Common/uncommon birthdays.

February 9, 2006 2:20 PM

When is the most common birthday? What about the least common?

Ok, I'm already assuming that February 29th is the least common birthday (for obvious reasons). When is the second least common?

I'm guessing the most common birthday falls in a range of dates nine months from Valentine's Day, but I have no experience with conception. Can anybody help me out?

(With the dates, not with conception.)

posted by viewofdelft to society & culture (36 comments total)

This site, should you choose to believe them, says the most common birthday for Americans is October 5, about nine months after New Year's Eve, based on a birthday database. Makes sense to me.
posted by mdevore at 2:30 PM on February 9, 2006

Do you mean just North American birthdays? Valentine's Day is quite culture-specific, I'm not sure that it would be a factor in births in China, India, most of Africa for example. I think a better way to correlate conception with certain times of year would be to consider the seasons; do people conceive more if it's cold or hot?
posted by nomis at 2:31 PM on February 9, 2006

In the US October 5th is the most common birthday and Mayy 22nd is the least common - apparently.
posted by rongorongo at 2:33 PM on February 9, 2006

Are you wanting to limit this to the US? Because other countries don't have Valentines Day, for example, and thus on a worldwide basis, it's probably pretty much a wash.
posted by jacquilynne at 2:33 PM on February 9, 2006

I've noticed (totally unscientifically) that birthdays of siblings tend to group together in the same month or so. My theory is that this is about 9-10 months after the parents anniversary. I've often wondered about this, but I have no empirical support for my theory.
posted by raedyn at 2:34 PM on February 9, 2006

This is month specific:
More people have birthdays in August than in any other month – 9.06 percent of the U.S. population. July is the second most popular birthday month (8.80 percent) and the fewest people celebrate in February (7.55 percent).
posted by meech at 2:37 PM on February 9, 2006

I went looking for the story about the spike in the birth rate 9 months after the 1965 New York blackout, and, of course, found that there was no such spike. From the Snopes article:

It is a common belief that the number of conceptions increases during natural disasters or crises that keep people confined within their homes for unexpectedly long periods of times. Nine months after such events — blackouts, blizzards, earthquakes, erupting volcanoes, ice storms, and even strikes by professional football players — reports about "baby booms" in local hospitals invariably appear in the media. However, these "booms" always turn out to be nothing more than natural fluctuations in the birth rate (or, in many cases, no variation in the birth rate at all). Of course, we never hear about these fluctuations when they are not preceded by some unusual event.
posted by bright cold day at 2:37 PM on February 9, 2006

Some of the sites that name October 5th as the most common b-day also point out that it's about 9 months after New Year's (not Valentine's Day)...so it might also have some international relevance.
posted by bachelor#3 at 2:38 PM on February 9, 2006

I'm not sure I buy into the Valentine's Day or New Year's Day phenomenon. People don't only have sex on those days. I don't even think that more people have more sex on those days. Maybe it has more to do with cold weather.
posted by lunalaguna at 2:41 PM on February 9, 2006

Here's a paper I found online which analyzed the birthdays of 2,000 people. Don't know if the statistics are valid (it was all females for one) but it does show an uneven gross distribution with birthdays clustered around July, August and September.

They had a peak on August 31st - 9 months after new years.
posted by vacapinta at 2:45 PM on February 9, 2006

Also from the Hallmark site (hat tip to meech), suggesting Americans do their conceiving in the autumn-winter ...

August Is The Top Month For Birthdays; Third Quarter Boasts Most Births

Rank Month Percent
1. August 9.07
2. July 8.80
3. September 8.62
4. October 8.60
5. March 8.51
6. May 8.30
7. January 8.25
8. June 8.15
9. April 8.12
10. December 8.07
11. November 7.96
12. February 7.55
posted by bright cold day at 2:47 PM on February 9, 2006

Huh. I don't see how New Year's specifically would necessarily have much to do with it. What people generally do on New Year's Eve is a) drink more than they usually would b) stay up far later than they usually would and c) be in the company of other people for a and b.

