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?
In almost all cases like this, there's going to be good, telling evidence besides your own mere existence. It's only when you build a subtly unrealistically large amount of information into the statement you're conditionalizing on that weirdness arises.
In this case, you have (for example) evidence of the form "I came out of a process of sexual reproduction that contraception can prevent." Even assuming with 100% certainty that I exist as part of my prior, this massively favors the no-contraception scenario. If it turned out that the coin landed heads and everyone used contraception starting in the 70's, my self-certain prior would make it extraordinarily likely that I am some sort of sui-generis swampman-type being, or something otherwise wild. Since in fact I have evidence that there is no such being, this serves as evidence that the coin landed tails.
Now of course the thought experiment can be made to explicitly stipulate that everyone definitively is born through normal sexual reproduction. So you might say I can't appeal to the epistemic possibility that I'm a swampman (or whatever) anymore, because you're asking me to conditionalize it away and thus assign probability zero to it. And then you can laboriously try to stipulate similar things for all the countless other aberrant epistemic possibilities besides swampman that would get me out of the dilemma in the above fashion.
But this is in fact extraordinarily unrealistic! I'm never going to be truly certain about empirical statements like that, and as long as I'm a little bit uncertain, the above reasoning goes through. So I don't mind biting the bullet, if you call it that, because the bullet only fires in a case (perfect knowledge of something I can never have perfect knowledge of) that I expect to be highly pathological anyway.
I think it's fair to treat SIA as a broken clock, which is correct twice a day. The case you've described in the post is where it produces the correct answer but it doesn't mean that this kind of reasoning is generally correct. And it definitely doesn't mean that there is no other way to produce the correct answer in case described in the post, except SIA.
Consider an alternative scenario. In 1970 a coin was flipped. On Tails everyone except your biological parents got a very effective contraception, while your parents can't use any contraception whatsoever, on Heads, you parents become infertile and the rest of the world extremely fertile and can't used any contraception anymore. Heads world has more people, but your in particular existence as a child of your parents isn't possible there. So the fact of your existence makes you very confident in Tails, in spite of what SIA claims.
The real rule has to be about investigation of the causal process that created you and seeing which outcomes made this process more likely. And in different settings your existence can depend on very different processes. Sometimes it's fair to treat yourself as a random person from a group of people, but there is no rule of the universe, that we know of, claiming that it always has to be the case.