As of today (or as my mother just reminded me, specifically as of 7:30 this evening), I am officially a year away from updating my “About the blogger” page to say that I’m in my early thirties.

So I thought that I’d write a post on birthdays that will sound very familiar to anyone that’s read Malcom Gladwell’s “Outliers”, which I wrote about here.

Accumulative Advantage

So here’s a portion of an infographic that I found:

Thanks this website
Thanks this website

Some key points:

  • At 3 years old, children can vaguely understand the difference between “on” and “under”.
  • At 4 years old, children can use basic grammar.
  • At 5 years old, children “speak clearly, tell simple stories with full sentences, and carry on conversations using more than 3 sentences”

And if we’re talking about a pre-school class where one toddler is born in January, and another in December, that’s effectively a year’s difference. And when you’re talking about 3 year olds and 4 year olds – the older toddler has had an extra third of the younger child’s learning experiences.

What that means:

  • More developed pre-schoolers catch on to things a little quicker.
  • So they get a little more attention from the teacher, and slightly better reports.
  • Which turns into confidence.
  • While the less-developed pre-schoolers get less attention.
  • That confidence gets reinforced regularly as the years go on.
  • And what was once a small difference becomes a great difference.

And we’re not even talking about the realm of sport – where a few months of age difference can be a dramatic difference in physical development.

Although that’s somewhat mitigated by this:

Thanks Matt Stiles
Thanks Matt Stiles

So admittedly, that’s referring to the US. And it’s also ranking birthdays by popularity (that is: one day may rank higher than another for actual births, but it could differ by 100 births or 100,000 births and this graph would look the same). However, even if you look at the actual birth count, it’s clear that there are more births in August-September than any other time of the year.

Which is to say that the average age in a year group is biased towards being less developed – so the end-of-year babies will tend to be slightly less below-average than an even distribution would imply.

But that’s Northern Hemisphere – and while couples seem to be especially amorous in the winter months (or festive season?) – it’s not clear that would bear out in the Southern Hemisphere.

So I went onto Stats SA, and found this:

Thanks StatsSA
Thanks StatsSA

So September is still the most popular month for giving birth – suggesting that Christmas season really is the season, regardless of the weather. But winter is also a good time – check out all those January and March babies*.
*February appears unpopular – but then, February also has three fewer days than both January and March. Which would explain some of that difference (even if not all!).

Speaking of the weather

I have one more series of studies that I want to mention – specifically, those that deal with the impact that the season of your birth can have.

Here’s a quote:

The association between season of birth and a number of physical and psychiatric conditions among individuals in non-equatorial regions has been firmly established. For instance, adult life expectancy (Doblhammer & Vaupel, 2001), body size (Phillips & Young, 2000), handedness (Martin & Jones, 1999), and rate of dyslexia (Livingston, Adam, & Bracha, 1993) have all been shown to relate to the season in which an individual is born. There is also evidence for an association of birth season with the rate of some psychiatric conditions including schizophrenia (Torrey, Miller, Rawlings, & Yolken, 1997), affective disorder (Castrogiovanni, Iapichino, Pacchi- erotti, & Pieraccini, 1998; Clarke et al., 1998), panic disorder (Iapichino, Pieraccini, Di Muro, Del Sole, & Castrogiovanni, 1997), autism (Bolton, Pickles, Harrington, Macdonald, & Rutter, 1992), and depressive or suicidal symptoms (Chotai & Salander Renberg, 2002; Joiner, Pfaff, Acres, & Johnson, 2002).

So apart from being associated with your body fat percentage and personality disorders, birth months also correlate with thrill-seeking behaviour:

Recent findings have demonstrated an association between season of birth and scores on the temperament scale of novelty seeking (Chotai, Forsgren, Nilsson, & Adolfsson, 2001; Chotai, Johansson, Hagglof, & Adolfsson, 2002; Chotai et al., 2003). Novelty seeking is defined as a tendency towards exploratory activity and intense excitement in response to novelty, impulsive decision making, and active avoidance of monotony or frustration (Cloninger, 1987).

And if you’re looking for the “why”, here’s another quote:

A number of factors have been suggested to be involved in the season of birth association, including seasonal variations in photoperiod and internal chemistry, external toxins, nutrition, temperature and weather effects, and maternal infection (Tochigi, Okazaki, Kato, & Sasaki, 2004; Torrey et al., 1997).

Is that not so interesting?

So perhaps astrology is really just a bad explanation for what should be a fairly expected phenomenon… Because a lot of who you are is switched on during pregnancy and your first few months of life. And depending on the season of your birth, you’re facing very different temperatures, food, illnesses, etc. All of which would have small impacts on your development. And small impacts become large differences. Just think of how easy it is for one bad math teacher to scar a child for life.

And when you’re looking at your personality and genes being generally influenced by:

  • The seasons; and
  • Your development relative to the development of your age group…

…then it’s really not that surprising that we observe common personality traits to people born at similar times in the year.

It’s just a thought.

Happy Tuesday 18th November.

Rolling Alpha posts opinions on finance, economics, and the corporate life in general. Follow me on Twitter @RollingAlpha, and on Facebook at