The post-holiday birth spike isn’t caused by any hard-wired biological drive — its causes are social.

Image credits Jill Wellington.

Nothing says Christmas cheer quite like getting a present — except, perhaps, getting it on. It’s an oft-remarked (but still funny) observation that birth rates tend to peak in September, nine months after the holidays. Up to now, research has focused on linking this post-holiday “baby boom” to seasonal changes in human biology. But a new paper from the Indiana University and the Instituto Gulbenkian de Ciência in Portugal raises an interesting question: what if this isn’t a result of biology, but of society?

Under the mistletoe

To find out, the team probed into the “collective unconscious” of web searches and Twitter posts to get an idea of how people feel during the holidays. Their analysis extended to almost 130 countries and looked at sex-related Google search terms from 2004 to 2014, as well as about 10% of all public Twitter posts over the same period.

The analysis revealed that interest in sex peaks “significantly” during major cultural or religious holidays (corresponding to a greater use of the word “sex” or other sexual terms in web searches). For the countries included in the study with available birth-rate data, this peak in sexual interest corresponded broadly with an increase in births nine months later.

“The rise of the web and social media provides the unprecedented power to analyze changes in people’s collective mood and behavior on a massive scale,” says co-lead author Luis M. Rocha, a professor in the IU School of Informatics, Computing and Engineering. “This study is the first ‘planetary-level’ look at human reproduction as it relates to people’s moods and interest in sex online.”

To make sure they weren’t picking up on an effect of biology and mistaking it for a social one, the team included multiple cultures in their study — and this correlation held. The greatest spike in sexual and sex-associated interest occurred during major holiday celebrations: around Christmas in Christian-majority countries, respectively Eid-al-Fitr, the celebration that marks the end of Ramadan, in Muslim-majority countries. The results for Eid-al-Fitr are especially significant because this holiday doesn’t have a fixed date and falls on a different day each year. However, the effect shifted in accordance with the holiday — a very strong argument in favor of its cultural roots.

Everybody’s getting festive

Moreover, the team included countries from both the Southern and Northern hemispheres. This was done in an effort to address a common limitation of previous studies on this subject, which tended to focus on smaller geographical contexts, usually in Western countries in the Northern hemisphere.

Despite the fact that seasons are reversed on opposite sides of the globe, the spikes in online sexual interest and birth rates occurred at the same time everywhere. The researchers thus conclude that the effects aren’t tied to a biological shift caused by changes in daylight, temperatures, or food availability.

“We didn’t see a reversal in birth rate or online interest in sex trends between the Northern and Southern hemispheres — and it didn’t seem to matter how far people lived from the equator,” Rocha said. “Rather, the study found culture — measured through online mood — to be the primary driver behind cyclic sexual and reproductive behavior in human populations.”

To understand what drives this increase, the team reviewed word choices in public Twitter posts (a method known as a “sentiment analysis”) which revealed that the causes are quite simple — as a whole, we just feel happier, safer, and calmer during the holidays. When they went looking for similar collective mood patterns at other times of the year, the researchers found that these too link to an increase in online sexual interest. Thanksgiving and Easter, however, seem exempt from this improvement in mood.

Rocha adds that it’s possible people feel a “greater motivation to grow their families” during the holidays. It’s not hard to see why — there’s a great cultural emphasis on children and gift-giving to children during this time of the year. Christmas especially evokes the ideas of birth, children, and family (baby Jesus), all of which helps put people in a loving, “family mood”, she explains.

The findings are more than an interesting tidbit. Rocha says that the results can be used by public health researchers who are trying to determine the best times to launch public awareness campaigns for safe sex in developing countries, which, more often than not, lack reliable birth-rate data.

“These types of analyses represent a powerful new data source for social science and public policy researchers,” she concludes.

The paper “Human Sexual Cycles are Driven by Culture and Match Collective Moods” has been published in the journal Scientific Reports.