This is Scientific Americans 60 Second Science.

I'm Karen Hopkins.

In the United States, there's a holiday that goes hand in hand with romance.

So much so that nine months later, there's a spike in the number of babies born.

Valentine's Day? Wrong. It seems that people in the US and other predominantly Christian

countries have been having some very Merry Christmases indeed.

That's according to a study in the journal Scientific Reports.

Scientists have long wondered why in Western countries birth rates spike in September and early

October. The prevailing hypothesis for this phenomenon postulates that there is a

biological adaptation to the solar cycles.

Louise Russia of Indiana University co-led the study.

He notes that nine months before this baby boomlet is the winter solstice,

and when the days grow shorter and the night grows long,

well, humans seem to turn to procreation for recreation.

However, this hypothesis was built on observations pretty much restricted to northern hemisphere countries and also culturally Christian countries.

And some data suggested there might be something cultural going on.

So for instance in Israel, it was previously observed that communities are associated with different religions have birth peaks at different times of the year.

To try to separate the cultural from the biological, Russia teamed up with Joanna Gonsalv Saa of the Galbanian Institute

of Science in Portugal.

Together, they comb through data on a planetary level,

comparing countries in the northern and southern hemispheres

and countries with predominantly different cultures, in this case Christian and Muslim.

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