科学美国人60秒:Holiday Cheer Leads to Birth-Rate Spike

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 U.S. and in other predominantly Christian countries have been having some very merry Christmases indeed. That’s according to a study in the journal Scientific Reports. [Ian B. Wood et al, Human Sexual Cycles are Driven by Culture and Match Collective Moods]

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.”

Luis Rocha 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 associated with different religions have birth peaks at different times of the year.”

To try to separate the cultural from the biological, Rocha teamed up with Joana Gonçalves-Sá of the Gulbenkian Institute of Science in Portugal. Together, they combed 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.

But they didn’t look at when babies are born. They looked to see when, during the year, people around the world Google the word “sex.” Joana Sá:

“What we found, first, was that Google…searches for sex on google…are a very good proxy for sexual appetite and sexual, offline sexual interest. And when we looked at close to 130 countries around the world, what we saw is that each country has a particular signature with peaks and valleys of interest in sexual content.”

Sá says that those patterns were most similar for countries that shared a similar culture…

“This means that if you live in a culturally Christian country, whether you live in the Northern Hemisphere or the Southern Hemisphere, you are more likely to have an increase in sexual appetite around Christmas. But if you live in a Muslim country, you are much more likely to conceive around Eid-al-Fitr, than at a different, at another time of the year.”

Now, if you’re still wondering what’s so special about these holidays…Rocha says interest in sex coincides with a particular mood…a finding he made with the help of Twitter.

“We collected close to 50 billion tweets and showed that, independently of the geography and independently of the particular culture, when we observed the surge of interest in sex we also observe the appearance of a particular mood that can best be classified as calm and happy and de-stressed mood that is maximized around Christmas for Christian countries and maximized at the end of Ramadan with Eid for Muslim countries.”

All of which makes the following year’s holiday family gathering just a little bit bigger.

—Karen Hopkin