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Comment by cacheyourdreams | Hacker News Reader

Aren't life expectancy at birth figures heavily skewed by infant mortality rates. I think this is quite a commonly misunderstood statistic for this reason. So while it's true that in the past a new born baby's chances of becoming a great grandparent were much lower than they would be today, that would mainly be due to the low chances of them ever reaching adulthood and becoming a parent at all, rather than the chances of parents living beyond 47.

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Exactly. While life expectancy from adulthood (say 20 yo) has increased (i.e. UK males have gone from expected average 60y to 80y between 1841 to 2011 [1]), it hasn't increased nearly as much as the life expectancy from birth (i.e. 33% vs 98% increase over that period).

[1]: https://www.ons.gov.uk/peoplepopulationandcommunity/birthsde...

The increase for a 40 year old is still nearly 14 years of extra life, though. That's a big difference.

This is such a rookie mistake to make (by the author). Can’t believe that people who write about this topic still don’t know this in this day and age.

I thought this as well, but I did a little research before responding, and it looks like even though this is broadly true, people still weren't living particularly long before the modern era. For example, in Ancient Greece, a man who lived to 15 would expect to live to 37-41 years, in Rome if a man made it to 20 they could expect to live to 60, in the late medieval if you made it to 25 you could expect to live to ~48 [0]. You still need to make it to 60 to be a great grandparent, assuming you and your kids are having kids at ~15 years of age (edit: and that might be a friendly assumption given how high infant mortality was).

[0] https://en.wikipedia.org/wiki/Life_expectancy

There's some uncertainty about this, and while not properly controlled for obvious reasons, a study of lives of men of renown in 5th and 4th century Greece found a median life expectancy of around 70: https://pubmed.ncbi.nlm.nih.gov/18359748/

Pretty sure the probability of making it to adulthood has never been below 50% excluding war, plague, or famine (which were common, so hard to normalize)

Wikipedia seems to suggest you might be wrong: https://en.wikipedia.org/wiki/Life_expectancy#Variation_over...

For example, for Ancient Rome it says “ while the ~50% reaching age 10 could expect another 40 years of life”.

Hmm I’m surprised.

But if 50% reaches an average age of, say 2, assuming it’s right skewed for those who died before 10; then an average life expectancy of 50 for the remainder means the average life expectancy overall just 26. That squares with the numbers stated I suppose.

Not only. Check the mortality rates. They've gone down for the higher ages. People really live longer past childhood.

I think it was not just infants but young kids as well but yeah.

"Infant mortality" refers to the mortality rate for those under 10 years of age.

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