One thing is held to cause another when in fact both are the effect of a single underlying cause. This fallacy is often understood as a special case of post hoc ergo prompter hoc.

Examples

1. We are experiencing high unemployment which s being caused by a low consumer demand. (In fact, both may be caused by high interest rates.)
2. You have a fever and this is causing you to break out in spots. (In fact, both symptoms are caused by the measles.)

Proof

Identify the two effects and show that they are caused by the same underlying cause. It is necessary to describe the underlying cause and prove that it causes each symptom.