Science Interviews


Mon, 3rd Nov 2014

The maths of coincidence

David Spiegelhalter, University of Cambridge

Listen Now    Download as mp3 from the show Supernatural Science

Have you ever thought about an old friend, only to have them call that same day? What about singing a song just before it comes on the radio? These spooky coincidences are surprisingly common when you look for them, but can be explained by simply psychology and maths, according to David Speigelhalter of Cambridge University. He explained to Chris Smith and the live audience why creepy coincidences aren't as creepy as we might think...

David -  I'm interested in strange things that happen to people, that people say, ďWhoa!  Fancy that!Ē and remember some of these things Ė the stories I've got.  People will remember for the rest of their lives some of the strange things that happen.

Chris -  Well, like what?

David -  We collect them.  Weíve got a website at Cambridge Coincidences and weíve got thousands of them.  Some of the most popular ones, and you could tell how old some of these are, how long theyíve remembered them, because they're about public telephone boxes.  I think we got four now where someoneís walking on the street, an old red public telephone box, it rings as they're going past it,  ďOh!  I might as well answer it.Ē  They answer it and itís for them.  Well, I wonder if anything strange has happened tonight.  I mean, there's about 70 people in the audience.  I've actually asked you to write down your birthdays so I've got them.  Now I wonder, would it be strange if two people in the room had the same birthday?  Is that strange?

Chris -  Hands up Ė this audience if you would be surprised if someone in this room had the same birthday as you.  This is a sceptical audience.  So, about 5 or 6.

David -  What happens if two people in the room walked in with the same birthday and happen to sit very close to each other?  Would that be quite cool?  Come on you miserable lot, that would be cool!.  Okay, stand up if you were born on the 27th of March.  Quite a long way.

Chris -  So, we have one lady and one man and they're on the opposite side of the auditorium.

David -  Yes, sit down.  Okay, stand up if you were born on the 6th of February.  Opposite sides of the room.  This is terrible.

Chris -  Itís a good hit rate so far.

David -  Yeah.  Stand up if you're born on October the 6th.  Not bad, but 6 or 7 feet away from each other.  Thatís not bad.

Chris -  We should point out, each time weíre getting about two people standing up.  Each time, youíve had a pair of people sharing those birthdates.

David -  Yeah, I'm not choosing these dates at random.  So, the sort of stories some people report to us, the classic coincidence.  I donít know anyone has had this when they meet a stranger and suddenly they find this connection with the stranger.  They went to the same school.  They know somebody in common is a usual thing.  I was just looking through the ones people have sent in just in the last week, somebody meeting somebody and finding out that they lived in the same house.  The other one is finding a connection with the one you know and then again, finding this very strange connection.  Weíve had one, a beautiful one, a married couple who discovered they were both born in the same bed.  They got married, yeah.  They found they were both born in the same little village in Germany which only got one little hospital and one little bed where all the babies were born.  Another one, people have just sent in a married couple who discovered that they'd both been in a hospital as children at both exactly the same time.  There's these wonderful strange ones where there's a picture of the husband when heís a boy sitting on the beach and there's his wife, just walking along behind on the beach.

Chris -  But isnít the whole point of every single one of these examples is notable because itís exceptional, whereas all of the other benign things that happen in peopleís lives that donít have these bizarre coincidences attached to them, people just ignore?

David -  Exactly.  There's so many million things that happen to us all the time.  There's inevitably going to be some strange things that happen.  For example, whoís born on the 2nd of September?  Not bad, about 6 feet, okay.  How about on 10th of January?  Miles away, that's useless.  You all deliberately sort of Ė thereís this powerful force.  Itís making people sit at opposite ends of the room if  theyíve got the same birthday.  But cool, there's a lot of people in here with the same birthday.

Chris -  David, isnít it a coincidence that none of the people with the same birthday are sitting together?

David -  Wait, I haven't finished yet.  Weíll see.  The other ones I love are where people rediscover objects.  It was lovely, someone sent one in recently where they had carefully stencilled on a lovely picture on their chest of drawers.  And then theyíve given it to a friend and the friend had gone away a couple of hundred miles, and years later, this woman had moved to this new town and she got a friend next door who invited her into her room and there was the chest of drawers Ė complete stranger, at that.  So, when these things happen to people, they go, ďWhoa!  Thatís really cool.Ē  People in the past have invented theories why these things happen.  A guy called Kammerer had this theory of seriality.  It was a sort force that caused these strange things to happen more than they should and Jung had this idea of synchronicity.  This is definitely to do with premonitions as well that we could feel things happening before they did.  We banned premonitions from our website.  I donít let them go in there.  Iíd only accept premonitions if people told me them before the event happened.  Afterwards, itís cheating.  Okay, 7th of March.  They're not sitting next to each other.  Pretty good, but not quite there.  Weíll see.  So yeah, those are sort of things, and then there's numbers.  There's people, the pin number keeps on cropping up, same pin numbers, someone wrote in and said, ďThere's the same pin number from my childís primary school and the bike lock at work.Ē  But these things happen.  The other thing is, we get a lot of people writing to us saying, ďThese things keep on happening to me.  Itís really actually quite disturbing.  These things happen to me all the time.  I keep noticing these connections between everything.Ē  I'm afraid Ė I do believe they do happen to some people more than others.  I really do.  They never happen to me.  Coincidences never happen to me.  I once had someone phone me up when I was on the train about a story about a bacon sandwich while I was eating a bacon sandwich.  But thatís the only time thatís happened to me.  It was so obvious, I couldnít miss it.  Now the point is, now why donít they happen to me?  I'm the sort of person who goes around, staring at the ground.  I never notice what's going on around me and I never speak to anybody at all.  I'm miserable.  I could sit next to someone in the train for hundreds of miles and not utter a word.  So, these things never happen to me.  But if you're the sort of person who sits next to someone in the train and starts talking to them, or you're the sort of person who actually notices what's going on around you, and then notices, ďI saw that person earlier in the day,Ē then thatís who they happen to.  The coincidences happen. So we have people, they happen to them all the time because they notice things and they talk to people.  I could sit next to my long lost twin I was separated from at birth and I would never know because Iíd just get up without speaking to them.  So, who knows how many things I've not noticed?  So, what I'm amazed at is not how many coincidences there are but how few there are.  I'm afraid I failed rather on the sitting next to each other.  Got quite close, but there were 6 pairs or 7 pairs of people that shared birthdays in this room, which I think is quite a good coincidence.

