Everyone loves the story of a good coincidence – whether it’s discovering your own long-lost copy of a favourite childhood book in a vintage flea market in Paris, or of travelling halfway round the world to find yourself bumping into your next-door neighbour.  Coincidences make us stop and think, and raise the question of whether life truly is random or whether there’s another force at work.

Most people have their own stories of coincidence to tell.  My husband and I studied almost the same subject at the same university, overlapping by a couple of years and almost certainly attending some of the same lectures.  However, we only met 6 years later in a different city. This story lends itself nicely to the romantic notion that fate brought us together! But, digging deeper, it’s not that surprising when you consider that we both studied software, we both moved to work in Cambridge which is well known for its high-tech industry, and Cambridge is small enough that after a few years here it seems everyone knows everyone else.  Suddenly the chances of us meeting here seem much higher.  Perhaps the most unlikely event in the whole chain is that I’m female and studied software!

With a world population of 7 billion people, we can expect an event with chance 1 in 1 million to happen to 7,000 people.  The fact that the event happens to those particular people is amazing for them individually, but the fact that the event happens at all is not. And this is one of the hardest things to get your head around when thinking about rare events – the chance of them happening at all is reasonably high.

In a recent broadcast of Radio 4’s “The Life Scientific“, David Spiegelhalter described coincidence as the ‘upside of risk‘.  If something terrible happens, it’s often the chain of a long sequence of unlikely events that eventually leads to that bad event. Coincidences on the other hand are also the result of a chain of unlikely events, but with a positive outcome.  In the same way as humans are naturally loss averse, we may overestimate the chance of a bad event, and underestimate the chance of a good event, giving coincidences more weight than they deserve.

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