Do humans have a mating season? Turns out we do—sort of.

Most animals mate at a certain time of year. One of our evolutionary advantages is we can continuously mate and have young. However, if you look at birth patterns, you’ll notice that most birthdays tend to occur at certain times of year. For a mammal with no official mating season, it’s surprising that the majority of births occur between July and September. September is the most common birth month in the US, according to one Harvard study, with September 16th being the most common day. Count back the months and you’ll realize that these babies were conceived around the holidays.

Some call this our “mating season.” But in fact, births peak two times a year, around the holidays, and again in late spring to early summer. How do we know this? More children are conceived at these times, more STDs are diagnosed and treated, and more condoms are purchased. Abortions also peak at these same six month intervals. A 2001 study suggests that late autumn and early winter are the best times of the year for sperm health, as they are when men are more likely to have a higher sperm count. While from August to October, sperm counts are at their lowest.

Researchers have also seen a particular pattern in Google searches. More sexually related searches occur during the holidays and in early summer than at other times of year. Dating terms are also more commonly searched, too. Researchers looked at patterns occurring over five years. They examined searches related to topics such as dating, pornography, and even prostitution.

The fact that babies are often born at certain times of year makes scientists think we may have something of a “mating season.” But are there biological factors at work or others?

In terms of psychology, during the holidays, when the weather begins getting cold, and we aren’t used to it yet, we may long for the physical warmth a partner provides. Women’s bodies are designed to keep their core warm, where the womb and organs reside. Ever notice a woman’s hands and feet tend to be cold in winter? This is why. As a result, the desire for a partner at this time of year, may be stronger for them. But yearning for physical warmth might be sublimated into a desire for romance, at least according to a 2012 study, published in the Journal of Consumer Research. More romance novels and movies are consumed during late fall and early winter than at any other time of year, the study concluded.

Since the holiday season is the most common time for people to couple up, it’s being called “cuffing season,” meaning you cuff yourself to someone, in the romantic sense. Around this time of year, we often focus on relationships, be they with friends, family, or someone special. Singles can feel lonely around the holidays. Plus, there are a ton of parties and social events and a lot of people don’t want to go alone. Family members too are known to nosily inquire about the love lives of single relatives, which can act as a motivating factor.

Even so, according to scientists, this isn’t exactly a mating season. For instance, a woman may be receptive to sex regardless of what time of the year it is. They ovulate not annually, but every 28 days. Evolutionary biologists aren’t exactly sure why humans have this unique ovulation mechanism. It conveys an advantage, to be sure. But why it developed is still a mystery. It may have been so the woman can shed her endometrial lining and ward off infection, thus preserving fertility. STDs were a scourge among our early ancestors. Another theory is that it was a way of reducing mating disputes between partners in the band or group.

Several factors may be in play, pushing people together at certain times of year.

Some studies suggest that the high birth rate occurring around June might be due to climate conditions. Sunlight exposure and warmer temperatures can help improve the rate of sperm production, and thus the likelihood of conception. Other studies suggest that hormonal or menstrual changes in women might increase the conception rate at this time. These are hard to prove however, as it’s difficult to reproduce seasonal conditions in a laboratory environment.

While humans can mate all year long, other female mammals have an estrous cycle. This is when they’re “in heat.” Changes in the animal’s physiology and behavior occur. It only happens once a year. But a woman’s sex drive can be active at any time of year. For this reason, some sex researchers reject the idea that seasonal changes affect humans.

Instead, the temperatures cooling down and people spending more time indoors together may mean couples cuddling up more and so a higher likelihood of things getting steamy. Some research has shown that infidelity is lower in winter months. Even so, on the whole, pregnancy isn’t very predictable. Though we may not have a hard and fast “mating season.” Being the complex creatures that we are, there could be environmental, social, biological, and psychological factors all working in concert, giving us a tendency to mate and conceive at a certain times of year.

To learn more about the quasi-human mating season(s), click here: