Randomness and Clustering: Is the Number of Twins in Timaru a Mystery?

By Darcy Cowan 04/08/2010

If you saw 3 News last night you might have caught the story about the bumper crop of twins born this year. The prologue to the story gave the impression of a mystery with words to the effect of  “Experts are at a loss to explain it”, I personally think this was a sloppy attempt to generate some “Experts are baffled” buzz around an essential pointless story that just filled up a slow news day.

Stuff also covered the story but without the mystery aspect, good thing because the stats given at the end of the piece kind of belie that approach.

“The previous year was another big year for twins with ten sets born out of 620 babies. In 2005 and 2006 there were 542 babies born, including six sets of twins. In 2004 and 2005 only two sets of twins were found among the 571 babies born and in the 2003 and 2004 year, there were sevens sets of twins and one set of triplets in the 557 babies born.”

So in other words the number goes up and down every year and this year just happened to be a cluster of births higher than average. Boring.

What’s the deal with randomness though and why are we so poor at recognising it? We tend to think of random events or locations as those that are approximately evenly distributed in time or space. This view of randomness however gives a false impression of what it means to be truly random.

Randomness is more a measure of unpredictability than it is of aesthetic impression. There are different ways of defining this property but one approach is to apply the criteria of an algorithm. An algorithm is essentially a series of instructions, the more instructions, the more complicated the algorithm. One such might be “1. from an initial number add 5, 2. repeat step 1.”. This would be an algorithmic representation of a sequence of numbers at regular increments of 5 eg 1,6,11,16,21.

Nothing random about that, the key here though would be that a sequence of really random numbers wouldn’t be able to be represented by an algorithm that was less complicated than the sequence itself, ie it would be it’s own algorithm and would not be able to be compressed any further.

What has this got to do with groups of twins? Well, if events such as the birth of twins are actually random (simplifying the world somewhat) then we would expect to see variations in the number of births in any one place. Based on this assumption we can look back at previous numbers to see whether this year is within the range we would expect.

Using the figures from the story and removing this year’s number and the year that only 2 twins were born as a possible outlier I get a range of between 0.5% and 2% of births being twins, with a high probability that normal variation will fall in this range. The percentage of twin births this year is 1.8%, high but apparently normal.

Now the sample size here is very small so I wouldn’t put too much trust in it but it is indicative that there is nothing really out of the ordinary going on here. According to the NZ Multiple Birth Association there were 900 multiple births last year in NZ (incl. triplets) this is about 1.4% of the 63,000 live births in NZ last year. So rough and ready these numbers may be but they aren’t too far off the mark, some places will be higher than average and others lower.

So when several rare(ish) events happen at the same time or place, consider; is this really unusual? What would we expect if it was just random?

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Filed under: Psychological, Sciblogs, Science, skepticism, Uncategorized Tagged: Multiple birth, New Zealand, Probability, randomness, Science

0 Responses to “Randomness and Clustering: Is the Number of Twins in Timaru a Mystery?”

  • Maybe the maternal age of mothers in Timaru is increasing – as it is in many places. The following paper may be useful to readers interested in twinning rates.

    Hum Reprod Update. 2008 Jan-Feb;14(1):37-47. Epub 2007 Nov 16.
    Dizygotic twinning.
    Hoekstra C, Zhao ZZ, Lambalk CB, Willemsen G, Martin NG, Boomsma DI, Montgomery GW.

    Department of Biological Psychology, VU University Amsterdam, van der Boechorststraat 1, 1081 BT Amsterdam, The Netherlands. c.hoekstra@psy.vu.nl

    The tendency to conceive spontaneous dizygotic (DZ) twins is a complex trait with important contributions from both environmental factors and genetic disposition. Twins are relatively common and occur on average 13 times per 1000 maternities, though the twinning frequency varies over time and geographic location. This variation is mostly attributed to the differences in DZ twinning rate, since the monozygotic twinning rate is relatively constant. DZ twinning is in part under genetic control, with mothers of DZ twins reporting significantly more female family members with DZ twins than mothers of monozygotic twins. Maternal factors such as genetic history, advanced age and increased parity are known to increase the risk of DZ twins. Recent research confirmed that taller mothers and mothers with a high body mass index (30>) are at greater risk of DZ twinning. Seasonality, smoking, oral contraceptive use and folic acid show less convincing associations with twinning. Genetic analysis is beginning to identify genes contributing to the variation in twinning. Mutations in one of these genes (growth differentiation factor 9) are significantly more frequent in mothers of DZ twins. However, the mutations are rare and only account for a small part of the genetic contribution for twinning.

    PMID: 18024802 [PubMed – indexed for MEDLINE]Free Article

    • Possibly but part of my point here is before trying to explain something we should spend some time to determine that there is actually an effect that requires explanation.

      I’m not familiar with the literature in twinning but I would be surprised if this small deviation was at all significant. Further the article is expressly interested in dizygotic twins or fraternal twins, I have no information as to how many of the twins in Tineru fall into this category. This is relevant as to quote the study
      “There are two types of twins, dizygotic (DZ) and monozygotic (MZ) twins, and their etiology is very different. ”

      For interested parties, the url for the full article quoted above is:

      Thanks for the comment.

  • This reminds me of one of Brian Dunning’s ‘Skeptoid’ podcasts, “The boys from Brazil”. He was commenting on a purported increase in births of twin boys from a small town in Brazil, which (in the press anyway) had been linked to visits by the infamous Josef Mengele. Shock, horror, more ebil experiments! Except – there wasn’t an increase in twin births…

  • Google tells me that the rate in that area was 15-20% of births resulting in twins. Now that’s a significant difference from the average.
    As you say though, the history of twins goes back much further than the period of time Mengele supposedly visited.