Why Northerners Are Born Earlier In The Year Than Southerners

Human birth rates peak at different times of the year, based on location, a huge new data set shows.

Researchers at the University of Michigan digitized 78 years of monthly birth records from every U.S. state to analyze when birth rates peak in different places.

Here's the modern era data from different states in the U.S. The yellowest states peak in early June, while the darkest green states, like Florida, peak in early November.

The striped state - Arkansas - is an outlier, with two birth peaks. This was more prevalent in earlier eras, specifically 1931 to 1945 (see chart below). You can see that the closer to the equator the state gets, the later in the year the birth rate peaks.

Within the U.S. the difference between Northern and Southern states is pretty striking.

Here's the bigger chart, with data from earlier years and a graph showing rate increase (over average rate - say, 10 % or 15 % more babies) compared to latitude.

The difference between "peak" and "off peak" baby months was much higher in the pre-baby boom era. Speculating, this could be an effect of modern medicine and birth control, or perhaps something like the advent and spread of modern technologies like air conditioning.

Interestingly, this pattern wasn't seen outside of the U.S. which means it might be a localized phenomenon or strongly correlated with social, economic and/or cultural factors.

This isn't a new idea: we've known that, in the U.S. birth rates peak around the fall. And we've seen similar peaks in other countries.

But we've never looked at it on a state level and we've never had data before comparing so many nations before. This is the largest birth data set ever compiled. More than 750 million births from more than 200 countries were included in the new analysis.

See the chart below, which contains data from more than 730 million births in the last 78 years from the Northern Hemisphere. The right-to-left axis is the month of the year - from March (3) to December (12). The bottom-to-top axis is the latitude from the equator of the country or state in question.

The researchers pose a few ideas for why this might be:

Given the robustness of birth seasonality as a global phenomenon of contemporary human populations, it is surprising that mechanisms driving these patterns remain poorly understood.

Demographers have implicated a host of social, environmental and physiological factors that may interact to drive birth seasonality.

While a consensus has yet to be reached, and mechanisms vary geographically, hypothesized drivers include income, culture, race, holidays, rainfall, cold winters and seasonally variable sperm quality.

The data was actually compiled for a new study in the journal Proceedings of the Royal Society B: Biological Sciences, to analyze how big of an effect vaccination time would have on measles epidemics. When there's a baby boom, there's a big new vulnerable community of babies, which the researchers say acts as 'kindling' to fuel epidemics.

"There are predictable times of year when we know there are going to be more infants being born, and we hope that in the future this information will be used to help control epidemics," she said.

They found that timing the vaccinations at the right time related to birth peaks could help alter the size of a measles epidemic.