The Post Hoc Ergo Propter Hoc fallacy is also known as “After this, therefore because of this”. It's a very common fallacy, easy to commit, and often difficult to detect. It's based on the assumption that if A happens after B, A was caused by B.
This is the fallacy that starts superstitions. When a baseball pitcher puts on green socks with orange stripes before a game and goes on to pitch a no-hitter, it's easy to see why he credits the socks with giving him “good luck”. If he doesn't wear the socks in his next game and plays horribly, it often reinforces the superstition. The pitcher falls into the Post Hoc Ergo Propter Hoc fallacy.
Like the previous fallacies we looked at, this is another one that is common in politics. For example, gun-control advocates will pass a law banning guns in a city, then violent crime involving guns goes down the next year. They assume that the law cause gun crime to go down. Likewise, gun-rights advocates may get a bill passed requiring law enforcement to issue concealed-carry permits to anyone who receives training and passes a background check. Shortly after the bill passes, gun crime goes down, and the advocates of “shall issue” laws point out that the increased numbers of law-abiding citizens carrying deadly weapons for defensive purposes caused criminals to think twice before committing violent crime, which caused the crime statistics to go down.
This fallacy is particularly tricky because sometimes ‘A' really does cause ‘B'. An obvious example would be that when the temperature rising after sunrise. In many places, especially desert climates, the sun rising really does cause an increase in temperature. The “Scientific Method” depends on the law of causation, and the reliability of experimentation to be reproducible. ‘A' must consistently be followed by ‘B', or the scientific method falls apart. This fallacy merely shows that ‘B' following ‘A' doesn't necessarily mean there is causation. Sometimes there are unseen factors involved that are the real cause. Perhaps every time you observe ‘A', ‘C' happens as well, and is the real cause of ‘B'. ‘A' happening before ‘B' just happens to be a coincidence.
Next Lesson: Appeal to Pity
