Why chance is no coincidence

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Science Editor

Really "spooky" coincidences, such as meeting a complete stranger at a party only to discover that you both went to the same school, are far more common than most people think, scientists have discovered.

Belief in the paranormal may be a result of our inability to assess how likely coincidences really are, according to the psychologist Susan Blackmore, of the University of the West of England in Bristol.

The more outlandish the coincidence, the worse people are at estimating the mathematically correct probability, Dr Blackmore found. In a study of 120 people carried out with Dr Robert Matthews, a visiting fellow at Aston University, she discovered that people use a simple but wrong mental rule for estimating probabilities.

Among the questions the researchers asked was: How many randomly chosen people need to be in one room before there is a 50-50 chance that two of them will share the same birthday? Most people think that, since there are 365 days in the year, the answer has to be about half that, say 186. In fact, the correct answer is 23. Similarly, when told that there are about 5,000 secondary schools in England, most of those questioned believed that 2,500 randomly chosen people would have to be assembled before there was a better than even chance of two of them having attended the same school. In fact, the answer is 85.

Their work has just been published in the scientific journal Perceptual and Motor Skills.

Many "coincidences" in real life are not really random at all because simple geography or social factors will tend to bring together people with shared traits. True coincidences happen where such obvious explanations do not apply, and that tends to be when people turn to paranormal explanations.

No "explanation" is actually needed, other than a better understanding of the fact that many things are more probable than most people think.

Dr Blackmore explained: "Humans typically acquire skill at something by constant practice, but we don't go around all day deliberately seeking out coincidences. If we did, we'd soon realise that we live in a sea of them, and would be far less surprised when they popped up."

Leading article, page 10