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This is the BBC. This

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outside the UK.

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Hello, it's Hannah Fry here and I am

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just dropping in your ears to let

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you know that my new series for BBC Radio 4,

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Uncharted, Tales of Data and Discovery,

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is out now. So for the next 15 minutes

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I'm going to take over this feed to

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give you just a little taste of the podcast.

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Enjoy the first episode.

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BBC Sounds, music, radio,

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podcasts.

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It's 1973 and Britain is

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in turmoil. The year begins

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with a stock market crash

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and quickly gets worse. By

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May, millions are on strike, miners

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and steel workers, postal workers and

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train drivers. Inflation is surging

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and whole industries are

0:54

on the brink of collapse. But

1:03

away from all of the societal unrest, unmentioned

1:06

in the news, something else is going

1:09

on. Something subtler and stranger.

1:12

Something that remained invisible until

1:14

many years later. In

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front rooms and hospitals across the country,

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an unusually high number of baby

1:21

boys

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were being born. We're

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not just talking about a few extra baby

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boys here. In 1973 to 74,

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the ratio of baby boys

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to baby girls born in England and

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Wales was higher than at

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any other time in the 20th

1:38

century. What

1:41

on earth was going on? I'm

1:48

Hannah Fry, a mathematician who studies

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patterns in human behaviour. And

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for BBC Radio 4, this is

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Uncharted, tales of

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data and discovery. This is a

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series about how graphs can help.

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help you to see the invisible, about

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how plots can be rich with hidden

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depths and unheard stories, and

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about how sometimes, if you know where

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to look, there is mystery and

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drama and intrigue to be found,

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all concealed within a few

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lines on a page.

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I was researching my book Sex by Numbers and

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I plotted out

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the sex ratio statistics from 2008 to 2012,

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and as far as I know, nobody had ever plotted

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that before, I've never ever seen it. It's

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just boring numbers in a big spreadsheet.

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But when you draw it, it is one of

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those graphs, one of those delicious graphs,

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where you just think, what is going

2:43

on here? This is extraordinary. This

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is David Spiegelhofer, a statistician

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and sometimes sexologist. He'd

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noticed something odd, something striking.

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Over the 20th century in the UK,

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in a few specific years, the

2:59

proportion of male births spikes

3:01

sharply. Now in general,

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we expect more boys to be born

3:06

than girls. The ratio is not quite

3:08

50-50. It typically hovers

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somewhere around 104 boys born for every 100

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girls. That is a weirdly

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neat trick of nature, because

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it just happens to compensate

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for the fact that boys are more likely to

3:23

die young, from developmental problems

3:26

or from accidents. So

3:28

the imbalance at birth evens out

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over time.

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But that ratio at birth is not

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fixed.

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If you look at each year across the 20th century,

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there are notable moments where

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the ratio of

3:41

boys to girls shoots

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up. The

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first spike arrives in 1919, of

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course just after the end of the First World

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War. The second is in 1944,

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towards the tail end of the Second World

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War. And in 1973, it's to

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its highest point ever. You

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might be tempted to dismiss this line

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as random variation, the messy

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uncertainty of biology writ large.

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And yet this is a pattern that you'll find in

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other

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places too. The sex ratio

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also peaks for the main combatants during

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and just after both world

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wars. You see it in France, in

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Germany, Austria, Belgium,

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Denmark,

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the Netherlands. This

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is such a strangely robust

4:29

observation that it has been given a suggestive

4:32

name, the returning soldier

4:34

effect.

4:35

The returning soldier effect is the observation

4:38

that more boys are born at the

4:41

end of wars. And it's

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just true. It is absolutely true. It's

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just a wonderful mystery. Why? What is

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going on? What indeed?

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There have been no shortage of theories,

4:56

but perhaps the most delightful comes from a German

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pastor in the 1700s

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by scrupulously collecting data from

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parish registers. J. P.

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Sussman explored the kind of questions

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which would eventually develop into the science

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of demographics.

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And he noted for the first time

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that consistently more

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boys were born than girls.

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And then, amid long

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dry columns of population statistics,

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Sussman detected the subtle

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hand of God. In

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his magnum opus, the divine

5:29

order in the changes of the human species,

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he set out the evidence for heavenly

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interference. Here was

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God in the data, quietly overseeing

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births and deaths, carefully managing

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the numbers of boys and girls born, taking

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and giving life according

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to some celestial algorithm, all

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to craft the ideal society.

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And thus, he argued, at the end

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of a war, God must be

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trying to replenish the male stock.

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a country, a divine rebalancing.

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It seems like the perfect explanation,

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but sadly one that doesn't hold up

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to scientific scrutiny. And

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so,

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centuries later, hoping for a

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more modern account, another

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man took up the past as mission, hoping

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to unearth its hidden secrets.

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I came to UCL about 30

6:26

years ago and one

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of the strange people who was

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in the department at that time was

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Bill. I mean, Bill was the kind

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of guy, I was busily working

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away, and I would take a break and

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I'd wander off and I'd bump into Bill. And

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there he was, standing in the corridor

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with a deerstalker hat, his

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trousers in his bicycle clips, and

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he had this wicked

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grin on his face, and

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he would come up to me and tell me some new

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facts that he'd found. This

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is Andrew Pomyankowski, or POM

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as he's known, a colleague and sparring

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partner of Bill, or William James.

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Bill passed away in 2022, and for many years, decades

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in fact, he

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carved out an incredibly niche area

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of expertise.

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He was interested in only one thing. He

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really was a man dedicated

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to one particular question,

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which is,

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what is the human sex ratio

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of offspring? How many males

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and females do couples have?

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And what might explain

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deviations?

