Do humans have mating seasons? This heat map reveals the surprising link between birthdays and seasons

Do humans have mating seasons? This heat map reveals the surprising link between birthdays and seasons
Nayomi Chibana

Written by:
Nayomi Chibana

Aug 15, 2017
do humans have mating seasons? the surprising link between peak birth months latitude and seasons around the world

Animals are by nature seasonal creatures: Deer mate during the fall, so their fawns are born just in time for the spring season when food becomes more abundant. Polar bears look for mates during late spring and early summer and give birth to cubs between December and January.

But what about humans? Are we the only animals that don’t have a true mating season? After all, humans have sex throughout the year, without worrying whether their babies will have enough food to survive the winter.

It appears we do—kind of.

This heat map created by Visme, based on the latest UN data on live births, reveals a surprising link not only between peak birth months and seasons, but also between peak birth months and latitude (which is the distance north or south of the equator, measured in degrees):

Do you notice a consistent pattern across high-latitude countries in the Northern hemisphere? The months with the greatest average number of births per day are July, August and September.

As you move down the list of countries, ordered from highest to lowest latitude, you can clearly observe that the peak birth months shift farther and farther to the right, occurring later in the year.

Once you’ve entered the middle latitudes, or the tropical zone, September and October become the highest-ranking months by average number of live births per day, followed by November and December, with some spilling over into the next year.

At the bottom of the list are the countries in the Southern hemisphere, which register the highest average number of births per day occurring between March and May.

When this data is translated into dates of conception, using a 40-week gestation period, we also see that the peak time for baby making is October in high-latitude countries such as Russia, Norway, Finland and Denmark, as seen in the graphic below made with Visme.

According to this same data, December is another peak conception month in a large percentage of countries, including the United States, United Kingdom, Australia, Canada, Mexico and Japan.

peak times for baby making around the world calendar
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What do these trends mean?

While it is easy to attribute this to the long-held assumption that colder temperatures lead us to find warmth in physical intimacy, possible scientific explanations are a bit more complex.

According to a study published in The Journal of Reproductive Rhythms, the ideal time of the year to conceive is when the sun is out for 12 hours and the temperature is between 50 and 70 ° F. For reasons that are not totally understood, these are the perfect conditions for conception, either because they stimulate sperm production or ovulation--or a combination of both.

The findings of other studies have also been consistent with the insights gleaned from this heat map. For example, a comprehensive analysis of human birth seasonality published in the Proceedings of the Royal Society B concluded that peak birth months occur later in the year the farther south you travel, as clearly seen in the graph below.

Boer Deng, in her article for Slate, also did the math and concluded that countries in northern latitudes were conceiving babies more often during the fall, while those farther south were doing so during the winter, which is also somewhat consistent with our findings.

Other studies such as this one on the seasonal variations in sexual activity concluded that there is in fact an “increase in sexual activity and unsafe sex occurring at or around the Christmas period.” Even an analysis of Google searches, as a proxy measure of sexual activity, indicates that queries related to sex and mating behaviors peaked during the winter and early summer.

All this seems to indicate that there is a confluence of factors, both environmental and biological, at work here and that, in the end, we may not be able to pinpoint one exact reason why these patterns exist.

What we can know for sure is that even though it appears humans may have a quasi-mating season, it is not really a true one as women are receptive to sex year-round and ovulate every 28 days, not annually. Unlike other animals, humans have concealed ovulation because they don't show any outward sign of biological fertility, which is still a mystery to scientists.



To create the heat map above, we filtered the latest UN data on live births by month to obtain figures between 2000 and 2015 (which is the time period with the greatest amount of data for all countries listed).

In order to account for differences in the number of days in each calendar month, we then calculated the average number of live births per day in each month and ranked every month of the year relative to each other.

For example, for the United States, the average number of live births per day for each month was calculated. Each month was then ranked relative to each other in terms of the average number of live births per day, from highest to lowest. Since the month of September registered the greatest average number of births per day for this time period, it was assigned a ranking of 1; August exhibited the second highest total, and so received a ranking of 2; and so on.

