Why are most kids born during July/August/September ?

[Tender Branson]

I've looked through monthly birth statistics of the US and Austria and in both countries the most kids are always born in either July, August or September.

http://www.cdc.gov/nchs/data/dvs/provisional_tables/Provisional_Table01_2010Dec.pdf

http://www.statistik.at/web_en/wcmsprod/groups/zd/documents/statueb/027679.pdf

What's the reason for this in your opinion ?

[Joe Republic]

Winter is cold.  Cold people tend to stay in and cuddle for warmth.  Cuddling leads to other things, like babies in July-September.

[Tender Branson]

Winter is cold.  Cold people tend to stay in and cuddle for warmth.  Cuddling leads to other things, like babies in July-September.

Now it would be interesting to see the monthly birth statistics of Florida, if this is the case there as well ...

[Tender Branson]

Interesting.

Even in the warm Florida, most births take place between July and September:

http://www.flpublichealth.com/VSBOOK/pdf/2009/Births.pdf

(page 35)

[Joe Republic]

Winter in Florida may be laughably warm by a lot of people's standards, but probably not to Floridians.

When I experienced my first winter here after moving from Ohio, I found it hilarious that seasoned locals were wrapping up tight in coats and scarves to cope with the 40° weather.  But given that six months later it was around 110°, it began to make a little more sense.

'Cold' to one person is 'warm' to another.

[Smid]

Christmas and New Year are nine months before, too.

Does the trend hold in the Southern Hemisphere, where it's Summer in December/January?

[Tender Branson]

Christmas and New Year are nine months before, too.

Does the trend hold in the Southern Hemisphere, where it's Summer in December/January?

Even in Australia, July-September are the months with the most births, says the ABS.

[minionofmidas]

Humans being mammals have a natural rutting cycle?

[Cincinnatus]

Humans being mammals have a natural rutting cycle?

This may have some truth to it.  Offspring born in warmer weather would be more likely to survive.

[jimrtex]

Interesting.

Even in the warm Florida, most births take place between July and September:

http://www.flpublichealth.com/VSBOOK/pdf/2009/Births.pdf

(page 35)

Summertime Mystery: More Born, Fewer Die in August

This says that many areas in Europe have a different pattern.

Also, seasonal variation used to be particularly pronounced in the South with a particular low in August.  One theory is that air conditioning has dampened the trough (ie it is not more conception in cold weather, but less conception in hot weather).

My understanding is that this year of the Water Dragon is particularly auspicious.  Seasonality of births might depend on whether parents planned ahead, or belatedly began yesterday.

[BigSkyBob]

Here is a better theory:

Historically, food was most plentiful starting in the spring, and going into the harvest.  That is, women would be thinnest at the start of spring, and have the greatest fat reserves around harvest time. Even today, fresh fruits and vegetables are more ready and cheaper in that spring-to-fall time frame.

[muon2]

If it's evolutionary, shouldn't it match to seasonal variations in East Africa?

[Ban my account ffs!]

I think research has been done and has found that hormones that might lead to sex are highest during autumn/early winter.  It is that way for many mammal species... and regardless of African seasonal variation, it has a small effect.

Another interesting thing:  While births rise and peak in summer, unplanned pregnancies, births to unwed mothers, and births to teens peak in winter/early spring.

The reasoning is that sex among young, unwed people is most likely to take place during spring or summer after having been shut in with the parents for much of the winter.

They studied this after they found that children born during winter were more likely to commit crimes, be depressed, or come from an untraditional or broken home.  In contrast, those with summer birthdays were more well behaved and tended to belong to more traditional families.

Of course, as with any statistic, while significant... only applies to a slight correlation.  Most kids born in December are just as well adjusted as those born in June.

[BigSkyBob]

If it's evolutionary, shouldn't it match to seasonal variations in East Africa?

Northern lattitude populations may have diverged. East Africa, probably, has an optimal time of conception based on factors such as rainfall patterns, animal migration, river levels, etc.

[Ban my account ffs!]

Christmas and New Year are nine months before, too.

Does the trend hold in the Southern Hemisphere, where it's Summer in December/January?

Even in Australia, July-September are the months with the most births, says the ABS.

It'd be interesting to see if that holds true among the aboriginal population... as compared to the white population that came mostly from the NH quite recently.

[memphis]

Those months may have higher rates, but it's certainly not most kids or even a majority of kids. The other 9 months greatly outnumber those three. Many species have a specific breeding period, so "most young" are born at one time of year. It's an interesting fact about humans that this is not the case for us.

[ilikeverin]

If it's evolutionary, shouldn't it match to seasonal variations in East Africa?

Yeah.  Even though I'm, well, me, I sincerely doubt this is at all evolutionary.  There really hasn't been enough evolutionary time since humans diverged that could explain this pattern.