Calculating How Many People Share Your Birthday

Calculating How Many People Share Your Birthday Birthdays are special days to each of us, but every so often we run into someone who shares our birthday. Its not an uncommon experience, but doesnt it make you wonder how many people do share your birthday? What Are the Odds? All things being equal, if your birthday is any day except February 29, the odds of you sharing your birthday with anyone should be approximately 1/365 in any population (0.274%). Since the world population as of this writing is estimated at 7 billion, you should share your birthday with over 19 million people around the world (19,178,082). If you are lucky enough to have been born on February 29, you should share your birthday with 1/1461 (because 366365365365 equals 1461) of the population (0.068%) and so worldwide, you should only share your birthday with a mere 4,791,239 people! Wait- I Should Share My Birthday? However, even though it would seem logical to think that the odds of being born on any given date are one in 365.25, birth rates arent driven by random forces. A lot of things affect when babies are born. In the American tradition, for example, a high percentage of marriages are scheduled for June: and so you might expect at least a small bubble of births to take place in February or March. Further, it seems likely that people conceive children when theyre rested and relaxed. Theres even an old urban legend, debunked by a Duke University study reported on the Snopes.com site, that claimed that nine months after the 1965 New York City blackout, there was a dramatic increase in babies born nine months later. That turns out not to have been true, but it is interesting that people would perceive it to be true. Show Me the Numbers! In 2006, The New York Times published a simple table titled How Common is Your Birthday? The table provided data compiled by Amitabh Chandra of Harvard University, on how often babies are born in the United States on each day from Jan. 1 to Dec. 31. According to Chandras table, including birth records between 1973 and 1999, babies are far more likely to be born in the summers, followed by fall, and then spring and winter. Sept. 16 was the most popular birthday, and the top ten most popular birthdays all fall in September. Not surprisingly, February 29th was the 366th most common day to be born on. Not counting that rare day, the 10 least popular days reported by Chandra to be born on fall on holidays: the 4th of July, late November (26, 27, 28, and 30, near Thanksgiving) and over Christmas (Dec. 24, 25, 26) and New Years (Dec. 29, Jan. 1, 2, and 3). That would seem to suggest that mothers have some say in when babies are born. New Data In 2017, Matt Stiles writing in the Daily Viz reported new data from United States births between for 1994-2014. The data was compiled from U.S. health records by the FiveThirtyEight statistics site- the original report is no longer on FiveThirtyEight. According to that set of data, the least popular birthdays are still around the holidays: July 4th, Thanksgiving, Christmas, and New Years. That data shows that those holidays even beat out February 29th, only the 347th least common day to be born on, which is pretty remarkable, statistically speaking. The most popular days to be born in the United States in this latest set of statistics? The top ten days fall in September: except for one, July 7th. If you were born in September, you were likely conceived over the Christmas holidays. What Does Science Say? Since the 1990s, several scientific studies have shown that there are, in fact, overall seasonal differences in conception rates. Birth rates in the northern hemisphere typically peak between March and May and are at their lowest between October to December. But scientists also point out that those numbers vary widely according to the age, education, and socioeconomic status and marital status of the parents. In addition, the health of a mother affects fertility and conception rates. Environmental stress does too:  conception rates plummet in war-torn regions and during famines. During very hot summers, conception rates are often suppressed. Sources Bobak, Martin, and Arjan Gjonca. The Seasonality of Live Birth Is Strongly Influenced by Socio-Demographic Factors. Human Reproduction 16.7 (2001): 1512-17. Print.Bronson, F. H. Seasonal Variation in Human Reproduction: Environmental Factors. The Quarterly Review of Biology 70.2 (1995): 141-64. Print.Chandra, Amitabh. How Common Is Your Birthday? The New York Times December 19, 2006. Print.Mikkelson, David Blackout Baby Boom. Snopes.com. Published July 31, 2009.  Stiles, Matt. How Common is Your Birthday? This Visualization Might Surprise You. Daily Viz September 17, 2017.