Shared birthdays

When researching the paternal side of my family, I was intrigued by my great-grandfather, Frank Healy. He was born to Irish Catholic immigrants who settled in Hudson, Columbia County, New York, and the St. Mary’s baptismal record in Hudson identified his birth date as 14 June 1864. This warmed my heart, because I was born 100 years to the month after him. Somehow knowing this made me feel a bit closer to the man my father was named after, but whom neither of us had ever met.

The statistical probability of sharing a birthdate with anyone, even one's own child, is 1 in 365. These are not bad odds, and certainly significantly better odds than holding a winning Powerball ticket. But what is the likelihood of multiple generations of parents and children, on both the maternal and paternal side, having shared birthdays?

While sharing the same birthday month 100 years apart is a fun coincidence, it’s not particularly impressive. But what is impressive, in a slack-jaw kind of way, is the plethora of shared birthdays that span multiple generations within my family.

On the maternal side of the family, my grandfather was born on June 4, his son Tom was born on June 4, and Tom’s granddaughter was born on June 4. I’m not sure what the statistical probability of this is, but if Tom’s child (not his grandchild) had been born on June 4, the odds would have been 48,000,000 to 1.[1]

What makes these June birthdays even more interesting is that my oldest daughter was born on June 5, ... one day before my birthday.

What makes these June birthdays even more interesting is that my oldest daughter was born on June 5, one day after her great-grandfather/great-uncle/second cousin … and one day before my birthday. So now we have consecutive or shared birthdays for four generations and five people of 4–6 June. I’m not a statistician, but I’ve got to believe the odds of this happening are pretty low.

But wait, it gets even better... On the paternal side, my father was born on his father’s birthday; my sister’s daughter was born on her birthday; my brother’s daughter was born on his birthday: if my daughter had been born one day later all three of us would have children who share our birthdays.

It would certainly appear that my family has a genetic memory that predisposes us to having children on or near our own birthdays. It’s fascinating to wonder what sort of memory could be seared into our DNA that would create such a strong propensity. Could the fact that there are shared birthdays on both the maternal AND paternal sides of our family have had something to do with the way my siblings and I share (or nearly share) birthdays with our children?

What’s the statistical probability of so many shared birthdays within multiple generations of the same family? Whatever it is, it’s probably right up there with the odds of winning the Powerball!

Note

[1] "Three generations of same family born on same day: Incredible odds of 48 MILLION TO ONE," The Express, 15 June 2017.