I can see how this would lead to wanting to get busy, but not so much acheiving conception...
posted by desuetude at 2:47 PM on February 9, 2006

Well when you consider the week between Christmas and new years it makes a bit more sense. People generally have a few days off if not the whole week giving more opportunity and people are travelling allowing for more chances of screwed up birth control.
posted by Mitheral at 2:53 PM on February 9, 2006

Whats odd is that the data seems surprisingly noisy - that is, a very popular day might occur right next to a very unpopular day. The most popular birthdays may not fall within the most popular months. That is, it seems to show structure at a gross level (some months are more popular than others) but is almost random at a day-by-day level.
posted by vacapinta at 2:54 PM on February 9, 2006

Some people are bad at math...In this thread alone August 31st and October 5th both were claimed to be 9 months after new years. Pretty large gap between the two, eh?

Random semi-disrail, My boyfriend was born on November 13th, so he's pretty sure his conception was Valentine's day.
posted by piratebowling at 3:01 PM on February 9, 2006

We have a strangely high rate of winter birthdays in my family (dad and nephew #2 are late November; sister and nephew #1 are December; BIL and nephew #3 are in January; I'm in February; most aunts/uncles/cousins/grandparents fall into the same time frame, too) -- my mom's the oddball with hers in July. The main thing is that it makes buying presents around the holidays a bit of a pain.
posted by scody at 3:04 PM on February 9, 2006

I like that May 22 thing - I always knew I was weird. Actually, my bday is May 21, and my (only) brother's is May 20th.

I've always heard the most common birthday is 9 months after New Years. Given a normal pregnancy is 40 weeks long, that means around October 7th.
posted by cgg at 3:12 PM on February 9, 2006

lunalaguna: The Valentine's Day/New Year's Day theory doesn't require you to believe that people only have sex on those specific days. It just means that the combined group of people who get pregnant are slightly more likely to get laid on those days than other days of the year.
posted by revgeorge at 3:12 PM on February 9, 2006

Southern Hemisphere perspective: the Australian Bureau of Statistics data for births by month over the period 1984-2004 (see table 6.4 on page 43).

Having graphed it, there doesn't seem to be one month that consistently spikes, but there are noticeable drops in the general level in February, June, and November. The peaks, such as they are, are in March, and September/October.
posted by bright cold day at 3:24 PM on February 9, 2006

can see how this would lead to wanting to get busy, but not so much acheiving conception...

Ahh, the youth of today, no sense of history. (I think Aristotle said something like that, too.)

The majority of people alive in America today were born before fully reliable and socially-respectable birth control methods were available (e.g. invention of the Pill, condoms emerging from disrepute). The sexual revolution of the 1960's really was a watershed event for social change, and wasn't just a bunch of smelly hippies shouting slogans and singing Donovan songs to get laid. Given that historical context, for most people's parents an increase in "wanting to get busy" was basically equivalent to an increase in "achieving conception".

An interesting set of statistics would be to see how or if the most common birth-date changes or loses its pre-eminence depending on age group. It seems quite possible that external social factors have had a statistically significant impact on the most popular day to be measured for a birthday suit.
posted by mdevore at 3:26 PM on February 9, 2006

Are you sure the least common birthday isn't February 30? :)
posted by TunnelArmr at 3:33 PM on February 9, 2006

Some people are bad at math...In this thread alone August 31st and October 5th both were claimed to be 9 months after new years. Pretty large gap between the two, eh?

Not larger than the gap in normal pregnancy limits of 38-42 weeks, which is what we are referring to here.
posted by vacapinta at 3:45 PM on February 9, 2006

Well when you consider the week between Christmas and new years it makes a bit more sense. People generally have a few days off if not the whole week giving more opportunity and people are travelling allowing for more chances of screwed up birth control.

When I was a divorce lawyer I had a client who had six children all born in different years, but between 24 and 27 September.

She blamed Christmas.
posted by essexjan at 3:48 PM on February 9, 2006

100% / 12 = 8.33%

Looking quickly at bright cold day's data, every month is within about 0.5% of that average. Seems like the variation could be explained by random chance, but IANAStatistician.
posted by InfidelZombie at 3:52 PM on February 9, 2006

Well, Oct 5 seems about right to me, since my birthday is Oct 4!

