Hello this is Tim Harford.

Thank you very much for downloading the shorter World Service edition of more or less.

First broadcast on Friday the 2nd of September.

This week we'll be talking about the mathematics of menstruation.

And if you've heard our Radio 4 edition of the program,

you will have heard this item before.

Hello and welcome to more or less on the BBC World Service.

We're your weekly guide to the numbers in the news and in life and I'm Tim Harford.

Now on more or less we don't shy away from examining any topic. We'll look at statistical

claims from across the world and across the political spectrum. We'll challenge government

policy, big business, charities. But today, well today we're going to be talking about

menstruation because why not? Charlotte, our producer, has spent the last few weeks

trying to answer this question. Do women's periods sink together when they live in

close proximity? The reason I was interested in looking at this question is because this is such a widely held view among people I know. This idea that when

women spend a lot of time together their bodies are somehow influencing each

other and they begin to menstruate at the same time.

It's certainly something my mom used to believe.

Now Charlotte, anecdotally, this is totally true, right?

I have.

You and you and fellow reporter Elizabeth did a straw poll of your friends and what

do they say?

It's pretty conclusive I'd say.

...