(Epistemic status: There are some lines of argument that I haven’t even started here, which potentially defeat the thesis advocated here. I don’t go into them because this is already too long or I can’t explain them adequately without derailing the main thesis. Similarly some continuations of chains of argument and counterargument begun here are terminated in the interest of focussing on the lower-order counterarguments. Overall this piece probably overstates my confidence in its thesis. It is quite possible this post will be torn to pieces in the comments—possibly by my own aforementioned elided considerations. That’s good too.)

I

George VI, King of the United Kingdom, had five siblings. That is, the father of current Queen Elizabeth II had as many siblings as on a typical human hand. (This paragraph is true, and is not a trick; in particular, the second sentence of this paragraph really is trying to disambiguate and help convey the fact in question and relate it to prior knowledge, rather than introduce an opening for some sleight of hand so I can laugh at you later, or whatever fear such a suspiciously simple proposition might engender.)

Let it be known.

II

Exactly one of the following stories is true:

Story One

Recently I hopped on Facebook and saw the following post:

“I notice that I am confused about why a nuclear war never occurred. Like, I think (knowing only the very little I know now) that if you had asked me, at the start of the Cold War or something, the probability that it would eventually lead to a nuclear war, I would've said it was moderately likely. So what's up with that?”

The post had 14 likes. In the comments, the most-Liked explanation was:

“anthropically you are considerably more likely to live in a world where there never was a fullscale nuclear war”

That comment had 17 Likes. The second-most-liked comment that offered an explanation had 4 Likes.

Story Two

Recently I hopped on Facebook and saw the following post:

“I notice that I am confused about why George VI only had five siblings. Like, I think (knowing only the very little I know now) that if you had asked me, at his birth or something, the probability that he would eventually have had only five siblings and no more, I would've said it was moderately unlikely. So what's up with that?”

The post had 14 likes. In the comments, the most-Liked explanation was:

“anthropically you are considerably more likely to live in a world where George VI never had more siblings”

That comment had 17 Likes. The second-most-liked comment that offered an explanation had 4 Likes.

~

Which of the stories is true?

III

It wasn’t a trick question; the first story was of course true, with the second false. (If you didn’t think it was a trick but still guessed otherwise, then I want you to tell me how the heck I can get discussions like the second story on my feed.)

Even if one disagrees with it as an explanation, invoking anthropics in the context of a nuclear exchange is a familiar part of the conversational landscape by now, but it most certainly is not in the case of George VI’s lack of further siblings. This smacks to me of a contextual bias; we treat nuclear exchanges as qualitatively different from George VI’s siblings, with respect to anthropics explanations, but I am suspicious of this distinction.***

The obvious defence in face of my suspicion is to point out that the qualitative distinction arises from the nuclear exchange being an ‘observer-killing’ event, whereas George VI having more siblings is not. In this case, ‘observer-killing’ would mean that the event seems incompatible with the observer’s existence (i.e. you wouldn’t exist in a world that had suffered a nuclear exchange since this would significantly affect world history), rather than that the nuclear exchange directly would have killed the would-be observer.

(At this point, I would like to remind you that George VI had five siblings; the same number as you presumably have fingers on each of your hands.)

(Also, note that the Facebook thread was talking about a nuclear exchange, not a nuclear extinction event. Indeed, I have often seen it claimed that there were never actually enough nuclear weapons in the world for extinction to even take place. I am not convinced this aside makes a difference to the justifiability of anthropic explanations, but I note it since it is at least as hard to justify anthropic explanations with a non-extinction nuclear exchange as it is with a potential extinction event.)

However, this defence does not hold when we examine the motivation for anthropic evidence in more detail. The reason a nuclear exchange seems observer-killing is that the history of a world with a nuclear exchange will not feature you being alive in 2014, for example due to economic or civilizational setback caused by the nuclear exchange. More precisely,

Probability(You exist | No nuclear exchange) > Probability(You exist | Nuclear exchange),

whereas

Probability(You exist | George VI had five (or fewer) siblings) = Probability(You exist | George VI had six or more siblings).

The idea of anthropics evidence is motivated by the epistemological principle that one must condition, in a Bayesian update, on all information (evidence) available to oneself, including one’s own existence. Anthropic evidence is unconventional; it arises from taking this epistemological principle seriously to an uncommon extent.

(Fix it in your mind that George VI had five siblings.)

However, taking this principle even further, and bearing in mind that you (now) know that George VI had five siblings, I might ask you now why George VI didn’t have more siblings.

…and since we now know that George VI didn’t have more siblings, we obtain

Probability(You exist [and know that George VI had exactly five siblings] | George VI had more than five siblings) = 0

since the ‘Evidence’ in our likelihood Probability(Evidence | Hypothesis) now includes your knowledge that George VI had exactly five siblings.

Oh look, I just made ‘George VI had five siblings’ an observer-killing event.

Or less sensationally: Insomuch as one can explain the absence of a nuclear exchange by one’s existence, you can now also explain George VI’s exact number of siblings, or any other part of one’s knowledge.

In fact, ‘anthropic effects’ or ‘survivorship bias’ is a fully general ‘explanation’ for why anything is the case rather than some contradictory fact of the matter. This is a strong form of actualism that, when presented in such terms, is generally rejected (or at least, which I would expect to be rejected by some of the people deploying anthropic explanations).

I am skeptical of counterarguments along the lines of ‘but you, the observer, completely fail to exist in the event of a nuclear exchange, whereas only some tiny part of your knowledge is lost if George VI has more siblings’. I am not sure on what grounds one would justify making a point of treating ‘being alive’ (which is a fuzzy human concept) as the relevant point of divergence, and exclude ‘being alive but slightly different’, and the whole line of argument reeks of the kind of anthropocentric epistemology that has lead to (for example) ‘Hrm, looks like the observation of a conscious entity causes wavefunction collapse.’

