What’s the Significance of Your Sign? A Guest Post

A reader named E. Allyn Smith, a Ph.D. student in linguistics at Ohio State University, wrote in with the following observations.

I’ve been thinking about birthdays a lot lately, because November 15th to the 27th is one of the annual “birthday rushes” in which I have eight birthdays in two weeks. It got me thinking that not only are there a lot of birthdays in that short period of time, but two of the days in that interim have more than one birthday. It reminded me of the birthday problem in mathematics (see more about this here) in which, once you have 23 people in a room, there’s a larger than 50 percent probability that two of them will have the same birthday.

In a similar vein, I became curious about a variety of properties of the birthdays I keep track of in my calendar. Here are some facts that I found (if I don’t have your birthday, feel free to leave it in the comments section “in the name of science”):

Total number of birthdays on my calendar: 51
Number of weeks in a year: 52

Of those weeks in a year, here’s the breakdown of how many birthdays I have in a given week (obviously this changes year to year, but not significantly):

0 people – 22 weeks
1 person – 15 weeks
2 people – 11 weeks
3 people – 2 weeks
4 people – 2 weeks

Number of days with more than one birthday: 5
Greatest number of birthdays in a two-week period: 8
Greatest number of consecutive weeks with no birthdays: 6

Upon discovering that so many weeks had no birthdays (in addition to the six-week stretch, there were also periods of four and three weeks at a time with no birthdays), I thought back to the horoscopes in magazines like Seventeen, in which they list your personality on the basis of your zodiac sign, as well as the compatibility (or lack thereof) between different signs. So I decided to see whether the birthday-dense times in my calendar correlated with my “compatible” zodiac signs, while the birthday-sparse periods correlated with my “non-compatible” zodiac signs.

The first thing I noticed was that my two “rush times” don’t fall into individual “signs” (for example, Nov. 15 to 27 spans both Scorpio and Sagittarius), but the next thing I noticed is that I’m supposedly not compatible with either of those signs. Let me point out that this group includes my college roommate (and we all know that you’d have to be pretty compatible to choose to live with someone in a dormitory room for a second consecutive year) as well as my brother. Meanwhile, the six-week no-birthday period extends from the last three weeks of Pisces through the first three weeks of Aries. Even in the weeks on either side, I only have one birthday, but I supposedly am compatible with Aries and Pisces.

Conclusion: either 1) zodiac “compatibility” is bunk (my personal favorite); or 2) I’ve managed to make friends primarily with those I’m not compatible with while simultaneously avoiding people with which I would be compatible. I reserve the right to do further research on this topic.

Marc P

If this were a movie, I'd refer to it as a "chick flick".

Women who know me, upon learning that I'm a Cancer, always tend to respond, "You are soooooo Cancer." Far out...

For what it's worth, have you found correlations between birthdays and holidays. My mother shot for the 4th of July, twice!, and narrowly missed. My brother and I have birthdays three days apart, July 8 and July 11, respectively. A fiend of mine, however, nailed it.


3 other people where I work share my birthday - 7/11
there are about 100 people in this facility, so that's 1/25.

make of it what you will


I tend to have the exact same two week rush in November. I like to attribute it to Valentine's day. Same thing with the end of August and Begining of September - Christmas and New Year's babies. So the Birthday rush may just be the work of probability. If there are more babies born around a certain time, I'd expect to know more people with those birthdays.


In a family of 4 people (2 adults & 2 kids), I am the only odd man out born in August. Everyone else is a December born!
Dont know what to make of it but its a fun talking point.


I was conceived on New Years Day - Birthday is Mid-September. Mom freaks me out by pointing this fact out and telling me the exact location. Too Much Information.


You're lucky she missed, Marc P. She probably wanted to name you Yankee Doodle Dandy P.


There are so many interesting things you could do with this data - why focus on zodiac signs? Sadly, I think it makes our whole gender look bad.
In the hope you will do something more scientific with your research I will throw myself into your birthday rush pot. I'm November 20th


Looking beyond one's sign, would be interesting to do some research to see if there are any seasonal effects that would influence a couple's tendency to conceive.

Perhaps as a result of spending more time indoors and together during a cold winter season? 9 months later, lots of birthdays?


My birthday is November 14 and my mother's is November 15. Of my eight closest friends at university, four of us have a birthday within a week. Everywhere I go, I feel like I meet more people with a birthday in mid-to-late November than any other time but:

a) I probably pay more attention to birthdays in November because that's when mine is (gah - cognitive biases).

b) As A said above, you'd think the Valentine's Day thing might play a role. I've heard my parents were in Brighton on Valentine's Day the year I was born. Definitely too much information!


Someone recently asked me "how many other people have the same birthday as me?" -- I replied "Roughly one in three-hundred-sixty-five. the person almost hit me. I ducked.

We tend to make a lot out of a relatively meaningless number. I do have a fair amount of trust in astrology, and know the zodiac fairly well. I also know that your sun sign is only one piece of your overall astrological makeup.

