Wednesday, April 23, 2014

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:


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.

20 comments:

  1. I've heard of a couple of instances of people planning their pregnancy to avoid it overlapping with summer. That doesn't quite fit the late-September bulge, though.

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  2. "Nine months after the summer holidays" doesn't cut it with you?

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  3. Why doesn't that then apply in the US data? July's mid-summer there. They'd then have an April boom.

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  4. Because as you described, the US has much stronger incentives to select based on school start dates. Anecdatally, I'm in my early 30's with school-age kids and I've never heard anyone in my cohort mention targeting a specific school start date.

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  5. Also, I've seen it suggested that the summer break in NZ, overlapping Christmas and New Year as it does, is a bigger deal (for non-students at least) than it is in the US? Especially given four weeks guaranteed annual leave, which I believe is not generally the case in the US. But I've lived my whole life here so I can't compare.

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  6. do better Eric, this is data drivel

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  7. It is.


    So your model has it then has it that those wanting a kid are mostly constrained by time in which to engage in production processes, that summer provides them that time in New Zealand, and that the school constraint in the US is sufficiently powerful to overturn the "summertime funtime" effect (or, perhaps, that hot and muggy US summers exert inhibitory effect)?

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  8. you are sacked Manitoba Eric, Canada , go home now and write about Canada birthdays, salary refused, go back, stop garbage, come back when a real man

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  9. I would want to see a repeat of Buckles and Hungerman using NZ data before making any predictions about planned vs. unplanned children. And I'm sure the inclination to procreate is a more complex function of season than just the available time. But yeah, basically.

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  10. Having December and June children the December was easier for birthday parties, which was noticed when we went to the US and the seasons where flipped, the June child loved getting to do outside stuff, and the Decemeber child hated being stuck inside. But we didn't plan the the birth timing.

    I did once hear a mother planning her births to avoid holidays (she happened to be a US native living in NZ)


    As a general real rule you really don't want your kids starting school in early December or late November, as they transition from kindergarten and then start school and it stops for six weeks and then start again.

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  11. But the above mentioned transition did occur for our December child, and she was just fine.

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  12. Many women want to avoid being *heavily* pregnant in summer, which fits the September bulge in NZ.

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  13. Sure, but July/August are the worst in many parts of the US, and there's a huge bulge there in those months, and US summer is way worse than NZ summer. So either NZ women hate summer pregnancy way more than do their American counterparts for some unknown reason, or it's due to something else.

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  14. Nice artifacts around the 13ths, 4th of July, Thanksgiving, and Christmas here in the States. Both of my kids were born on 13s.

    The effect might become even more pronounced here, as California very recently began moving it's cutoff dates from Dec 1 to Sept 1. That means my daughter, born in October, would be waiting a very long time to start, but since we've opted to pay for private school instead of sentencing her to the purgatory that is LAUSD, she'll be starting Kindergarten this fall, just before her 5th birthday (as will my son, born in Nov.).

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  15. Anecdotally I was aware that there might be a bulge around the Sept/Oct period; as a kid I knew a lot of folk with birthdays around that time. I have to confess I'd always assumed that this was at least in part due to an increase in intercourse around the Xmas/New Year window, where perhaps feelings of happiness and goodwill led to more shagging. And it is possible that extra alcohol consumed around this time may lower inhibitions a bit, although I doubt this is a significant contributor. I suppose Spring is a nice time to have kids, not too cold, not too hot.
    When we embarked down the path to parenthood we didn't attempt to pre-determine potential birth dates, we let nature take its course. Elizabeth arrived in late June, and #2 is due early July. Our winters here are a bit chilly and rainy, but nothing compared to the snows in some parts of the US and Canada. I don't really see there being a particular difficulty with mid-winter births here in NZ.

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  16. A reference (from your Otago friends) here http://www.otago.ac.nz/christchurch/otago014801.pdf - which also refers (briefly) to some Research in 1980s (I think) by McDonald showing that birth month plus teacher decisions could have an affect on kids' progress - for kids born between April and July.

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  17. Thanks. Would be neat to see more recent work incorporating a larger span of months.

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  18. Presumably the least common are to do with them being on public holidays, and therefore births triggered by medical intervention are less likely due to medical staff being on holiday.

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  19. Yes, and that's common across the US and NZ: nobody wants to give birth around Christmas.

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  20. Much more A/C in the US than in NZ

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