The heat map was then created using a color scale corresponding to the rank value of each month, from 1 to 12. This way, the color-coded visualization would allow for easy comparisons across countries and hemispheres. Otherwise, the vast disparities between live birth numbers from one country to another would generate a color scale too broad in scope that would not allow for quick visual comparisons by viewers.


Your Turn

What are your thoughts on these conception and birth trends? Do you think other external factors besides climate and season are involved? Let me know in the comments section below!

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    About the Author

    Nayomi Chibana is a journalist and writer for Visme’s Visual Learning Center. Besides researching trends in visual communication and next-generation storytelling, she’s passionate about data-driven content.

    25 responses to “Do humans have mating seasons? This heat map reveals the surprising link between birthdays and seasons”

    1. Edward Coleman says:

      Fascinating and definitely fun! I wish you’d divided births per month by the number of days in the month so that, for example, you wouldn’t have the February problem. Otherwise I love it! Thanks for your thoughtful analysis and visualization.

    2. Lots of work went into this. Impressive. Very interesting findings too.

    3. Antje says:

      Is there also a difference in weight, size, sex, number of siblings … observable?

    4. Anastas Pashov says:

      What about a tendency of birth dates in close relatives to cluster in season/month? This is a personal observation – I have been collecting examples in the last several years but for a statistical study one would need access to big data of both births and family relations. This is an interesting connection between genetics and season of birth which would suggest to some extent inference of biological/psychological typology from the date of birth. Could it be that some genotypes favor their own mating season? Is this related to culture and social relations? And, no, I don’t justify astrology (neither homeopathy for that matter). Just currious…

      • Interesting hypothesis, Anastas. I’ve never thought of the possibility of those correlations before. Have you published something on this topic yet?

        • Anastas Pashov says:

          No, my studies are in a totally different field. This is just an observation without a statistical backing. I pay attention to this fact whenever I see another confirmation but it may be also a classic case of confirmation bias :). That is why I am wondering which public source I can tap into for the relevant data.

    5. Ian says:

      What isn’t taken into account here is the beginning of the school year – e.g. UK is September; people want their kids to be older when they start school because they get an advantage by a few more months development prior to first attending. This is a problem when you miss the September date by one day and become the youngest, and thus most disadvantaged. Not that I’m bitter or anything, I caught up eventually.

    6. Hiro says:

      Lots of social factors play into this. For instance, many countries have Christmas and/or New Year’s holidays for workers, which means a lot of free time to relaxxx… lol

    7. Sampat says:

      India is the second most populous country and largest diversified country. yest either you idiot chose to omit it from statistics or those UN pot heads decided to ignore Indian live births…
      Either way this whole statistical analysis is bull shit without one of the largest contributors.

    8. Excuse me says:


    9. HMM says:

      Yeah where’s Africa

    10. Jimmmygreit says:

      This is an interesting study. The study can be flipped around to give an insight on the possibility of the time people are having sex most. The seasons and festival in the countries can possibly offer additional information.

    11. Benjamin Dover says:

      it features “Reunion” but doesn’t feature BRAZIL

    12. Zee says:

      Very interesting indeed. Good job you guys. I wonder what the data is across gender

    13. Matt says:

      How’s this for a wild idea. People are more relaxed and have more time for sex when they are on Christmas holidays.

    14. Jun says:

      Very, very interesting. Not only the climatic, environmental, and demographic aspects to consider but also the cultural, religion, and spiritual perspective. Though there is a correlation between culture, spirituality, and the environment, sometimes culture and spirituality can also dictate trends. Anyhow, this data can be useful for birth classes such as Bradley Method or Birth for men and others.

    15. Ben says:

      All very interesting and great visualisation (I love the ordering by northern/southern hemisphere)! Does the fact that countries such as Australia and New Zealand peak in the earlier months imply that this isn’t a Christmas effect, more a summer effect? i.e. do more people want their kids to have birthdays in hotter weather so they can one day enjoy outdoor celebrations?

    16. MOROCCO says:

      where is Morocco.
      this is not around the world , this is around the countries that YOU chose.

    17. Kellty Housman says:

      If you apply Bedfords Law to this data, then that would mean that most people would be born in January, October, November and December (30%) then February would be the next highest, but I am speculative about this because there are only 12 months in a year and it takes at least 50 sets of numbers to have get more accurate data. What are your thoughts?

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