I too have noticed that sometimes random chance seems to not be random and a bunch of people in a similar birthdate range will somehow be thrown together. I would recommend taking a different approach than trying to figure out why this statistical improbability exists. Instead ask yourself "Why am I noticing this probability. What about this is important to me and why do I feel the need to address it?" Statistical improbabilities happen around us all day long. We notice them for one reason or another. More important than the improbabilities occurrence, is why we happen to notice that one, and how we can use that awareness to better survive the world.



in trying to top Mark P, here's my July rush:

mom-7/7; brother-in-law 7/8; his daugher 7/8; dad 7/28; three of my best friends growing up-7/6, 7/18 and 7/29

my wife an I planned very hard for our kids to NOT be born in July.


Of my extended family, on my mother's side, there are approximately 45 people. There are 6 birthdays, three anniversaries in the Dec-Jan timeline. There is a similar pattern in May. Three birthdays and two anniversaries, although what's also interesting is that that is just one family and each of them have a name that is a derivative of the other: Steve, Stephan and Stephanie.

Another unique pattern: on my maternal family side there is also a birth pattern in 3s. I am the second of three children born in 1983. My sister is the three of three born in 1987. Both patterns arise from the three eldest children in my mothers family. I also have three cousins born of my mothers' other three siblings who have the same pattern, but younger. I have three cousins born from each of the three relatives all born in 1999.

Another pattern is arising from the children born of my mother and her siblings: all of the grandchildren are being born every year for the past three years.

For the pot: my birthdy is august 12.



Over a span of about 20 years (elementary school to my mid-20s), I had four "best friends" whose birthdays all fell between June 4 and June 13.


Sorry, but this posting is exactly the opposite of what I expect to see on this blog. Do linguistics have to learn basic (i.e. freshman or even high school) probability.

Here's the result of less than 30 seconds on excel:

Let's take the first result: 22 weeks without a birthday.

Any one week will not have a birthday with probability (51/52)^51 = 0.3715.
The expected number of weeks without birthdays is thus 52*0.3715 = 19.3

The standard deviation of that expectation is

Thus, the observed 22 weeks is not even one standard deviation away from the expectation. No puzzle at all.

Sorry, but this posting just illustrated the foolishness of looking at data superficially.


Dr. Troy Camplin

According to the Zodiac, I'm not supposed to be compatible with my wife, but we are incredibly compatible. Go figure.


Growing up, 4 of the 22 people living on my culdesac (with only 5 houses) had a birthday on July 6.


I was born the day after Christmas. Most of my birthdays are between November and January. I also noticed that I can make friends effortlessly with people born between late Dec and early Jan.
My guess is that babies born in winter are more likely to stay indoor and adapted to certain ways of human contacts and enjoy more indoor activities. That's how we are compatible.
But then again, my college roommate was born in August, in the Caribbeans.

Marc P

@dd #22
Nifty, but you missed the point of the essayist's query.

"...I decided to see whether the birthday-dense times in my calendar correlated with my 'compatible' zodiac signs, while the birthday-sparse periods correlated with my 'non-compatible' zodiac signs... Conclusion: either 1) zodiac 'compatibility' is bunk (my personal favorite); or 2) I've managed to make friends primarily with those I'm not compatible with while simultaneously avoiding people with which I would be compatible. I reserve the right to do further research on this topic."

I pleaded ignorance with respect to Zodialogical considerations of compatibility (#1) but obliged her request: "(if I don't have your birthday, feel free to leave it in the comments section 'in the name of science')". I suspect the majority of the other posters responded likewise. On the other hand, less than a handful of posters responded directly to the query.

For your effort, you get a Mulligan.



My daughter's birthday is Nov 23rd, definitely a Valentine's baby (plus my birthday is the 16th of Feb., therefore making Valentine's weekend a little busier for me than most, probably).

My twin step-siblings are the 24th and 25th of Nov., and my youngest step-brother is the 4th of Nov. All are probably Valentine's babies (even the 4th, which is also the "Long Island Iced Tea" baby).

We also have a string of Spring-time-conceived babies in my family (Dec. 31, Jan. 1, Jan. 21, Jan. 30, Feb. 16). My wife's a late Summer-conceived baby, but then her and her older brothers are spaced very closely together, leaving very little time between the birth of one child and conception of the next, until she was born, anyway. I believe her youngest brother is another Valentine's baby, and he's significantly younger (her parents really wanted a girl after having the first boy, luckily for them it only took until the third kid to have one).

Since the births of children are largely determined by human behavior, we can see patterns in the birthdays that have little to do with random happenings. I'm sure that if other cultures have celebrations similar to Valentine's day during different times of the year, there will be a similar clustering of birthdays 40 weeks later, and people tend to conceive in the spring and winter (when they're cuddling a lot and then finally getting out in the lovely spring weather).


Joseph R

In the extended family 23 people) on my wife's side there are two duplicate birthdays (mine 3-27 and my son's 7-8). Only one month has no birthdays -- June. There are also many birthdays within 3 days of each other 5-6 and 9, 8-25 and 26, 9-21 and 24, and 12-4 and 6.

For your further research here are the spousal pairs
4-30 and 3-27, 8-25 and 9-21, 1-29 and 2-05, 12-4 and 5-9, and 8-26 and 3-27. I have no idea about the compatibility of these by astrological sign.