Shiny happy people means more... boys?

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Happy people give birth to boys.

That sounds crazy, but there's a fair amount of evidence that positive or negative events can change the ratio of boys to girls being born.

Hosting the 2010 World Cup caused a boom in male babies in South Africa nine months afterward, according to a recent paper by researchers in Johannesburg. Conversely, the death of Princess Diana or the 9/11 terror attacks were shown to reduce the percentage of males compared to total births — the so-called M/F ratio.

Economics can also shift the ratio. Declines in male babies have been correlated with higher unemployment rates, and according to another recent paper, the ratio plummeted going into the Great Recession — suggesting that the M/F ratio functions as a lagging indicator of economic well-being.

The Big Crunch took a look at the data.

The paper found that the ratio fell significantly in the second half of 2007 compared to the first half — but that's not surprising given that the birth ratio tends to be higher in the first half of the year.

In general, humans tend to give birth to slightly more boys than girls, and in the U.S. more boys tend to be born during the early spring and summer months than the autumn and winter. The reasons for that seasonality are not well-understood.

More importantly, the paper found that the decline in the second half of 2007 was statistically significant compared to the year after and "almost significant" compared to the year before — indicating that the start of the recession may have had an effect. (Technically, the recession did not start until December 2007, but warning signs like plummeting home prices, downgraded bonds and lender bankruptcies took place months before.)

Looking at our chart above, it looks like the ratio was also lower in later years, which may or may not be economics-related.

The Big Crunch looked at the same numbers in a different way, using rolling averages to smooth out the variation. A three-month average more clearly shows the seasonal variation in the ratio, and a yearlong average shows a clear dip in the 1 ½ years afterward when the recession was at its worst.

The change may seem small, but those small percentages add up fast. The dips in the second halves of 2007 and 2008 may be only a little more than a tenth of a percentage point deeper than the four years before, but that's thousands of fewer boys than would have otherwise been born.

So what makes this strange indicator work?

There are two explanations. The first is that "coital frequency" goes up when people are happy. That could make a difference because women are more likely to have a boy if they conceive at the edges of each fertile period — and hitting those windows is more likely the more you try.

The other hypothesis is that stress causes pregnancies to fail after conception, and that male pregnancies could be more fragile and less likely to make it to term — perhaps for evolutionary reasons.

That idea is supported by data that show that declines in the M/F ratio often show up three months after a stressful event, not the nine months you'd expect if it was simply a matter of increased conceptions.

Either way, this indicator isn't one that will be much use for measuring the state of the economy. U.S. birth data only show up after a substantial lag — the most recent final data available now is from 2013. And it is also affected by other variables that cause it to change over time — like the age that women give birth.

For the curious, here's the yearly birth ratio data going back to 1940. The ratio has been declining over time in the U.S., perhaps due to demographic changes and Americans giving birth at older ages. While it may not always line up with economic data, there are noticeable dips around the times of the 1991 and 2001 recessions.