Re: Objection to the Doomsday Argument?

Robin Hanson (hanson@econ.berkeley.edu)
Fri, 14 Aug 1998 16:08:29 -0700

Hal F. writes:
>Many times I've convinced myself that I have a refutation for the DA,
>only to decide on further consideration that it is more difficult than
>it seems.

I've just spent the last 24 hours reconsidering the Doomsday argument, including Leslie's book, Nick's paper, and the two articles Nick cites that I could find on the web (at: 

http://www.hedweb.com/nickb/140797/doomsday.html
http://xxx.lanl.gov/abs/gr-qc/9407002
http://www.cs.monash.edu.au/~jono/TechReports/analysis2.ps )

The basic argument is that, all else equal, we should not expect to be temporally among the first, say, 5% of creatures "like us." So given exponential growth with a sudden end, that end is near.

I'm currently pretty negative on the doomsday argument.

  1. Even when it is valid, the argument suggests "change," and not necessarily "doom". Creatures like us can no longer exist because their descendants are different, but not destroyed.
  2. Our concept of "like us" can be biased by where we are in time. No fair picking the last big intelligence change, and then defining "like us" to be anything at least as smart as we are. Better to exclude smarter creatures as well as dumber.
  3. It's actually hard to formalize the argument so that the implications are big.

For example, Oliver & Korb show that accepting one's birth rank as a random draw from some total population, with a uniform prior over that total population, an observation of a low rank changes the expected value by less than a factor of ten. Kopf, Krtous, & Page also show that expected values don't change much for a power law prior on total population, unless the power is near minus two.

As another example, Leslie admits in his "shooting room" example that if the probability of "doom" is constant with time independent of population size, the doomsday argument fails. (The expected population is infinite here, so stuff depends on how you slice it.) Finally, Nick shows how the argument gets weakened as one allows for alien creatures "like us" who won't get hurt by our local doom.

I suspect that the more these models get elaborated with our detailed knowledge relevant for forecasting "doom" or change, the less important the doomsday type selection effect would be.

4) Nick explains well how the doomsday argument assumes that one was guarenteed to show up as a creature "like us" at sometime in our universe, regardless of how many such creatures this universe produces. If one instead assumes that the probability of finding oneself in a universe is proportional to population of that universe, the doomsday argument evaporates. This later assumption seems much more reasonable to me, and to Kopf, Krtous, & Page. I buy it even if the different universes are only "possible" rather than coexisting in some way.

In summary, I don't buy a basic assumption of the doomsday argument, and even if I did the implication seems only a moderately higher chance of change, not doom.

Robin Hanson
hanson@econ.berkeley.edu http://hanson.berkeley.edu/ RWJF Health Policy Scholar, Sch. of Public Health 510-643-1884 140 Warren Hall, UC Berkeley, CA 94720-7360 FAX: 510-643-2627