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Q: Birthdays over a year - Evenly spread or not? ( Answered,   0 Comments )
Subject: Birthdays over a year - Evenly spread or not?
Category: Reference, Education and News > General Reference
Asked by: lucky2beon-ga
List Price: $20.00
Posted: 24 Nov 2003 18:00 PST
Expires: 24 Dec 2003 18:00 PST
Question ID: 280242
Someone who believes in Astrology told me that if you plotted all the
birthdays of all the people on Earth, you would see clumps of
birthdays that match up with the signs of the zodiac in some fashion.
i.e. some Zodica signs would have more birthdays that others.

I say that it is purely random. If there is any clumping, it is due to
specific reasons, like maybe in Sweden there are more children born
nine months after the long dark winter when there is more time to,
well, make babies. But all in all, I feel that any day in the year has
exactly the same amount of people born as any other day.

I would like to see some numbers that show when people are born and
why the numbers are as they are.
Subject: Re: Birthdays over a year - Evenly spread or not?
Answered By: juggler-ga on 24 Nov 2003 21:27 PST

Here's a good web page that discusses this issue:

"An Analysis of the Distribution of Birthdays in a Calendar Year" by
Roy Murphy, hosted by

As discussed on that web page, the researcher found there was a slight
seasonal variation in U.S. births.  Specifically, births were a little
bit higher in the July-to-October period while births were slightly
lower in the March-to-May period.
"..there is more variance in the data than can be accounted for by
chance. An examination of the histogram shows significant seasonal
variations. The months July - October show higher than expected births
and March - May show the most significant decline in births. Perhaps
the most reasonable explanation is that conceptions are up in the
months of October through January and down in June through August. You
be the judge."
"An Analysis of the Distribution of Birthdays in a Calendar Year" by
Roy Murphy, hosted by

The same slight seasonal variation was reported in this newsgroup
post, hosted by Google Groups:
"Subject: Re: data on on distribution of birthdays by month US '96"

Again, the same trend (i.e., higher July-October birth rate) is
reported on the web site

"Average Daily Birth Frequencies in the United States, 1978?1987
January    .0026123
February   .0026785
March      .0026838
April      .0026426
May        .0026702
June       .0027424
July       .0028655
August     .0028954
September  .0029407
October    .0027705
November   .0026842
December   .0026864 "

Ivar Peterson's MathTrek, hosted by

The basic trend is also mentioned in passing in an article on
"...Halperin explained that previous studies have confirmed a
disproportionate amount of babies in the United States are born in the

As you suggest, the reason for any variation is probably very simple
(i.e., more conceptions in the winter due to more long, cold nights).

Many people assert this on a "common sense" basis. For example:

"It's like they say, there's usually more babies born in September
because they're conceived in the winter months when it's cold."

There have also been studies that suggest the same explanation. 
"Wintry nights spark summer births" hosted by the BBC:

As for any astrological or astronomical connection with birth
distribution...  An internet search suggests that some people claim a
connection between the full moon and birth rates.  See:

"More Babies are Born on Full Moons: Fact or Fiction?"
By Dr. Jay DiLeo, M.D, hosted by

A scientic study conducted in 1994 found that "scientific analysis of
data does not support the belief that the number of births increases
as the full moon approaches, therefore it is a myth not reality."
"Labor ward workload waxes and wanes with the lunar cycle, myth or reality?"
source: PUBMED, National Libary of Medicine, hosted by


search strategy:
"distribution of birthdays" months
"seasonal variations", births
"more babies" born seasonal

I hope this helps. If anything is unclear, please let me know via the
"request clarification" feature. Thanks.
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