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In the 1970s, the number of boys versus girls born reached an all-time high in England and Wales, amidst mass strikes, surging inflation and industrial chaos. But for a long time, the reason why was a total mystery.

There are always more boys born than girls

Even in a normal year, around 104 boys are born for every 100 girls. But what’s more fascinating – and tragic – is that this ratio evens out to around 50:50 over time, because boys are more likely to die young from accidents or developmental problems.

The birth sex ratio is a fluid, ever-changing thing. In some years, the number of boys born compared to girls is even higher.

Spikes in the number of boys typically happen at the end of wars

This phenomenon has been dubbed the “returning soldier effect”. It can be seen after both world wars, when more babies were born and even more of them than usual were boys. In fact, soldiers coming home after World War One fathered the highest number of babies in the 20th century, with an incredible 1.1 million births in 1920.

This pattern isn’t only limited to England and Wales – data from other countries shows the same thing. France, Germany, Austria, Belgium, Denmark and the Netherlands all saw an increase in the ratio of new-born boys to girls in the years after the world wars.

But while it makes sense that more babies would be born when soldiers came home to their wives, girlfriends and lovers, that doesn’t explain why so many of them were male.

Pastor JP Süssmilch was the first to notice that more boys are born than girls, after painstakingly poring over parish registers. His explanation for the returning soldier effect was a celestial one: he believed that God was carefully rebalancing the population to make up for the men lost in battle. However, this neat theory collapses under scientific scrutiny.

One man dedicated his life to solving the mysteries of birth sex ratios

Statistician Bill James, who died in 2022, made it his mission to understand why some people have more male children and others have more females. With an honorary appointment at University College London, he pursued answers with remarkable single-mindedness for decades.

James searched for evidence in academic books and journals, cycled around London looking for new data sources, and corresponded at great length with friends and colleagues around the world. He considered a huge range of potential influences on sex ratios, from stress to parental age and paternal height, and his findings were often published in journals.

When it came to the question of why more boys were born after wars, James had a particularly persuasive hypothesis: he thought it was down to the fact that people in these years had sex more often. Here’s why…

When conception happens early in a woman’s cycle, they’re more likely to have a boy

Bill James had observed that the chances of a woman having a boy or a girl change very slightly throughout her menstrual cycle. If she gets pregnant earlier on, she’s marginally more likely to give birth to a male child. (This might be to do with changing hormones or acidity levels at different points in a woman’s cycle.)

The theory goes that during times when lots of people are having sex more often, such as when men have just got back from a war, women are more likely to get pregnant sooner, so more boys will be born. On an individual level, these variations are so small they’re irrelevant. But across hundreds of thousands of pregnancies, the tiny differences in probability stack up.

But what about 1973 and 1974?

No one was returning from war in the early 1970s, so why did the number of boys peak then too? It appears to come back to the same thing: amidst strikes, spiralling inflation and sky high energy prices, people were having a lot of sex. David Spiegelhalter explains that at that time teenage pregnancy surged, the age of marriage dropped and social attitudes loosened, all of which indicates that young people were more sexually active. And, as he puts it, “A lot of sex means more boys.”

These fascinating insights all stem from a few data points on a graph. To hear more thrilling tales from the world of statistics, listen to the full series of Uncharted now.