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.