From The Failure Of The Failure Of Contraception To The Failure Of The Failure of SSA (And Other Alternatives To Sia)

If you think that your existence gives you a reason to think your parents did not use effective contraception, you should adopt SIA

I’ve written roughly 1 quadrillion articles about the self-indicating assumption (SIA) of anthropics, the view according to which one should, based on the fact that they exist, think that there are more people in existence, all else equal, on the grounds that if there are more people then it’s likelier that any particular person would exist. But I think it’s a pretty important topic. If SIA is right, then one should believe otherwise odd theories according to which every possible person is created. Anthropics is—with the possible exception of infinite ethics—the trickiest and, to use the technical term, most mindfuckey field of philosophy. Anthropics is dark, mysterious, and underexplored, with enormous implications about the fundamental nature of reality yet virtually no one seriously thinks about it.

Here, I’ll continue my practice of writing for SIA. My position is that SIA is the best view and it isn’t particularly close. All the objections to SIA are totally wrong, and SIA is supported by extremely powerful arguments. I feel about SIA roughly like David Deutsch feels about the many worlds interpretation—puzzled that there’s still a debate about it.

Suppose that in 1970 a coin was flipped. If it came up heads then everyone in the world would have had to use extremely effective contraception. If it came up tails, they wouldn’t. Given that you exist, I submit that you should think the coin probably came up tails. But why?

SIA has a natural answer: a world where people use effective contraception predicts the existence of fewer people and thus makes my existence less likely. But what’s the alternative explanation if you reject SIA?

One possibility is that you make the reference class include both past and future people, and so therefore, because most people born before 1970 were born to parents who didn’t use effective contraception, you should think that probably you were born to parents who didn’t use effective contraception. But this fails for two reasons.

First, the case can be modified so that there was no one birthed before 1970. Perhaps the world was made in 1970 and there were no preceding generations that were born in any way.

Second, this seems to have other odd implications. For instance, imagine that in 1970 a time machine was invented and then a coin was flipped. If it came up heads, then everyone went back in time and had babies before 1970, while if it came up tails then they had babies after 1970. One could argue that you should think it came up heads in the same way—for the reference class is mostly people born before 1970. But this is clearly wrong, and I submit it’s wrong for the same reason as it is in the contraception case.

Beyond this, I can’t think of any other solution other than biting the bullet. But biting the bullet is crazy! To see how crazy it is to just accept that in the contraception coinflip case, you should be indifferent between the two hypotheses, note that if you knew that your parents and only your parents flipped a coin and only used effective contraception if the coin came up heads, you should think it came up tails. But this means that the odds you should give to have been birthed by your parents using effective contraception will depend on whether other people also choose to base their contraceptive decisions on a coin flip. But this is nuts! The odds I should give to my parents having used effective birth control when having me obviously has nothing to do with the odds that other people’s parents used birth control to have them.

I can’t see any good way out of the puzzle for the person who rejects SIA. Am I missing something?