A dowser finds water or a golf ball after using his dowsing rod. He claims the rod led him to the find. Did it? I doubt it, but it is true that one thing came after the other.
A gambler blows on the dice before he rolls them. They come up a winner. He thinks his blowing on the dice affected the outcome of the roll. Did it? Probably not, but it is true that one thing came after the other.
A woman claims that a vaccine caused her child's autism. Why? I don't know, but it is true that the diagnosis came after the shot.
A man claims that his knee pain diminished significantly after receiving acupuncture treatment. What caused his pain to lessen? I don't know, but he thinks it was the acupuncture.
A woman's headache went away after taking a homeopathic potion for headaches. Why did her headache go away? I don't know, but she thinks it was due to the homeopathic pill. It is true, though, that the one came after the other.
Desiree Jennings was a young cheerleader when she became the poster child for the anti-vaccination movement based on her claim that a flu shot caused her dystonia. Her evidence? She started showing symptoms ten days after she got the shot. She once had her own website (www.desireejennings.com) where she wrote:
On August 23, 2009, I received a seasonal flu vaccine at a local grocery store that drastically, and potentially irreversibly, altered my future. In a matter of a few short weeks I lost the ability to walk, talk normally, and focus on more than one stimuli [sic] at a time. Whenever I eat I know, without fail, that my body will soon go into uncontrollable convulsions coupled with periods of blacking out.
Each day is a battle to control the symptoms triggered by the flu vaccine and a reminder that my life will never be the same. I set up this site to tell my story and warn people of the neurological side effects than can result from vaccinations; especially knowing that in the majority of cases, these stories are seldom heard outside of immediate families and friends.
I hope everyone that reads my story will heed my warning and think very carefully, including seeking out consultations with your family doctor, before making the decision to receive a vaccination.Jennings claims that about ten days after she received the seasonal flu vaccine, she developed a severe respiratory illness that required hospitalization. Shortly after that she had difficulty speaking and walking, with involuntary muscle contractions and contortions. Her symptoms were relieved, she claimed, by walking backward or by running.
There is no known way that the flu (which is what probably hospitalized her) or a flu vaccine could cause dystonia and there is not a single case in the medical literature of such a thing ever happening. Still, there is always a first time, I suppose. But getting bogged down in that discussion is a red herring because it is very unlikely that Jennings suffered from dystonia, much less that the flu vaccine caused her symptoms.
The post hoc ergo propter hoc (after this therefore because of this) fallacy is based on the mistaken notion that because one thing happens after another, the first event was a cause of the second event. Post hoc reasoning is the basis for many superstitions and erroneous beliefs. The examples of poor causal reasoning listed above were each probably combined with preconceived ideas about such things as a causal connection between astronomical events and tsunamis, dowsing and finding things, superstitious actions and outcomes on dice or cards, vaccines and autism or other disorders, acupuncture and pain relief, and homeopathy and headaches.
Post hoc reasoning is one of the most common cognitive biases and one of the more difficult to overcome because the personal experience of immediacy seems to intuitively justify the making of a causal connection. After all, when you hit your finger with a hammer or bump your head on a kitchen cabinet door, you know what caused the pain! When you watch somebody else do the same thing, you're not surprised that you don't feel any pain.
As noted above, if one already has a belief about a causal connection between two unrelated things, it is natural that she would confirm her bias by seeing a sequence of events as an example of her belief in action. Contrary to what some people might think, making hasty conclusions about causal connections is not a sign of stupidity or idiocy. It is natural and the norm, which is why it is so difficult to overcome.
It may be true that your engine blew up two days after you lent your car to your brother-in-law, but it isn't necessarily the case that he did anything to the car that caused the engine to fail.
It may be true that you aced your physics test after forgetting to shave, but you would be foolish to think that not shaving had anything to do with your score on the test.
Just because one thing happens after another does not mean that there is any causal connection between them. On the other hand, it is not a coincidence that your car won't start after you filled the gas tank with water. Sometimes when one thing happens after another, it's because the first thing caused the second. What justifies making a causal connection is knowledge. That knowledge can come from experience or from experiments. When a doctor prescribes a medication for a urinary tract infection, she bases her treatment on knowledge. When the patient gets better and thinks the medicine helped in her recovery, she is not committing the post hoc fallacy because her connection between the two is justified. Unfortunately, many people think they have knowledge (about dowsing, vaccines, astrology, etc.) when all they really have is misinformation.
Scientists have developed various ways to test causal claims. For example, many people believe that vaccines cause autism. Yet, study after study has not found what should be found if vaccines cause autism. Vaccinated children should have a significantly higher rate of autism than non-vaccinated children, but they don't. Nor do dowsers find water at a greater than chance rate when tested under controlled conditions.
postscript: Since posting this blog entry I have received several more stories. Rather than post each one, I suggest you substitute the following items for psychiatrist, alkaline diet, prayer, aromatherapy, dophin therapy, and acupuncture:
- Jin shin Jyutsu
- hypnotic regression
- thought field therapy
- magnet therapy
- emotional freedom technique
- aura therapy
- angel therapy
- mega-mineral therapy
- colloidal silver therapy
- urine therapy