Causal basis of the ice cream-shark correlation fallacy

Most people that have ever taken a stats course have encountered the ice cream-shark correlation fallacy. The fallacy states that whenever ice cream sales rise, so do shark attacks.

Obviously the purpose of this nonsensical statement is to embody the dangers of blindly attributing causation based on correlations. In other words, just because two variables are highly correlated does not mean that one causes the other. In statistical terms, we say that correlation does not imply causation. Although a commonly used example in introductory stats courses, one rarely (for obvious reasons) encounters a discussion of putative causal mechanism for this causation, i.e. why is ice cream consumption positively correlated with shark attack frequency? One could of course simply accept the somewhat boring text book explanation of a common response to an unobserved variable, i.e. ice cream sales and shark attacks both increase during summer. Alternatively one could come up with more colorful mechanisms for this correlation. BBC has an online lesson about causation where readers have gone wild submitting comments proposing alternative mechanisms. Here are a few (a few have been a bit cleaned up and reworded to make “more sense”)…

• Eating ice cream makes you tastier.
• Sharks like ice cream but find it really difficult to buy them, so they resort to eating humans in the hope they will still have ice cream inside them.
• Ice cream sales rise in hotter weather, which also forces sharks to get their Aircon serviced. This is very expensive and angers the shark. Angry shark equals more attacks.
• Eating ice cream will cause people to urinate a large quantity of lipids (fats) similarly to seals and other fatty sea mammals, which happen to be the sharks’ favorite prey.
• When there is a horrible shark attack on the beach, people stay out of the water and have an ice cream instead.
• It is easier to swim away from a shark if you are not trying to hold on to an ice cream.
• Ice cream sales rise in hot weather, people go to the beach in hot weather, there are sharks at the beach, people steal the sharks’ deck chairs, the sharks attack them.
• During warm weather more people swim in the sea. Warm weather also heats up the sea which brings the sharks closer to shore.
• Warmer weather brings people to beaches where people buy ice cream, sharks see ice cream, sharks become jealous. The sharks want the ice cream, and attack people to get it.

This is from the “Mario’s Entangled Bank” blog ( http://pineda-krch.com ) of Mario Pineda-Krch, a theoretical biologist at the University of California, Davis.