Birthday Statistics

Each spring I'm hit by a deluge of birthdays to attend.  The deluge tapers off in July.  This made me curious: are my friends more likely to have spring birthdays?  I did some digging and found the answer: overwhelmingly, yes. First, the control group.  Here are average US birthdays by month (2009 census):

As one would expect, US birthdays average 8.33% per month (as 100% divided by 12 months = 8.33%).  Now, below are my friends' birthdays by month:

Fully 25% of my friends are born in May and June.  And three of my friends share my exact birthday, June 18th.  When you consider the math of the Birthday Problem, this seems unlikely.  What are the odds of four individuals in a set of 167 sharing the exact same birthday?

By way of control group, only two of my other 167 friends share the same birthday with each other.

DATA SET

To obtain the data above, I took my total set of Facebook friends and parsed 180 that I feel a genuine connection with (as many Facebook friends are acquaintances).  Of 180 friendships, I was able to scrape birthday data for 176 of them.  Creating the chart above.

THE BIG QUESTIONS

Why am I nearly twice as likely to have a friend born in the spring than the summer?  Why am I nearly three times as likely to have a friend born in June as a friend born in January?

Is this random chance or do other people notice trends among their friends as well?

THE SCIENCE OF BIRTHDAYS

Turns out, science has spotted many birthday correlations, none of which are properly understood.  For instance, children with autism are 16% more likely to be born in winter months. ¹ Spring babies are at a 17% higher risk of suicide.² A mother's exposure to sunlight (read: vitamin D levels) during gestation may be a significant factor in fetal development. For instance, both MS and Schizophrenia are strongly correlated to babies who came to term during winter months, or in northern latitudes with lower levels of sunlight.^{1 2} If vitamin D can have such a marked effect on fetal health and development, is it possible that brain and personality may be effected as well? Since photoperiodism can effect the brain chemistry of adults (fully 10% of Alaskans suffer from Seasonal Affective Disorder), can daylight itself be a factor?

Other bizarre birthday statistics:

* US teen mothers are more likely to give birth in January than any other month ³ * February babies have a higher likelihood of narcolepsy ⁴ * Pilots are more likely to be born in March ⁴ * People with autumn birthdays have the longest lifespans; spring birthdays have the shortest. A person born in October will outlive a person born in March by an average of 215 days.⁴ * June and July babies consistently have the highest likelihood of short-sightedness⁴ *September babies get the best grades and test scores in school.⁴

Conclusions

I think astrology is malarkey. But is it possible that birth month affects personality? Is my statistical sample of 167 friends simply too small to be meaningful? It is interesting to me that among my very best friends, spring babies are still over-represented, with a distribution mirroring the chart above. Possibly science will begin to formulate explanations for the statistical correlations between birth date and personality, health, and aptitude.

Anthropogenic Fibonacci Sequences

As folks know, the Fibonacci sequence and its corresponding Golden Ratio can be observed throughout nature, from the arrangement of leaves on a stem to the spiraled florets on the head of a sunflower.  But what about man-made Fibonacci sequences?

Here are some Fibonacci sequences I have observed that are created strictly from humans being human beings:

CHANGI AIRPORT, SINGAPORE 6:00 A.M.

Arriving for an early flight, I witness a terminal opening for the morning.  The first security guard enters the security check.  He walks through the metal detectors, passes his bags through the x-ray, and then runs the metal wand over his body.  Then clips on his security badge.

Thus screened, the first security guard performs the same operation on the second security guard.  While the new guard clips on his badge, the first guard screens a third guard.  Now the first two guards screen two more while the third clips on his badge.

Factoring in the pause time while each newly screened guard clips on his badge and turns on equipment, I realize that the rate at which security guards pass each other through the security check is a Fibonacci sequence.

THE KISSING DISEASE

The incubation period for mononucleosis - the time between exposure to the contagion and the appearance of symptoms - is roughly one month.  Once exposed to the virus, a person carries it for life and can theoretically pass it on for several years.

Thus, imagining a population in which (1) the "Kissing Disease" is introduced by a single person and (2) every person kisses exactly one new person each month, the spread of mono throughout the population is a Fibonacci Sequence.

1. MONTH ONE: 1 carrier; 1 incubating
2. MONTH TWO: 2 carriers; 1 incubating
3. MONTH THREE: 3 carriers; 2 incubating
4. MONTH FOUR: 5 carriers; 3 incubating

CHRISTMAS FIBONACCI

I witnessed the following Fibonacci Sequence at a Midnight Mass on Christmas Eve.  In a special ceremony, the minister turned off the church lights and distributed unlit candles to every member of the congregation.

The minister's candle was the only lit candle.  He used it to light the first candle in the first pew.  While that person's candle flame gathered strength, the minister lit a second person's candle.  Now two people could light candles while the third person's flame gathered strength.  Soon there were eight people with lit candles and five who could light other people's candles.  Thus the brightness of the dark room accelerated in accordance with the Fibonacci sequence.