Birthdays

American kids tend to be born June through September. I'd thought this was generally due to parents planning to hit a September school enrolment cut-off day: in many US states, if your child turns 5 at or before the start of school in September, the kid's enrolled; otherwise, you've got another year of daycare to worry about. Buckles and Hungerman showed that seasonality in births is due to deliberate timing: women who were trying to conceive showed strong seasonality, while those for whom births were unexpected showed no seasonality.

In New Zealand, your child starts school on the fifth birthday. If the child is born before a cut-off date (end-March, but later at most schools), he or she will start straight into Year 1. If the birthday is later in the year, the kid starts in Year 0 then either progresses to Year 1 at the start of the new school year in February, or continues in Year 0 until ready for Year 1. 

Incentives facing Kiwi parents are then a bit different. Since your kid is in school on the fifth birthday no matter what, you don't have to worry about hitting that barrier. But you might want to avoid a protracted stay in Year 0 unless you want your child to be old for his class. If you want to have your kid start straight into Year 1, you'd time your birth for the Kiwi summer or autumn; if you want your kid to start in Year 0 and dominate his later classmates on the rugby pitch, you'd time it for a spring Year 0 start. 

StatsNZ today put up a table showing the most common birthdays and linked to a great visualisation of the US data. In both cases, the heatmap shows the frequency of particular birth dates.

Here's the US:

Learn About Tableau

And NZ:


 
Where Americans tend to be born June-September, Kiwi births cluster September-October. Those kids would get a short start in Year 0 before progressing to Year 1 when school starts in February. 

We hadn't really considered school timing when beginning the Ira and Eleanor production processes. I expect that the Kiwi data reflects deliberate timing decisions like those found in the US. I'm just a bit curious what's underlying those decisions. Kiwis avoid June the same way that Americans avoid January, but the only sense I can make for Kiwi preferences for spring over fall is differences in school timing. But there are disadvantages to being part of the cluster. Maternity wards have only so much capacity. If you're giving birth at the same time as everybody else, you're likely going to be pushed home rather more quickly than you'd like.

Pointers to the relevant literature, or Kiwi common knowledge, are welcome.