How many Irish cousins: according to Tim Urban – Part 1 in Series

Tim Urban wrote an entertaining blog post in 2014 on calculating a ballpark number of cousins based on your country’s average birth statistics. His formula breaks down the totals by degree of cousin i.e. 1st/2nd/3rd/4th and outward.

Tim calculates numbers for USA, UK, Canada and a few other countries, but not Ireland – so I figured I’d crunch the figures for green.

Tim’s formula iswhere “n” is the fertility rate and “d” is the degree of cousin.

The 2013 fertility rates from NationMaster report fertility rates of 1.9 for the U.K, 2.01 for Ireland, and 2.06 for the U.S. Here are the totals for the three countries, with Ireland in the middle line.

Figures for 2013

For now, ignore the eye-watering totals at fifth cousin and beyond. I took one look at the predicted total of 4.1 for First Cousins and rechecked my calculation for a mistake. It just looked suspiciously low to me. My reaction wasn’t based solely on my own family. From the life-cycle of weddings, christenings, and funerals, you tend to have a passing familiarity of the family structures of your friends and neighbours.

A moment’s thought reminded me that Ireland’s birth rate has dropped in recent decades from one of the highest in Europe to closer to the average. The nearest published rate I could find for my birth year was 1970’s rate of 3.87 which predicts 22 first cousins using Tim’s formula. Quite the difference! The same publication reported fertility rates of 2.08 for 1989 and about 3.9 for 1950.

Here are the figures for Ireland only for those years:

figures Ireland 1970 and other yearsNow you can shift your eyes right, and look at the totals of more distant cousins. Again, I thought I’d got the calculations wrong, this time because those numbers were so big. Thankfully, John Reid of Anglo-Celtic Canada Connections has taken Tim’s formula and applied across a wide range of fertility rates, so a spot check stopped me from fretting.

I’ve seen other reported projections of numbers of cousins, including one from Ancestry and one from 23andMe. I’ll address them in other blog posts and compare them with Tim’s.

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