At Christmas, we’re happy and Googling porn—9 months later, there’s an uptick in births.

‘Tis the season to be frisky. According to a new study this week in Scientific Reports, Christmas in the US often sees a spike in Google searches for sexy time topics and a surge in Tweets dripping with language denoting happy, care-free moods. The online frolicking is followed by a seasonal uptick in births about nine months later in September. And the US isn’t alone in this trend: the study shows other majority-Christian countries see similar climaxes in Internet romping around celebrations of the birth of Jesus. And majority-Muslim countries see them around Eid-Al-Fitr, a joyous religious celebration marking the end of Ramadan.

The authors of the study suggest that the online data may show, once and for all, that cultural factors—i.e. happy holidays—explain the yearly cyclical patterns of human reproduction. For decades, researchers have debated whether we’re seduced en masse by such cultural factors or whether collective moods are swayed by biological factors—responses to changes in daylight, temperature, and so on—or if it’s actually some mix of the two.

Global explanations for baby boom cycles have been tricky to figure out. Many countries have spotty data on births, and the study authors note it’s difficult to glean conclusions from things like condom sales and upticks in sexually transmitted diseases. But the advent of Google and Twitter data, they say, is a gift to human reproductive studies.

The authors, led by Luis M. Rocha at Indiana University and Joana Gonçalves-Sá of the Instituto Gulbenkian de Ciência in Portugal, suggest that the worldwide links between births, collective moods, and interest in sex “provide strong converging evidence for the cultural hypothesis.”

Further, they conclude:

“The observed peaks of interest in sex occur around family-oriented religious holidays, across different hemispheres and cultures, and the measured collective mood on these holidays correlates with interest in sex throughout the year, beyond these holidays.”

Oh come all ye faithful

For the study, Rocha, Gonçalves-Sá and colleagues analyzed Google Trend data between 2004 and 2014, normalized by search volume. The researchers looked at changes in searches related to sex, mostly search terms like “sex videos,” “sex porn,” “porn,” and “video sex.” Of the 70 majority-Christian countries that celebrate Christmas on December 25, fully 91 percent saw a significant increase in sex searches the week of the holiday. Notably, the researchers did not see a six-month shift in search-term patterns in majority-Christian countries in the Southern Hemisphere. That might have been the case if sex interests were linked to weather- or season-related biological factors rather than the “holiday effect.”

The authors found a similar increase in sex searches around Eid-al-Fitr in 77 percent of 30 Muslim-majority countries examined. This trend is particularly favorable for the cultural over the biological hypothesis because the Muslim calendar does not follow the solar calendar.

That said, the researchers didn’t see such trends around other religious and cultural holidays, such as Easter and American Thanksgiving.

The authors turned to Twitter to try to understand the collective moods that linked specifically to the arousing holidays. Parsing language in Tweets from seven countries (Australia, Argentina, Brazil, Chile, Indonesia, Turkey, and the US), the researchers scored the use of words that evoke feelings of calm vs. excited, controlled vs. in-control, and sad vs. happy. They found that during holidays with upticks in sex searches, public moods shifted relatively strongly to “happy” bins and away from “sad” ones. They also saw shifts toward “calm” bins but no clear pull toward “in-control” or “controlled.”

The study only reveals correlations, but the authors speculate that Christmas and Eid-al-Fitr specifically put people in a loving, family mood. "Perhaps people feel a greater motivation to grow their families during holidays when the emphasis is on love and gift-giving to children,” Rocha hypothesized in a media statement.

Though researchers will need to conduct more studies to back that up, Rocha argues that the online data studies are valuable. “These types of analyses represent a powerful new data source for social science and public policy researchers."