Actually I have noticed that there seems to be a lot of people with birthdays close to mine. Two of my good friends and one of my cousins are within a day, for example. But I'm sure I would notice the same thing no matter what my birthday happens to be.
posted by PercussivePaul at 4:30 PM on February 9, 2006

Well, everyone talks about February having fewer birthdays, and I'm wondering if that's the whole month of February, which is only 7.73% of the year (28.25/365.25).

It is still slightly underrepresented in bright cold day's statistics, but much less so.
posted by JMOZ at 4:30 PM on February 9, 2006

InfidelZombie: Based on that data, there are 12.4% more births in August than in December. (Note: Both these months have 31 days.) Assuming the data is based on a large sample, this is definitely a significant difference, not just random chance.
posted by mbrubeck at 5:31 PM on February 9, 2006

I was always under the impression that, although fairly evenly distributed, birthdays occur more in the months of July-October because people have more sex in the winter. Not because of any holidays, but just because they spend more time indoors out of the cold, and not so much time working. Granted, this was more prevalent in the farmland days, but it still seem reasonable.
posted by team lowkey at 6:21 PM on February 9, 2006

12. February 7.55

Of course February is at the bottom of the list, it's the shortest month, so there are less birthdays than in other months!
I do feel somewhat special for having a birthday in the least common birthday month, but even more so for posting in this thread on my birthday.
posted by easternblot at 8:12 PM on February 9, 2006

Many happy returns easternblot!!
posted by bright cold day at 8:24 PM on February 9, 2006

an increase in "wanting to get busy" was basically equivalent to an increase in "achieving conception".

Um, I meant that the desire to have sex and the abillity are by no means the same thing, particularly when drunk and/or tired.
posted by desuetude at 6:25 AM on February 10, 2006

Not all pregnancies last nine months. Some go over. Multiple births frequently are 4-6 weeks early. Some kids just like the womb too much and often refuse to leave on their own.
posted by camworld at 8:01 AM on February 10, 2006

I was born on November 15; my sister was also born in November. But our due dates were only one day apart--October 26 and 27. Doesn't necessarily rule out Valentine's Day...I've never asked my parents.

I think they deliberately concieved us. To me, Valentine's Day seems like a more likely day to deliberately concieve a child, and New Year's seems like a day to accidentally concieve.

There are other things to consider than just holidays. I'd think more babies would be born in late summer and fall, as a result of staying in more during those long, cold, winter nights....

Also, isn't it true that there's usually a big spike in births about 40 weeks after a big event such as a blizzard or a blackout? Nowhere to go...no tv, no video games, not even lights to read by...

Not to mention there's a whole generation defined by the fact that their parents had a lot of sex for a few years :)
posted by lampoil at 8:50 AM on February 10, 2006

My birthday is November 3rd. My roommate back in college... his birthday is November 3rd. When we were living together, we both dated girls whose birthdays were... November 3rd.

Now I'm dating a girl whose birthday is... November 2nd. My friends think I have a problem. :)
posted by educatedslacker at 12:52 PM on February 10, 2006

educatedslacker - I bet those were fun bday parties.
posted by raedyn at 1:57 PM on February 10, 2006

I'm interested that most people are assuming spikes in birthdates are related to accidental conception. It seems entirely possible to me that, if there is a pattern, it is the result of a deliberate attempt to time conception so that babies are born at a specific time of year.

Take a look at the list that bright cold day posted. The top four months are August, July, September, and October.

Perhaps people want to give birth in May or June, so they won't be stuck inside with their baby for the first few months... but since the odds of conceiving are apparently only about 15-20% each month, most people wouldn't be successful in the first month or two of attempted conception, and it would therefore end up giving birth in July, August, September, or October.

If this theory is correct, then we'd expect to see different trends in the Southern Hemisphere, where people would be shooting for October or November births, and would end up giving birth in December, January, or February. Anybody have any information for Australia?
posted by yankeefog at 6:58 AM on February 11, 2006