What Is Post Hoc Fallacy? | Examples & Definition

The post hoc fallacy is a common error in reasoning in which one event is assumed to have caused another based solely on the timing of events.

Its name is derived from the Latin phrase post hoc ergo propter hoc, meaning “after this, therefore because of this.” Arguments that commit this logical fallacy ignore every variable except the order of events, often leading to erroneous conclusions.

Post hoc fallacy example
A CEO assumes that hiring a new celebrity spokesperson directly caused a subsequent increase in stock prices without considering other factors, such as broader market upswings.

Post hoc errors highlight the importance of rigorous analysis. Understanding the post hoc fallacy is essential to making evidence-based decisions and policies in domains such as science, business, and politics. Recognizing this fallacy helps in distinguishing between mere correlations and actual causation, a distinction critical in research and strategic planning.

What is post hoc fallacy?

The post hoc fallacy erroneously assumes causation between two events solely because one event follows the other in time. As an informal fallacy, the post hoc fallacy renders an argument unsound.

Post hoc reasoning arises from the human inclination to identify patterns and rely on heuristics, which are mental shortcuts designed to simplify complex problem-solving processes.

The post hoc fallacy is a type of causal fallacy, all of which assume a causal connection without sufficient evidence on the basis of timing or correlation. Another example of a causal fallacy is the cum hoc fallacy, which occurs when an argument assumes that simultaneous events must have a causal relationship.

Why is the post hoc fallacy a problem?

Post hoc reasoning can lead to serious errors in analysis and decision-making. Arguments that commit the post hoc fallacy can sometimes reach correct or partially correct conclusions, but the reasoning behind the argument is nevertheless unsound. A sound argument requires reasoning and evidence beyond the mere sequence of events.

A common error based on post hoc reasoning is the assumption that an alternative medicine treatment has cured an illness simply because a patient’s health improved over time. This reasoning fails to account for spontaneous remission and any conventional treatments undergone at the same time.

Superstitious beliefs are often rooted in post hoc reasoning. For instance, the post hoc fallacy may have been a source of traditions such as good-luck charms and evil-eye amulets. The originators of these superstitions may have observed that they seemed to have better experiences when wearing certain objects. Such traditions persist despite lacking scientific evidence because of the human tendency to take comfort in seeking patterns and causal links.

A variety of other pseudoscientific assumptions can arise from observing a sequence of events and mistakenly inferring a cause-and-effect relationship without evidence. For example, a full moon was once thought to be a direct cause of erratic behavior and mental instability, often termed “lunacy.” This theory may have been based on specific observations about humans behaving differently after a full moon.

Why does the post hoc fallacy occur?

The post hoc fallacy often results from faulty reasoning that stems from cognitive biases. The illusory correlation bias is particularly relevant. Illusory correlation refers to the human tendency to assume there is a relationship between two events or variables because they occur together or in a sequence.

Another contributor to post hoc reasoning is confirmation bias, the tendency to search for and remember information in a way that confirms preexisting beliefs. In the context of the post hoc fallacy, confirmation bias can cause people to remember and attach significance to instances in which a sequence of events appears to confirm a cause-and-effect relationship.

Although the post hoc fallacy typically occurs because of faulty reasoning, it can also be used deliberately as a persuasive tactic. For instance, if a newly elected politician were to claim credit for an improvement in unemployment rates, that would probably be a disingenuous, deliberate use of post hoc reasoning.

Post hoc fallacy examples

In business, a variety of tests and systems have been developed to guard against faulty post hoc reasoning. Businesses strive to establish cause-and-effect relationships based on evidence to avoid making incorrect assumptions based on the order of events.

Post hoc fallacy example in marketing
A marketing manager attributes a sales increase solely to a recent marketing campaign, overlooking other factors such as market trends and seasonal demand.

In this example, the marketing manager makes the mistake of using post hoc reasoning. To avoid making similar mistakes, marketers can employ techniques such as A/B testing and sales funnel analysis. These methods compare the campaign’s effects against a control group, isolating its specific impact from other factors.

Similarly, in science and medicine, rigorous standards for testing help prevent post hoc reasoning from creating false impressions.

Post hoc fallacy example in health research
A marketing manager attributes a sales increase solely to a recent marketing campaign, overlooking other factors such as market trends and seasonal demand.A newly developed drug is claimed to cure a virus because most patients who used the drug recovered.

This is an example of post hoc reasoning that could lead to harmful results if drugs weren’t required to undergo rigorous testing. Without controlled experiments, other factors influencing recovery, such as additional treatments or natural recovery processes, would not be accounted for, making the causal link between the drug and recovery rates speculative.

Frequently asked questions about post hoc fallacy

How can I identify a post hoc fallacy?

Post hoc fallacies can be recognized by the following attributes:

  • A causal relationship is asserted with certainty
  • The fact that one event happened first is the only evidence that suggests it caused the next event
  • No evidence is provided
  • Other contributing factors are ignored or underestimated
What is the difference between post hoc fallacies and hasty generalization fallacies?

Post hoc and hasty generalization fallacies both involve jumping to conclusions, but there is a difference between the two.

The post hoc fallacy could be considered a subcategory of the hasty generalization fallacy that focuses specifically on causation and timing.

What is the difference between the post hoc fallacy and the non sequitur fallacy?

The post hoc fallacy and the non sequitur fallacy are sometimes conflated, but they are fundamentally different.

  • Non sequitur is Latin for “does not follow.” A non sequitur is an invalid deductive argument whose conclusion doesn’t follow logically from its premises because of its faulty structure. Any formal fallacy that doesn’t fall into another, more specific category can be called a non sequitur fallacy.
  • Post hoc ergo propter hoc is Latin for “after this, therefore because of this” and refers to an informal fallacy in which causation is assumed based on the chronological order of events. It is also called “post hoc fallacy.”
What is an example of post hoc fallacy?

The following scenario is an example of the post hoc fallacy:

A country introduces new environmental regulations. Shortly afterward, there is a downturn in the economy. Some politicians argue that the new regulations caused the economic decline, neglecting other global economic factors at play.

The argument is fallacious because it assumes that the order of events is sufficient to prove causation. Although it’s possible that the regulations affected the economy, they can’t be assumed to be the main or sole cause of the economic downturn without further evidence.

What does post hoc ergo propter hoc mean?

Post hoc ergo propter hoc is a Latin phrase meaning “after this, therefore because of this.” It refers to the logical fallacy of assuming that because Event B follows Event A, Event A caused Event B. This error is often referred to as the post hoc fallacy.

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Magedah Shabo

Magedah is the author of Rhetoric, Logic, & Argumentation and Techniques of Propaganda and Persuasion. She began her career in the educational publishing industry and has over 15 years of experience as a writer and editor. Her books have been used in high school and university classrooms across the US, including courses at Harvard and Johns Hopkins. She has taught ESL from elementary through college levels.