Humans think they are special: That we can just breed whenever we want and nine months later pop out a baby. But that doesn’t seem to be completely true.
Birth rate varies by the time of year with one exception: The time of year when birth rates are the highest seems to be determined by your latitude, generally falling in the warm (but not blistering hot!) months, such as late spring or fall.
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 locations are colour-coded based on their affiliation, either with Europe, the Americas, or specific U.S. states (light green triangles). You can see how birth rates peak later in the year in states that are closer to the equator.
For example, in the most northern areas of the world, birth rates peak in June. In the more southern areas of the world, the peak hits in October or even November. As you can see this is especially prominent in the different US states — Northerner birth rates peak in June, while Southern states peaks in November.
We don’t know exactly why, but a few ideas include: Income, culture, race, holidays, rainfall, cold winters and seasonally variable sperm quality.
The chart is from a new study in the journal Proceedings of the Royal Society B: Biological Sciences. The researchers say it’s “the most extensive spatiotemporal data set on human births to date.”
They actually used the data to analyse the best time of the year to vaccinate for measles, based on when birth rates peak in developing nations.
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