I’m taking a mid-week break from all the big topics to share a 2014 post on birthdays. Specifically, it’s about the scientific evidence for the influence of your birthdate on your life/personality/behaviour. I mean, it’s not astrology – but it might help explain why people believe in it. I’m also using the broad definition of ‘economics’ here – which allows me to apply it to anything that deals with human behaviour #blogginglicence
So let’s start.
Age and Accumulative Advantage
So here’s a portion of an infographic on child development that I’ve found:
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”
Here’s an observation: if we’re talking about a pre-school class where one toddler is born in January, and another in December, that’s equivalent to a year’s difference in age. And when you’re talking about 3 year olds and 4 year olds – the older child 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.
- The less-developed pre-schoolers, on the other hand, 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.
Why isn’t it more noticeable?
Well, here’s a chart I found:
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…
The Southern Hemisphere’s birth patterns
So I went onto Stats SA, and found this:
Clearly, September is still the most popular month for giving birth. Although 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.
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).
But also, it seems that 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).
Interesting, right? So perhaps astrology is really just a bad explanation for what should be a fairly expected phenomenon…
Because a chunk of who you are is determined 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 those factors will have an impact on your development. And even small impacts can become large differences over time.
So when your personality and genes are 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.