October 1996
by Robert Novella

Coincidences are ubiquitous events in everyone’s lives. Some are trivial, like being dealt a flush in poker, but others really grab our attention, like thinking of a friend you have not seen in years only to have them call on the phone moments later. What these events have in common is our intense desire to explain them, a belief that there is a special reason things happen the way they do. These explanations can range from a lucky rabbit’s foot to a psychic link with a friend. What most people do not know or do not want to believe is that coincidences, even remarkable ones, are not all that surprising. In fact most are inevitable occurrences with no special significance at all.

There are many simple reasons why most people misinterpret coincidences. Humans have a poor innate grasp of probability, we believe that all effects must have deliberate causes, we do not understand the laws regarding truly large numbers, and we easily succumb to selective validation – the tendency to remember only positive correlations and forget the far more numerous misses.

Probability was not a very popular class when I went to college. This is unfortunate, because understanding probability can give one the power to more greatly appreciate the significance, or lack of significance, of many everyday events. A poor understanding of probability and statistics, common in our society, causes people to be more amazed than they should be when confronted with coincidences, hence the easy jump to a metaphysical explanation.

For example, what are the odds of two people sharing the same birthday in a room containing twenty three people? Many people I have spoken with say it must be one in thirty or more. Surprisingly, to most people, it is only one in two. Not knowing this has caused many people to conclude that since their experience seemed so unlikely, perhaps they share some special link or that a supernatural force brought them together.

The true significance of bizarre coincidences can be understood more fully with what is called the law of truly large numbers. This widely accepted law of statistics states that with a large enough sample size, even the extremely unlikely becomes likely, and therefore any outrageous thing is bound to happen. A telling example of this occurred a few years ago when a New Jersey woman won two lotteries within four months. Newspapers widely reported it to be a one in seventeen trillion coincidence. Technically speaking, this is correct, but it is misleading because it is based on too narrow a perspective. The chances of a specific person winning two lotteries after buying two tickets is indeed trillions to one, but the odds of someone winning among the millions that play is only one in thirty. This is the essence of the law of truly large numbers. Unusual occurrences become highly probable when enough people are involved. This perspective lifts the shroud of mystery and puts the focus where it belongs, on science.

Human memory is not like a tape recorder, faithfully recording everything experienced. Dramatic experiences leave a much more indelible mark on our minds than any other. This leads to a phenomenon called subjective validation, more commonly known as selective memory. Therefore it is only natural to remember unusual experiences, even Alzheimer’s victims with significantly impaired short term memories can more easily recall recent events that were out of the ordinary. Returning to our friend who calls soon after you think about him, this event becomes much less striking if we consider how many times we have thought of friends who did not then call.

A common ploy used by psychics (often called the Jeanne Dixon effect) is to make dozens of predictions knowing that the more that are made, the better the odds that one will hit. When one comes true, the psychic counts on us to conveniently forget the ninety nine percent that were way off. This makes the correct predictions seem much more compelling than they really are. This is a conscious or deliberate form of subjective validation, or, put more simply, fraud.

Our coincidence detection abilities have been finely honed through the ages by evolution and natural selection. Being able to spot significant correlations between events would lend an important survival advantage to our ancestors which would then be selected for through the generations. We can speculate, therefore, that man is hardwired to look everywhere for patterns and connections. Modern culture, however, with its myriad connections between events and people, activates these abilities at every turn, causing us to continually posit explanations and invoke strange forces that do not exist.

It is not my contention that all coincidences have no meaning and should be ignored, indeed truly unlikely events may have some underlying significance and the search for their causes would be a laudable endeavor. However, the vast majority that we experience turn out to be much more probable than they appear, if analyzed critically. When this is taken into account, along with our propensity for selective validation, our desire to believe in something akin to fate, and our coincidence detection hardwiring, the true deceptive power of coincidence is realized.

References:
1) Kolata G. 1-in-a-Trillion Coincidence, You Say? Not Really, Experts Find. The New York Times/Science Times Tuesday 2/27/90
2) Paulos JA. Coincidences. Skeptical Enquirer Vol. 15,No. 4, Summer 1991 pgs 382-385

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