Argument Detective A Field Guide to 15 Logical Fallacies
#9 of 15 Make an Unsupported Leap

Post Hoc and False Cause

Also called: post hoc, correlation is not causation, causal fallacy, after this therefore because of this

False-cause reasoning assumes that one event caused another without enough evidence, often because the events happened together or one happened first.

A careful definition

False Cause is the broad category, and Post Hoc is one common form. Sequence and correlation can provide clues, but they do not settle causation. A causal claim needs evidence that rules out coincidence, reverse causation, shared causes, and other explanations. "Post hoc ergo propter hoc" means roughly "after this, therefore because of this."

Family — Make an Unsupported Leap: Does the conclusion actually follow from the reasons?

The move

  1. A happened before B, so A caused B.
  2. A and B occur together, so A caused B.
  3. A third factor C that could influence both is ignored.

Why it can feel convincing

  • Human minds naturally search for patterns and explanations.
  • A single cause is easier to remember than several interacting factors.
  • A story with a clear cause feels satisfying.

Clear examples, diagnosed

Clear example

The lucky socks

"I wore my lucky socks, and then our team won, so the socks caused the victory."

What went wrong:

  • The only link offered is that the socks came before the win.
  • Sequence alone does not establish a mechanism.
  • Player performance, strategy, and chance are all ignored.
Repaired: "We won while I wore those socks. That is a coincidence unless the socks somehow affected play, so the credit belongs to preparation and performance."

Clear example

The garden and the scores

"The school planted a garden in September, and reading scores rose in spring, so the garden caused the increase."

What went wrong:

  • Two events are connected only by timing.
  • Many other changes during the year could explain the scores.
  • No mechanism links a garden directly to reading gains.
Repaired: "Reading scores rose after the garden was planted. To claim a cause, we would need to rule out other changes and test the effect carefully."

Where the boundary is

Not a fallacy

A controlled comparison

"The class compared similar plants, changed only the amount of light, repeated the trial, and consistently found slower growth in the low-light group."

Controlled, repeated evidence that rules out other explanations can support a causal conclusion. Careful cause-and-effect reasoning is not the fallacy.

Borderline case

Sleep and grades

Students who sleep more often earn higher grades.

Questions to ask:

  • Could sleep help learning, or could the causation run the other way?
  • Could schedules, health, or study habits affect both sleep and grades?
  • Is this correlation a starting point for investigation rather than proof?

Not automatically fallacious when

  • A mechanism connects the cause and the effect.
  • Controlled or carefully designed evidence rules out alternatives.
  • The claim uses cautious language such as "may contribute."
  • Multiple forms of evidence point in the same direction.

Diagnostic questions

  • Did the supposed cause actually happen before the effect?
  • Could the direction run the other way?
  • Could a third factor cause both?
  • Was the relationship repeated or controlled?
  • Is the claim stronger than the evidence?

Spot it. Prove it. Repair it.

The argument

"I changed my pencil and then solved the problem, so the pencil made me better at math."

Spot Notice the possible problem

This looks like a false-cause claim.

Prove Point to the exact failure

  • The only connection is that the new pencil came before the success.
  • Practice, the specific problem, or focus could explain it instead.
  • One sequence is not evidence of a cause.

Repair Rebuild it fairly

Repaired: "I solved the problem after switching pencils. Let me repeat the comparison and consider other changes before claiming the pencil helped."

Use this for your project

Essential question

How do we tell a real cause from two things that just happened together?

Where to hunt for examples

  • Superstitions and lucky-charm stories.
  • Advertisements claiming a product caused a result.
  • Headlines that turn a correlation into a cause.
  • Everyday "ever since I started X" explanations.

Questions to research

  • What is the claimed cause and effect?
  • Did the cause clearly come first?
  • What other explanations are possible?
  • Was the relationship tested or controlled?

Project products to try

  • A causation-detective board with sequence, correlation, mechanism, alternative causes, and evidence needed.
  • A rewritten claim that softens an overstated cause.
  • A diagram of a third factor influencing both events.
  • A short experiment testing a simple cause-and-effect idea.
Slippery Slope

False cause misjudges a link that already happened; slippery slope predicts a future chain of links.

Anecdotal Fallacy

A single story often carries a hidden false-cause claim about what produced an outcome.

Sources and attribution

Content status: Original to this project. This page's explanations, examples, non-example, borderline case, and repair activity were written for Argument Detective.

Sources and further reading