Birth rates often peak in September, and a new study may have uncovered one reason why: the Christmas period seems to spark our in interest in sex.

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Researchers reveal that our interest in sex rises at Christmas, and this correlates with a peak in birth rates 9 months later.

Having conducted an analysis of Google search terms and social media posts from almost 130 countries over a 10-year period, researchers found that there is a significant peak in sex-related search terms during major religious or cultural celebrations.

According to study co-author Luis M. Rocha, of the School of Informatics and Computing at Indiana University in Bloomington, and team, these peaks in sex-related terms correlate with happier and calmer moods, as well as peaks in birth rates 9 months later.

The team notes that the study is the first "planetary-level" analysis of the link between online sexual interest and mood and human reproduction.

"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 Rocha.

The researchers recently reported their results in the journal Scientific Reports.

Utilizing Google Trends and Twitter

The team reached its findings by analyzing Google Trends data, which enabled the researchers to look at the number of searches for sex-related terms across 129 countries from 2004 to 2014.

Additionally, the researchers looked at the number of sex-related Tweets that had been posted on the social media platform Twitter across seven countries — including the United States, Brazil, and Australia — between late 2010 and early 2014.

The analysis revealed that the number of sex-related search terms and Twitter posts peaked during religious and cultural holidays, including Christmas and Eid-al-Fitr, which is the religious holiday celebrated by Muslims to mark the end of Ramadan.

Interestingly, these increases in sex-related search terms and posts corresponded with peaks in birth rates in countries that celebrated such holidays, and these correlations remained even after accounting for geographical locations.

According to Rocha, "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."

Strengthening their findings, the scientists point out that the date of Eid-al-Fitr changes each year, yet the observed link between greater online interest in sex and peak birth rates followed these date variations.

What's behind the increased interest in sex during religious and cultural celebrations? According to the researchers, it's down to how these celebrations impact our mood.

Mood may heighten sex interest at Christmas

As well as investigating sex-related terms, the researchers took to Twitter to conduct a "sentiment analysis" — that is, they used Tweets to pinpoint certain moods.

This revealed that we tend to feel happier, calmer, and safer over religious and cultural holidays — particularly during Christmas and Eid-al-Fitr — and that these moods correlated with the increased online interest in sex.

"We observe that Christmas and Eid-Al-Fitr are characterized by distinct collective moods that correlate with increased fertility," says Rocha.

"Perhaps people feel a greater motivation to grow their families during holidays when the emphasis is on love and gift-giving to children. The Christmas season is also associated with stories about the baby Jesus and holy family, which may put people in a loving, happy, 'family mood.'"

Luis M. Rocha

In many countries that celebrate Christmas, including the United Kingdom, September is the most popular birth month, and Rocha and his colleagues believe that their findings may help to explain why this is.

The team also says that its results could help to streamline safe sex campaigns in developing countries.

"The strong correlation between birth rates and the holidays in countries where birth-rate data is available — regardless of hemisphere or the dominant religion — suggests these trends are also likely to hold true in developing nations," says Rocha. "These types of analyses represent a powerful new data source for social science and public policy researchers."