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Some people have more

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male offspring and some have

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more male offspring.

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That was his big obsession,

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and it really was an obsession.

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at

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University College London, which meant that

8:02

he could access journals and books and use

8:04

an academic address. His partner

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for almost 60 years, Susan Crichton,

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records how dedicated he was to his research

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to the exclusion of much else.

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He worked in his really squalid basement flat

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and he worked, he had a typewriter, a manual

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typewriter which he kept going for years before

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I persuaded him to move on to a word processor.

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And he was so individual, he'd

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write his papers, he'd type them out and

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then he'd take them to the journals

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that he was trying to get them into. And people

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seemed to like this, people like him turning up on

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the door with a paper. And he got

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a lot of publishers, he was totally committed.

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But you see, he didn't have anything else to do. He managed

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to avoid having to teach, he managed

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to avoid all administrative things, he

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definitely avoided cleaning.

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Bill would cycle around London to sniff

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out new data sources. He corresponded

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with a huge number of friends and colleagues around

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the world in long handwritten letters

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and would

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return to his basement office to pore

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over the evidence that he'd gathered. Could

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it be something to do with the effects of stress

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that

9:13

explained the ratio of boys to girls born?

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Or maybe parental age? Was

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it all down to differences in the father's height?

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He considered them all, but in the end,

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to explain the returning soldier

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effect in particular, he favoured

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one theory more than the others.

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A theory that was all about

9:34

sex.

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Let

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me transport you back

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to 1980. The Armistice which

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ended World War I was signed in November.

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Over the next year, more than 3 million

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British soldiers were demobilised and

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made their way home.

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Think of this vast

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number of newly demobbed soldiers

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being reunited with wives and girls. friends

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after months or years surrounded by

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suffering and death. Well, you

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might expect both parties to be pretty

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jubilant, deeply relieved and

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fantastically horny. And

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okay, that is speculation. It's difficult

10:18

to measure,

10:18

of course, but it is true that

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those soldiers returning in 1918

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and 1919 produced the highest

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number of babies born at any point

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in the 20th century. The

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more than 1.1 million babies born in 1920 remains

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a

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standout, an all-time record

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for the UK.

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So what about 1944? The Second

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World War came to an end in the following year,

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but it turns out that in 1944 there

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were large numbers of soldiers on temporary

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leave, and the lead-up to D-Day

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saw an extraordinary number of new

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marriages.

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But okay, so what? Even if it's true, how would

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the frequency of sex make any difference

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to the number of boys being born?

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Well, Bill had an answer. He'd

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spotted something subtle in the data,

11:08

something which convinced the statistician David

11:10

Spiegelhalter.

11:12

My belief is, follows the work of

11:14

Bill James at University College London, who

11:16

identified that the

11:19

sex ratio is dependent on when in

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the cycle the conception occurs.

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And so if a conception occurs

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early on in the cycle, the ratio

11:28

of boys to girls is higher than later

11:31

on in the cycle. So why should,

11:34

at the end of wars, conceptions

11:37

tend to occur early on in the cycle?

11:39

Because there's more sex.

11:41

So essentially, if you are having a lot

11:43

of sex throughout a period

11:46

of fertility, you're more likely to get pregnant

11:48

early on, because if

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you're pregnant early on, you can't get pregnant later on.

11:52

It turns out that the chances of a woman conceiving

11:55

a male or a female child very

11:58

subtly change

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on when in her cycle she conceives

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slightly earlier and it's ever

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so slightly more likely to result in a

12:07

male child. This might

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be because of the gently fluctuating

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hormonal environment or the delicate

12:13

changes in acidity which will affect

12:16

X and Y sperm differently. We

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are talking about tiny, tiny

12:21

changes to probability here. Certainly

12:23

too small to be seen at the level of an individual

12:25

woman, so you can't use this

12:28

to game the system to your own advantage.

12:31

But fragile as it is,

12:33

the signal is there. It's

12:35

a whisper but it's there. And

12:38

once you scale up to the size

12:39

of a population, there is a pattern

12:42

that starts to become clear. At

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the end of a war, people have much

12:47

more sex than normal, women get

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pregnant slightly earlier and there is

12:51

a spike in the number of baby boys

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who are born. And

12:55

yet

12:56

what of 1973? 1973 wasn't

12:59

a war, there were no returning soldiers.

13:02

Are we to believe that rising inflation, strikes

13:05

and sky-high energy prices were

13:07

turning people on? I mean,

13:10

kind of, yes.

13:12

A lot of things were going on in 1973. It

13:14

was a time of intense sexual activity in young

13:16

people. There was a surge of teenage pregnancy

13:19

and widespread use of contraceptive

13:22

pill was still not there and

13:24

the age of marriage reached its lowest

13:26

ever point. All this says that there was

13:28

a huge amount of sexual activity in this newly

13:31

liberated time among young people

13:34

and a lot of sex means more boys.

13:40

So there you have it, the

13:43

oscillations of this simple line built

13:45

from tiny shifts in the probability of

13:48

conceiving a boy or a girl hold

13:51

countless unheard stories of human

13:53

history. The passion of the soldier

13:55

and his lover reunited for some brief

13:57

period of leave, the jubilation of

13:59

people peace after years of devastating

14:02

war, or a population embracing

14:04

new-found sexual liberation amidst

14:07

political turmoil.

14:09

The faint echoes of a nation's

14:11

libido flexing with the

14:14

force of history.

14:16

I told you graphs were interesting.

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That is it for this episode, but there are

14:23

ten fascinating stories in this

14:25

series, even if I do say so myself.

14:28

So search for Uncharted and subscribe

14:30

on BBC Sounds Now or wherever you

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get your podcasts.