The Popular Annual Myth Of December 11th

Today is December 11th, and the chances are that at some point today you've seen at least one of the many articles doing the rounds informing you that December 11th is the most fertile day on the calendar in the UK. There are articles (albeit old ones) in The Telegraph, The Daily Mail, The Guardian, and The Times being shared around Facebook, all making the claim that more babies are conceived on December 11th than any other day of the year, and that apparently this happens year on year.

Despite appearing in reputable newspapers, my initial instinct was that this story is poppycock, and that what we are seeing with this account is lazy, uncritical journalism. Save for one of those extremely rare coincidences that one expects every now and then when the law of large numbers throws up some irregular statistical patterning, the probability of the same day each year being the most fertile day of the year is vanishingly small. There are just too many variables and complex interlinking causes for such a peculiar pattern to emerge, and I could definitely smell a rat. 

So after a bit of probing, I found out that December 11th is not consistently the most fertile day on the calendar in the UK, this was simply an example of the researchers distorting the evidence. It turns out on closer inspection that they merely did a survey collating people’s birthdates and found that September 16th is the most popular within that survey period (and by slight margins). This is not the same thing as saying that year on year December 11th is the most fertile day on the calendar in the UK - the search space consisted of only a small, unrepresentative sample that would hold to no patterned regularity once broadened.

One thing of interest though

Even though it turns out that the specific date reported is untrue, it is apparently true that December is the most fertile month of the year on a consistent year by year basis, and that is something that may still be of interest to readers here. The fact that there is consistently a particular month of the year when most sexual unions occur ought to be no surprise really, particularly if there are regularities in human behaviour that could easily cause such a statistic to be true.

I can think of three reasons why December might be the most fertile month of the year:

 

One is that colder air helps to improve sperm quality. Actually, I didn't think of that one - it was reported after the media consulted a biologist. What the biologist didn't mention, though, is that by itself that's not compelling, because the 'colder air' factor should only narrow it down to winter months, not specifically December.

 

Two is that quite a few parents plan pregnancies in December so that their children’s birthdates are in September, which increases the probability that their children are among the oldest in their school year (giving those children an advantage).

 

Three is that early to mid December is when the country has lots of Christmas office parties and work nights out*, which might amount to an increase in sexual activity in early to mid December. 

So my best guess is that it’s a combination of the colder weather, school planning, and Christmas conjugation that gives us the statistic that December is the most fertile month on the calendar each year. 

 

* Note that Christmas office parties usually happen on Fridays and Saturdays, and those days are different dates each year, which increases the spread of probability away from any specific date in December.