The post hoc fallacy

This fallacy is committed when unconnected sequential events are linked together and the assumption is made that the first event caused the second and/or subsequent events.

The Post Hoc fallacy takes the following form:

1) Event A occurs; then event B occurs.

2) Therefore, event A is the cause of event B.

This fallacy also manifests itself as a bias towards jumping to conclusions based upon coincidences.

Examples:

• A footballer striker has performed poorly this season. Then he wears golden laces in his boots one day and scores a hat-trick. Those laces must be good luck…he keeps on wearing them!
  
  
• Jane has a cold. She takes homeopathic "cold-eeze". One week later the cold has gone. She's now recommending cold-eeze to all her friends.
  
  
• Amanda buys a lottery ticket from a different shop than usual. She gets 4 numbers and wins £50. She decides to go out of her way to buy her tickets from that shop again.

Many false conclusions are formed due to post-hoc reasoning. It is the source of many superstitions and old wives' tales.

A post-hoc fallacy can be spotted by showing the effect to have been: coincidence; likely to have happened anyway; or that event B had a different cause to event A.