Saturday, 23 February 2013

Why Are There More Pregnancies In December?




Do you remember recently when it was all over the newspapers and on the TV and radio recently that December 11th is the most fertile day on the calendar in the UK?





I do remember; there were articles in The Daily Mail, The Guardian, and The Times, and it was a major news bulletin on BBC and ITV.  The claim was that more babies are conceived on that day than any other day, and apparently this happens year on year.  As someone who attunes his thinking to probability and economics, I smelled a rat straight away, but due to ambiguity of information I couldn’t get to the bottom of this without further enquiry.  Save for one of those extremely rare coincidences that one expects every now and then with the law of large numbers, I felt pretty certain that one single day (and the same day) every year would not consistently be the most fertile day.  There are just too many variables and complex interlinking causes for such a peculiar pattern to emerge.

So after a bit of probing, I found unsurprisingly that the claim was a lie, or at the very least, a bit of dishonest journalism (fancy that).  They claimed that December 11th is the most fertile day on the calendar in the UK but it just wasn’t true, it was 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.

Why is December the most fertile month?
Yes, it turns out that although the specific date reported is untrue, December is the most fertile month of the year on a consistent year by year basis.  This is interesting in itself, although perhaps not as headline grabbing as claiming it to be the same single date every year.  I fancy that the sort of person who would write a Blog like Philosophical Muser is the sort of the person who would find this interesting, because people all over the country are having sex every day – so, that there should be one consistent month in which most babies are conceived is still worthy of attention.

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).  And 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 conjugal bliss 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.


No comments:

Post a Comment

/>