IV

I realize that the extreme interpretation of ‘condition on all information’ that I have invoked here looks very pedantic, and one suspects that it might be more Clever than wise. After all, even if the argument presented for anthropics evidence is refuted by my considerations, there might be other legitimate processes for harvesting anthropics evidence that do allow ‘survivorship bias’ to be an appropriate explanation for ‘why wasn’t there a nuclear exchange’.

Another angle of offence on anthropics explanations in this context gives me more confidence that anthropic explanations are inappropriate (and thereby more confidence that the particular argument I gave above holds):

Even if one insists that there is a relevant qualitative difference between a nuclear exchange and George VI’s siblings, according to some criteria, one can think of any number of similar questions that do not feel anthropics-appropriate but which fall on the same side of the criteria as a nuclear exchange.

For example, if one thinks the qualitative difference is the possibility of civilizational collapse or economic setback, then one can explain the absence of World War III without reference to history, merely with one’s existence. That might, in fact, seem legitimate, but then we have to explain why the first two World Wars took place.

Similarly if one thinks the qualitative difference is that more people existed for one’s anthropic soul to be epiphenomenally bound to in worlds without a nuclear exchange than worlds with one; again one has to explain why we are in a model where any setbacks have happened.

It seems very ‘mysterious’ that when not talking about Conventionally Anthropic-y Thingies like nuclear exchanges, we try to explain questions of history or geopolitics or fertility using the typical, direct, causal considerations of the relevant fields. But when nuclear weapons come up, we switch into Anthropic Thinking and abandon, say, geopolitical or game theoretic explanations in favour of observer selection effects.

V

Another perspective that views anthropic explanations unfavourably is a pragmatic account that begins by considering what we want when asking a question like the one posed in the Facebook post. It seems to me that the post is basically asking for one of the following:

(A) Coincidence: Evidence that the absence of a nuclear exchange was coincidental, in the sense that there were no identifiable causal factors for the nuclear exchange not happening, or at least that any such factors are not relevant (e.g. not helpful for making predictions about future calamities, not helpful for understanding international relations, etc.)

(B) Faulty model: Evidence that there are relevant factors that the original poster overlooked or weighed incorrectly that, if weighed correctly, would decrease the probability one would give at the start of the Cold War for a subsequent nuclear exchange

In case (A), we want to know so that we can confirm that there is nothing actionable to be learned from the incorrect prediction. In case (B), we want to know so that we can learn from any systematic mistakes that might crop up in our understanding of such situations. An anthropic explanation advances neither of these projects.

Imagine that you are drenched if and only if somebody turned on a sprinkler next to you. We could represent this graphically by ‘Somebody turns on sprinkler’-->’Water is sprinkled towards you’-->’You get soaked’-->’You outragedly ask why you are soaked’, with each of the successive conditional probabilities being one (certainty). It would not be a particularly useful reply to the question (“Why am I soaked?!”) for someone to say, “Well, in every model in which you’re not soaked, you don’t think to ask that question.” It is trivially true that there is perfect correlation between these two events, but this is not the causal information being sought. The observation that you ask the question if and only if you are soaked encodes a correlation, but this is not the aspect of the graph we’re interested in. In fact, this observer selection explanation arises entirely from regular, causal, useful factors downstream.

So long as you believe that your weight of experience is distributed among your instantiations according to some prior on initial conditions and causal laws governing the redistribution of weight of experience thereafter (for example, if one starts with a prior over quantum wavefunctions then allocates weight of experience within each wavefunction according to its evolution and the Born rule), then anthropic explanations are lossy compressions of causal explanations, as with the sprinkler; insomuch as any event is meaningfully explainable, there must be a causal explanation.

(This is equivalent to the deep point that the Doomsday Argument or the Great Filter only rephrase our priors in ways that seem significant, so that updating on them is a mistake of updating twice on evidence—or, more accurately, updating on one’s priors!###)

Now, there might be rules for allocating weight of experience that somehow favour anthropic perspectives. But there seems to be no reason to expect any unbiased set of rules to favour anthropic perspectives specifically, or even to support anthropics rather than penalize it, and my conjunctive probability that there exist such unbiased supporting sets of rules and that the people explaining the absence of a nuclear exchange with those unbiased rules in mind…is not very high.

VI

I’ve given several reasons to be skeptical of anthropic explanations like that quoted early in this post. Even if the considerations here are non-exhaustive or incomplete, they suggest that the matter is more involved and less clear-cut than many seem to believe.

On the other hand, possibly the pattern of agreement in the comments on the Facebook post was about showing off understanding (or even just having heard of) anthropic arguments, or rewarding Cleverness, rather than endorsement of anthropics as the One True Explanation. Maybe I’m not actually much less confident than others about anthropic explanations, and I misread the situation?

***There is at least one unmentioned line of potential redemption that I see for the instinct to treat these cases as qualitatively different, but for the sake of allocating attention to the points raised here, in the interest of not jumping a few rungs up the ladder, and because I have not explored that avenue so well, I shall pass over it. Ideally I shall explain the line in question eventually, but first I would prefer to build up the preceding rungs. If I see anyone raise it anyway, I shall publicly award them many Knave points.

###This also leaves an opening; since we are not Bayesian reasoners, it might be practical to try to construct priors after-the-fact from considerations such as observer selection effects. But then we are in murky enough territory that it is not clear why we should give much weight to observer selection effects compared to the countless other types of evidence we can learn from—or indeed that we should take selection effects into account at all. This point deserves further thought, though.