What if we all died at forty?

Tyler Cowen forwarded this question from Akka:

If it was certain that when we sleep on our 40^th birthday, we wouldn’t wake up, how different would the world be? Economically? Culturally? Will it be more peaceful? More left leaning?

Many of the commenters try to draw conclusions to this fantasy by going back to the time when human life expectancy was much lower, but I think there’s a huge difference between having a low life expectancy (but also some chance of living to an old age) and the certainty of dying at 40.

To me, the certainty scenario is more like getting married on Saturday (so a bachelor party on Thursday), or having been accepted at a good college (and so blowing off the second semester of your senior year).

I think the overall reproductive rate would be very low, because one of the reasons to have children is historically to have them take care of you in your old age, when you can’t take care of yourself. If old age isn’t going to come, some of the motivation for having children disappears.

Can we draw a better analogy?

I keep trying to use plants (annuals, biannuals) and animals (17 year cicadas) as analogues, but failing because humans have “choices”.

But maybe this is yet another case in which human “free will” isn’t really the defining difference it seems to be at first.

Plants/animals with defined life cycles tend to be very concerned with reproduction. But I think I am looking So, at first, we might think that if human life were limited to 40, we would be extremely concerned with reproduction.

But maybe this is looking at this from the wrong end. If annual/biannual plants had a defined, relatively short lifetime and weren’t very concerned with reproduction, they would be extinct by now or rare. So perhaps with a shorter, defined lifetime humans would have very low reproductive rates and, if not going extinct, be rare: occupy less of the planet’s resources. Might be a good thing for the overall health of the planet — fewer but more hedonic humans.

But, if course, within a few generations it would only be the offspring of those who were extremely concerned with reproduction who would be around; so perhaps we WOULD evolve in a similar manner to biannual plants and become obsessed with passing along our own genes.