80 False Cause: Post Hoc Ergo Propter Hoc

In general, the false cause fallacy occurs when the “link between premises and conclusion depends on some imagined causal connection that probably does not exist” (Hurley, 2015, p. 149). Post hoc ergo propter hoc — often called simply the post hoc fallacy — is one specific variety of this fallacy; it is also a type of non sequitur (Latin for “it does not follow”), because the conclusion simply does not follow from the mere fact that one event happened after another. The Latin phrase translates literally to “after this, therefore because of this.” The fallacy is committed when one argues that a causal relationship exists between A and B mainly because A happened before B, without providing actual evidence of causation. Crucially, it presupposes that two consecutive but independently occurring events are causally connected, projecting a relationship that may not exist.

HOMER: Not a bear in sight. The Bear Patrol must be working like a charm. LISA: That’s specious reasoning, Dad. By your logic, I could claim that this rock keeps tigers away. HOMER: Oh, how does it work? LISA: It doesn’t work. It’s just a stupid rock. But I don’t see any tigers around, do you? HOMER: Lisa, I want to buy your rock. — The Simpsons, “Much Apu About Nothing”

As Lisa’s rock illustrates, temporal sequence alone is never enough. By the same logic, it could have been her beaded necklace or Bart’s skateboard that was keeping the tigers away. Consider Sheldon’s words from an episode of The Big Bang Theory titled “The Electric Can Opener Fluctuation”: “No, Mother, I could not feel your church group praying for my safety. The fact that I am home safe does not prove that it worked, that logic is post hoc ergo propter hoc.” Sheldon notes that his safe return is not sufficient evidence that the prayers caused it. Athletes often commit the same fallacy when they attribute winning to an article of clothing they wore; by the same reasoning, they could attribute the win to the shampoo they used that morning. Everyday superstitions follow the same pattern: “I lost my wallet the afternoon after I kicked my neighbor’s cat; therefore, my unethical act caused me to lose my wallet.” Causes do precede their effects, but that alone never suffices to prove a causal connection. What is needed is actual evidence that two events are causally linked; we must not project a causal relationship onto unrelated events simply because they occur in sequence.

The fallacy appears frequently in political discourse. A classic form: “Smith was elected Governor in May. In June, crime rates skyrocketed. Clearly, Smith must have caused crime to increase.” Senator Mitch McConnell in 2015 attributed an improving economy to the election of a Republican Congress: “The uptick appears to coincide with the biggest political change of the Obama Administration’s long tenure in Washington ….” Yet the economy had already been improving before the elections, and structural economic changes do not manifest within two months of a party taking office. Similarly, a Fox Nation headline once read, “Stocks Tumble Worldwide after Obama Speech,” clearly implying the president’s remarks caused the decline. Many people also blame or credit sitting presidents for gas prices, ignoring the multitude of global economic conditions that actually drive them. A more succinct version of the same flawed logic can be found in claims like, “The financial meltdown happened after Republicans took office, so we can blame the Republicans for the crisis we’re in.” The mere sequence of events does not establish which policies, if any, were responsible.

The post hoc ergo propter hoc fallacy has also had serious consequences for public health. The fear that the MMR (measles, mumps, and rubella) vaccine causes autism began with a 1998 Lancet article by Dr. Andrew Wakefield and colleagues; the study was later retracted after numerous flaws were uncovered. Yet celebrity Jenny McCarthy became a vocal opponent, attributing her son’s autism to the vaccine. Because autism is often diagnosed in early childhood — around the same time the MMR vaccine is administered — many people inferred causation. Repeated studies since then have found no causal connection (see, for example, Myers & Pineda, 2008). This misattribution is a textbook example of the fallacy: the mere temporal proximity between vaccination and diagnosis was taken as evidence of causation, despite a complete absence of valid causal evidence. The resulting decline in vaccination rates led to a resurgence of preventable diseases, endangering not only unvaccinated children but also those who rely on herd immunity due to autoimmune disorders.

The “gateway drug” hypothesis sometimes employs the same fallacy. While it is true that people who use hard drugs often started with marijuana, that sequence alone does not demonstrate that marijuana caused the later use. The Institute of Medicine of the National Academy of Sciences acknowledges that marijuana use typically precedes other illicit drug use, but also notes that underage smoking and alcohol use typically precede marijuana use; there is “no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs” (Szalavitz, 2010). To infer causation from sequence is to commit post hoc ergo propter hoc.

The fallacy is not limited to dramatic claims. A casual glance at social trends can produce the same error: “Teen pregnancies have decreased since the creation of the Internet. So, it is clear that the Internet prevents teen pregnancies.” Like all post hoc reasoning, this mistakes correlation in time for causation, ignoring a host of other possible factors. The core misunderstanding of cause and effect — assuming that because one event follows another, it must be the result of that earlier event — underlies every instance of the fallacy. Only careful causal reasoning and evidence beyond mere sequence can establish genuine connections.