It got me thinking…

Last night on the tube home, I overheard someone say that they have five grandchildren but that because two of them share a birthday, it’s easier for them to remember the birthdays because there are fewer dates to remember. 

Maybe it was because I had forgotten to take my book with me but this simple remark started me thinking about whether it was actually less information for him to remember, given that two of his grandchildren share a birthday. 

Let’s assume that his grandchildren have names beginning with the letters A, B, C, D and E.  It’s just easier :-)  In its simplest form, we have the following table of birthdays…

Grandchild Birthday
Amy Date 1
Belinda Date 2
Colin Date 3
Donald Date 4
Englebert Date 5

Each one of the Date items is a number between one and 366.  Of course there’s a year associated with it too, but from what I could overhear, the guy wasn’t saying that two of his grandchildren were born on the same day – simply that they share a birthday.

So, we know that two of the Date items above have the same value.  But does that actually make it easier to remember?

I’m avoiding couching this discussion in terms of variables and pointers, because this is about how people remember information rather than how computers store information.  People tend to remember facts – some facts are harder to remember than others, admittedly, but it’s not as easily quantifiable as looking at the number of bits which need storing to remember something.

So, this guy knows that two of his grandchildren share a birthday.  We’ll presume that remembering this fact doesn’t take much space at all.  But which two share the birthday?  This is something he does have to consciously remember.  Let’s presume that Englebert and Donald share a birthday.  By knowing this, you’ve not removed the need to remember something about Englebert’s birthday – but you’ve simply replaced the need to remember a specific date with the need to remember that his birthday is the same as Donald’s. So the number of facts which need remembering is actually the same in this case – but is the piece of information “Englebert shares his birthday with Donald” more or less information to remember that “Englebert was born on 15th March” (or whatever).


This isn’t so much a question of computation, it’s a question of psychology, I think.  Subjective evidence makes me think that remembering dates is quite a hard thing to do.  People tend to remember roughly in the year when something happens, but remembering the date tends to take a few minutes.  However, the human brain does have a natural ability to spot patterns (even where none exist, but that’s a whole different discussion) and so remembering that “D and E share a birthday” is something the brain is going to find relatively easy.

So, it looks as though the guy was right – it probably is easier to remember your five grandchildren’s birthdays if two of them share a date.

But what happens if we extend the problem.  Let’s presume that we have so many friends that pretty much every day of the year is someone’s birthday within that group.  It’s fairly easy to see how it now doesn’t really help to remember coincident birthdays as the set of birthdays from which to choose is actually almost as large as the number of days in the year.  So in the general case, this isn’t going to help us remember birthdays.

However, there are some birthdays we don’t forget.  We tend to know the birthdays of our parents, siblings and children.  So that’s going to be around half a dozen birthdays throughout the year to which we can pin other things.  “Sheila has the same birthday as my mother” is quite easy to remember, and your mother’s birthday is largely imprinted in your mind so doesn’t need consciously remembering in the same way a friend’s birthday does. 

Of course, to take another degenerate case if everyone you know has the same birthday, then it becomes a case of remembering only two facts – 1. everyone has the same birthday and 2. the date of that birthday.

Of course, the distinction between something which you need to consciously remember and something which is a mental landmark in your memory is not quite that clear cut.  Some things are easier to remember than others, and once you have more than a few friends with a particular birthday, that day will start to stick in your mind as “the day on which lots of my friends have their birthday”. 

So, the way to remember your friends birthdays is to seek out friends who share birthdays – preferably sharing those birthdays with close family members or other memorable dates such as Christmas or 29th February.

Alternatively, just buy a calendar :-)

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