Science Puzzle
The Umbrella Blame
A study shows that on days when lots of umbrellas are open, it is almost always raining. The link is real and consistent.
A shopkeeper decides that opening umbrellas must be bringing the rain, so he bans them from his street to keep the sky clear. What is wrong with his reasoning?
The Answer
He has the arrow of causation pointing the wrong way. Rain causes people to open umbrellas, not the other way around.
This is different from the classic lurking-variable trap where a hidden third factor drives both. Here the two things really are directly connected, but the direction runs opposite to what the shopkeeper assumed. Banning umbrellas will not change the weather one bit.
Whenever two things are linked, it is worth asking not only whether a third factor is driving both, but also which one came first and could actually be causing the other.
The principle: Reverse causation. A real link between two things can run the opposite direction to the obvious guess, so always ask which one is doing the causing.