By Amanda Macmillan /
December 22, 2017
TIME Health
For more, visit TIME Health.

More babies are born in September than any other month in the United States, which means that nine months prior—right around Christmas and New Year’s—is the most popular time of year for conception. Many scientists think this spike in fertility is a biological response to changing seasons. As temperatures drop and nights get longer, the theory goes, humans begin to hunker down, partner up and think more about sex.

But a new study in Scientific Reports suggests that all the post-holiday baby bumps have more to do with culture and society than with biology. Using data from all over the world, researchers at Indiana University and the Instituto Gulbenkian de Ciencia in Portugal found that interest in sex peaks around major holidays, regardless of the season.

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To investigate mood and interest in sex, researchers looked at Google Trends data from 2004 to 2014, and Twitter data from 2010 to 2014, in nearly 130 countries. In predominantly Christian countries, they found that web searches for the word sex were highest around Christmas—even in countries in the Southern hemisphere, like Australia and Argentina, where Christmas takes place in the summer.

In majority Muslim countries, web searches for sex spiked around Eid-al-Fitr, a major holiday that marks the end of Ramadan. This was particularly interesting, say the researchers, since Ramadan is based on a lunar calendar and is observed during different seasons, depending on the year.

The study is the first “planetary-level” look at human interest and desire as they refer to sex and reproduction at different times of the year, says co-lead author Luis Rocha, professor of informatics and associate professor of cognitive science at Indiana University. And it offers strong support for the idea that interest in sex peaks during major cultural or religious celebrations, he says.

Online interest in sex doesn’t just mean people searching for pornography. “We saw an increase in people searching for general sex knowledge—including medical terms, terms about contraception, and so forth,” says Rocha. “And that increase correlated very well with the increase in births nine months later.”

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The researchers can’t say why, exactly, sex is on the brain more than usual during the holidays, but there are a few compelling theories. Their Twitter analysis offers some additional clues about what we’re thinking of at this time of year. Interest in sex correlated with an increase in tweets that used vocabulary related to feeling happy, safe and calm.

“Every time this mood manifests itself on Twitter, it leads to more searches for sex,” says Rocha. “We can only speculate right now, but it may be that when people are feeling happier and less anxious—at the end of the year and around the holidays, in this case—they are more likely to think about starting a family.”

Other theories have it that the holidays are a time for celebrating, social gatherings, and increased alcohol consumption—and that people who don’t want to be alone may be more likely to seek out partners during the season.

But the new study did not find similar trends in birth rates following other major holidays, such as Thanksgiving in the United States or Easter in Germany and France. “These other holidays involve food and family and drinking, so it seems like it is something more than just those elements,” says Rocha.

Another possibility, he says, is that Christmas and Eid-al-Fitr are both very family-oriented; both involve giving presents to children, and Christmas centers around the birth of Jesus. “Maybe people feel more of an urge to grow their families when they are in this type of environment,” he says.

Rocha hopes his team’s future research will answer some of these questions. The findings may also have public health and policy implications: They could help officials plan more effective campaigns around safe sex at certain times of the year, he says, especially in developing countries lacking reliable birth-rate data.

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