This logical fallacy is committed when someone rejects or discounts extensive evidence in favor of an isolated or personal experience. This is often used as the basis for the overgeneralization fallacy and is linked to the Post hoc ergo propter hoc (correlation/causation) fallacy, a fallacy that assumes a causal relationship where none exists.

Cognitive bias causes anecdotes to be very compelling. If the audience wants to believe in something, they will often cling to anecdotes as evidence even if there is no other evidence or documentation for the phenomenon. Urban legends are prime examples of how anecdotes can circulate rapidly and repeatedly and how they can get exaggerated and embellished over time.

Anecdotes are often given more credence than they should due to the availability heuristic. This causes people to overestimate how common something is based on how easily they can think of an example. People are more prone to remembering exceptional events, and thus anecdotes are more often the exception and not the rule. In fact, because the experience isn’t typical, it is almost guaranteed that they are more likely to remember the exceptional event causing it to seem more prevalent than it actually is. In addition, most people don’t take into consideration the vast numbers of “unexceptional” stories that are more in line with reality because they rarely hear about them or, if they do, they don’t remember them because they are so typical.

People often use this fallacy when it comes to science and medicine. For the general population, it is often easier to believe a testimonial of someone “just like you” than the nuances of dozens of complex scientific studies that have been accumulating for years. Even though science is almost always more reliable than anecdotes, people will tend to accept a relatable isolated example, often cherry-cherry picked for its adherence to a particular belief, over statistical data backed up by extensive evidence.

When it comes to science, explanations are only valid if they can be rigorously tested and are falsifiable. For something to be accepted by science, it needs to be based on facts or careful study, not casual observations, and should come from scientific observers (those who understand bias, evidence, and testing). Scientists will often use anecdotes as a starting off point for conducting research and suggesting hypotheses, but they are not used to validate evidence. As scientists will often say, “The plural of anecdote is not data.”

Often people use anecdotal evidence to confirm their belief or unbelief in a supernatural being. If you pray to God to heal your grandmother, and she gets better, that is not proof that God intervened; likewise, if she doesn’t get better, that doesn’t mean that God doesn’t exist.

Anecdotes are not always a fallacy. When it is offered as just another piece of data to consider, not as proof one way or the other, when it is part of a larger collection of consistent anecdotes (inductive reasoning, which is valid if there is enough evidence), and when it is used to counter an overgeneralization (i.e. “All smokers will get cancer” can be disproved by “my chain-smoking grandma died of a car accident at 93 and never got cancer”), anecdotes can be useful.

Examples:

I know a Scientologist who is a millionaire. Therefore Scientology creates millionaires.

I thought about my grandmother and a few minutes later she called. That proves telepathy is a fact.

I heard of a boy on the news who remembered a past life. That proves that reincarnation is real.

There’s abundant proof that God exists and is still performing miracles today. Just last week I read about a girl who was dying of cancer. Her whole family went to church and prayed for her, and she was cured.

Yeah, I’ve read the health warnings on those cigarette packs and I know about all that health research, but my brother smokes, and he says he’s never been sick a day in his life, so I know smoking can’t really hurt you.

According to statistics, smoking causes you to die young. But my Grandmother Sally smoked like a chimney and lived until she was 95, so clearly the statistics are wrong.

As a rebuttal, one might simply point out that they met a man on the way home who said that anecdotal evidence doesn’t prove anything.

There’s abundant proof that drinking water cures cancer. Just last week I read about a girl who was dying of cancer. After drinking water she was cured.

For instance, someone who claims to have had an encounter with a supernatural being or alien may present a very vivid story, but this is not falsifiable.

My uncle Chet told me once that he used to get welfare even though he had a good job. Therefore, we need to get rid of the welfare system because it’s being defrauded way too much.

My cousin was in a car accident and wasn’t wearing his seatbelt. He was thrown from the car and walked away without any injuries. So we don’t really need seatbelts.

“On the one side are scientists who have been unable to find any causal link between the symptoms of autism and the vaccine preservative thimerosal, which in the body breaks down into ethylmercury, the culprit du jour for autism’s cause. On the other side are parents who noticed that shortly after having their children vaccinated autistic symptoms began to appear. These anecdotal associations are so powerful that they cause people to ignore contrary evidence: ethylmercury is expelled from the body quickly (unlike its chemical cousin methylmercury) and therefore cannot accumulate in the brain long enough to cause damage. And in any case, autism continues to be diagnosed in children born after thimerosal was removed from most vaccines in 1999; today trace amounts exist in only a few.” (Scientific American, “Wheatgrass Juice & Folk Medicine: Why subjective anecdotes often trump objective data”, August 2008, Michael Shermer).

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