It’s more than just eggnog, mistletoe, chilly days, and long nights.

Given that September is the month with the highest number of births in the United States, the most common time of year for conception is nine months earlier, right around Christmas and New Year’s. There is a widespread belief among scientists that this increase in fertility is a natural reaction to seasonal changes. According to the theory, people start to cozie up, form relationships, and think more about sex as the weather gets colder and the nights grow longer.

However, a recent study published in Scientific Reports contends that culture and society, rather than biology, are more to blame for all the post-holiday baby bumps. Regardless of the time of year, interest in sex peaks around significant holidays, according to research from Indiana University and the Instituto Gulbenkian de Ciencia in Portugal.

Researchers examined Google Trends data from 2004 to 2014 and Twitter data from 2010 to 2014 in nearly 130 countries to investigate mood and interest in sex. Even in Southern hemisphere nations like Australia and Argentina, where Christmas falls in the summer, they discovered that word sex searches on the internet peaked around Christmas in nations with a large Christian population.

Web searches for sex increased around Eid-al-Fitr, a significant holiday that signifies the end of Ramadan, in countries with a majority of Muslims. According to the researchers, this was especially intriguing because Ramadan is based on the lunar calendar and is observed during various seasons depending on the year.

According to co-author and Indiana University associate professor of cognitive science Luis Rocha, PhD, the study is the first “planetary-level” examination of human interest and desire with regard to sex and reproduction at various times of the year. It also provides strong support for the idea that sex interest peaks during major cultural or religious celebrations, he claims.

People looking for pornography aren’t the only ones who are interested in sex on the internet. “We saw an increase in people searching for general sex knowledge, including medical terms, contraception terms, and so on,” Rocha says. And nine months later, there was a strong correlation between that rise and the rise in births.

The researchers are unsure why sex is on people’s minds more than usual during the holidays. However, their Twitter analysis provides some additional insight into what we’re thinking about at this time of year. Interest in sex was associated with an increase in tweets containing vocabulary associated with feelings of happiness, safety, and calm.

According to Rocha, when this mood appears on Twitter, more people search for sex. “At the moment, we can only make wild guesses, but it might be that people are more likely to consider starting a family at the end of the year and around the holidays when they are feeling happier and less anxious.”

According to other theories, the holidays are a time for celebration, social gatherings, and increased alcohol consumption, and those who don’t want to be alone may be more inclined to look for partners during the season.

However, the new study did not discover comparable trends in birth rates after other significant holidays, such as Easter in Germany and France or Thanksgiving in the United States. It seems like there is more to these other holidays than just those three things because they involve food, family, and alcohol, according to Rocha.

He suggests that another explanation is that both Christmas and Eid-al-Fitr are very family-oriented holidays that emphasize gift-giving to children and celebrate the birth of Jesus. “Perhaps people feel more compelled to grow their families in this type of environment,” he speculates.

Rocha hopes that future research by his team will provide answers to some of these questions. The findings may also have implications for public health and policy: He argues that they could aid officials in developing nations that lack access to accurate data on birth rates in planning safer sex campaigns during specific times of the year.