Chris -  Any questions for David on the science and study of coincidence?

Anne -  Anne from Canada.  I'm just wondering how you go about saying this.  Are you using numbers and statistics and probability to study how coincidence happen orÖ?

David -  Yeah, I'm a statistician.  I work in the math department here.  So, where possible, we try to do the maths.  So, the people sharing birthdays, I can work out exactly what the chances of various pairings in this thing is.  I know that if there's 23 people in a room, there's a 51% chance to them sharing the same birthday and so on, and so on.  So, I can do all these calculations and sometimes on the pin codes, you can do the maths.  Does anyone share birthday with their parent, same birthday as their mom and dad or anything like that?

Anne -  Anyone got a birthday this year?

David -  Anyone got a birthday shared with a brother or sister?  Who isnít a twin.  I donít care.  That doesnít count.  Thatís cheating.  Because weíve got I think 5 examples of people with us, 3 siblings, all with the same day, but born in different years.  So, they were all born on each otherís birthday.  The chance of that happening, we know is 135,000 to 1, which is rare.  Itís extremely unusual for it to happen, except that it happens every year in this country because there's 160,000 third children born every year in this country.  And just by chance, there'll be one Ė you can expect one to match, have an older brother and sister born on the same day as well.  So, they come up in the news.  If you Google Ď3 births in the same dayí theyíll come up all time and the Daily Mail always gets the odds wrong.  It says itís 48 million to 1, which it isnít.

Chris -  Ladies and gentlemen, David Spiegelhalter.


(Callum) -  Callum from (Stratum).  Are you destined for each coincidence?  So, if you keep crossing paths with someone else, is it destined for you to always meet with them or mix with them?

David -  Whoah!  Thatís a big question.  Thatís quite a tricky one really, because you're really talking about almost predestination, is everything we do actually sort of pre-decided and we donít have to choose ourselves, there's no randomness and no freewill, which is a reasonable argument that some people have got.  The fact that we donít know what's going to happen itís just because we donít know, it is pre-decided, we just donít know what it is.  I donít have a too strong opinion on that at all.  What I say is that the fact that we donít know means that it might as well be random because we donít know, and we donít want to know.  Who would like to know what they're going to get for Christmas?  Yes.  All the kids want to know what they're going to get for Christmas.  Come on.  Live with uncertainty.  But the grownups donít, no, look at this.  Adults are quite happy about it.  Letís say, if I were a great all powerful being that could tell the future and I could tell you how long you're going to live.  Who would like to know?  Yes, look at these kids.  They want to know.  Why do you want to know?  I would say, the young people are the ones who want to know.  Why do you want to know?

Sophie -  Sophie from Blenheim.  I want to know because then if you're going to live a really long life then you could plan it.  But then if you weren't, then you could think, well, what's the most important thing I need to get done in my life and do that first.

Chris - Run up a big bill on the credit card.

David -  Exactly, well done.  If you knew you could plan it Ė so, thatís a very sensible thing to do but most people Ė notice that as you get older, you're actually prepared to embrace a little bit more uncertainty in life and you quite like it.  So, I think thatís a really interesting age profile in that.  But unfortunately, I'm not an all-powerful, all-knowing being, so I can't tell you.


Subscribe Free

Related Content


Make a comment

What I will say is that one notices things that seem out of the ordinary, but they may not be as uncommon as one might think. 

And, the odd events become more noticeable and memorable. 

So, thinking of the sex of children in genetics. 
For a family with 2 children, half of the families will have a boy and a girl.  But, half will also have either 2 boys, or 2 girls.  It is not necessary to postulate a "predisposition" to have boys or girls for such a random event.

Even with 4 children, 1/16 will have 4 boys, and 1/16 will have 4 girls, or 1/8 of the families will have all same sex children. 

What about 10 children?  1/1000 families with 10 children will have all boys, and 1/1000 will have all girls.  Certainly not an impossible coincidence.  Random events could account for the numbers with no supernatural components. CliffordK, Tue, 4th Nov 2014

See the whole discussion | Make a comment

Not working please enable javascript
Powered by UKfast
Genetics Society