be true that [A] is genuinely a necessary condition of [B], yet, nonetheless, be a fallacy to conclude that [A] caused [B], where control of [A] is impractical, irrelevant, or impossible.”
— From “Post hoc, ergo propter hoc” (Woods & Walton, 1977)
Finally, there are additional fallacies that either involve causation or are often used as parts of fallacious arguments that involve causation. These include the following:
- Texas sharpshooter fallacy. This is a logical fallacy that occurs when outcomes are analyzed in an inappropriately selective or de-contextualized manner, in order to give misleading results and imply that causation is responsible for something that actually occurred due to chance.
- Regression fallacy. This is a logical fallacy that occurs when someone incorrectly interprets regression toward the mean as being caused by something other than chance.
- False dilemma. This is a logical fallacy that occurs when a limited number of options are incorrectly presented as being mutually exclusive to one another or as being the only options that exist, in a situation where that isn’t the case.
Related concepts
In addition to the logical fallacies that are associated with the post hoc fallacy, there are also other concepts that are associated with it and with causal fallacies in general.
One such concept is the adage that correlation does not imply causation, which denotes that just because two variables are correlated with one another, that doesn’t necessarily mean that one causes the other.
Another related concept is that of the cargo cult, which is described as follows:
“In the South Seas there is a Cargo Cult of people. During the war they saw airplanes land with lots of good materials, and they want the same thing to happen now. So they’ve arranged to make things like runways, to put fires along the sides of the runways, to make a wooden hut for a man to sit in, with two wooden pieces on his head like headphones and bars of bamboo sticking out like antennas—he’s the controller—and they wait for the airplanes to land. They’re doing everything right. The form is perfect. It looks exactly the way it looked before. But it doesn’t work. No airplanes land. So I call these things Cargo Cult Science, because they follow all the apparent precepts and forms of scientific investigation, but they’re missing something essential, because the planes don’t land.”
— From Caltech’s 1974 commencement address, by physicist and Nobel laureate Richard Feynman
Finally, another related concept is called post hoc rationalization, which is described as follows:
“There is some empirical support for the following broad claim: most explicit practical reasoning and justifications we offer to others or ourselves are rationalizations, and we instead act on instincts, inclinations, stereotypes, emotions, neurobiology, habits, reactions, evolutionary pressures, unexamined principles, or justifications other than the ones we think we’re acting on. Then – and this is the crucial part of the claim – we tell a post hoc story to justify the actions that are better explained in these alternative ways. The challenge, then, is that explicit, conscious reasoning about what to do is very often post hoc rationalization.”
— From “Post hoc ergo propter hoc: some benefits of rationalization” (Summers, 2017)
Origin of the fallacy
Mentions of the post hoc ergo propter hoc fallacy go back at least as far as Aristotle’s Rhetoric (written in the 4th century BCE), where he says the following:
“Besides genuine syllogisms, there may be syllogisms that look genuine but are not; and since an enthymeme is merely a syllogism of a particular kind, it follows that, besides genuine enthymemes, there may be those that look genuine but are not…
Another line [of argument that is a Spurious Enthymeme] consists in representing as causes things which are not causes, on the ground that they happened along with or before the event in question. They assume that, because B happens after A, it happens because of A. Politicians are especially fond of taking this line. Thus Demades said that the policy of Demosthenes was the cause of all the mischief, ’for after it the war occurred’.”
— From “Rhetoric” by Aristotle, in Book II, Chapter 24 (as translated by W. Rhys Roberts)
Summary and conclusions
- The post hoc fallacy (from the Latin post hoc ergo propter hoc, meaning “after this, therefore because of this”) is a logical fallacy that occurs when someone assumes that because one event occurred after another, then the second event must have been caused by the first.
- An example of this fallacy is the claim that if the rooster crowed immediately before the sun rose, then that means that the rooster caused the sun to rise.
- To respond to this fallacy, you can ask the person who used it to elaborate on their reasoning, or you can explain to them why this type of reasoning is fallacious (potentially using relevant examples)
- To avoid using this fallacy, you should identify situations where you’re suggesting that there’s a causal link between two events, outline your reasoning while asking yourself what supporting evidence you have, and make sure that you’re not arguing (even implicitly) that just because one event followed another, then the first event must have caused the second.
- Remember that even if an argument uses the post hoc fallacy, that doesn’t necessarily mean that the associated causal relation doesn’t exist.