Thanks to Gavin, Vinicius, Fernando and Bruno for comments on early drafts.
The Cryonics reductio: If “pure time preference” is rational (i.e., if an experience – e.g., eating a cupcake - is more valuable now than in the future), then cryonics (i.e., freezing your brain to wake up in the future where lifespans are way longer) is irrational: when you wake up (say, in 100y), your future longer life will be way less valuable than in the past. Similarly: if a person is frozen and wakes up in the future (like the guy in Futurama), one will suffer a great harm.
(Seriously, this is my main point; is this reasoning persuasive? Would anyone say "yeah, cryonics is nonsense because my life does not have relevant value in 2300?")
Maybe I should add: I'm not advocating for cryonics. As far as I know, there might be many objections against it; but I don't think the fact that it would "take me to the future" is one of them - quite the opposite, it's one of the best arguments for it, since the present sucks.
Maybe I should add_2: This is not about discount rates per se, but about time preference / temporal neutrality. I don't want to turn this into a long argument about discount rates (this was supposed to be a shortform post, and then the caveats ran out of control), as I believe others have done a better job than I could do here (and here, and here, here, etc. - I'd say social discount rates, or SDR, is one of those subjects where, despite being ignored by the general public, there's a long high-quality literature no one can expect to review in one lifetime). In summary, a “pure time preference” rate would be just one of the parameters for computing your discount rate (with something like a Ramsey formula) - besides the marginal elasticy of utility and the risk of death / extinction, which no one denies are valid parameters .
I googled “pure time preference” + “cryonics” and found no argument / example like the one above . Well, it is not exactly a new argument. The overall rationale of this objection to pure time preference has appeared before in the literature in the next paragraph; and I vaguely remember cryonics adovates suggesting that one of the pros of cryonics would be to make you more interested in the long-term future – as there is a relevant chance that your existence will occur in it. But I think that framing this reasoning clearly is important, since in philosophical arguments a lot of weight (too much, maybe) relies on what intuitions an example appeals to. I wouldn’t say it is very “down-to-earth”, though I guess it’s likely more digestible to the lay person as a matter of individual decision-making than the arguments in the literature mentioned below.
(Very) similar (and formally way better) arguments:
- Tyler Cowen’s sci-fi example in Stubborn Attachments (right after ft. 50 in my unpaged draft): if we build a spaceship that travels close to lightspeed, it’s no use to worry about breaks, since the travelers will likely stop only in the distant future (given relativity) – so we might just let them die.
- Wiblin & Ord’s Tutankhamun argument: the Pharaoh’s life was worth billions of lives in the present. Along the same line, Greaves mentions that a consistent and general pure time preference rate implies past experiences are more valuable.
- Sarah’s example (i.e., time preference violates the Pareto principle), according to Cowen and Greaves (section 7.1).
- Philosophers vs economists on discounting (Carl Shulman, 2012)
Maybe I'm repeating myself: even if you don’t think cryonics is quite appealing for you now, it’s unlikely that you think that living in the future would be bad for you in any way. I think this example (or any example that actually makes someone consider the idea of living in the future) highlights that what lurks behind temporal discounting is the fact that we won’t live in it, because we have very limited lifespans: if there’s a chance of dying tomorrow, and no chance of surviving more, say, 100 years (part of it in slow decay), a “reasonable allocation” of your resources and experiences along your existence might include a preference for the present (and possibly hyperbolical discount). And, of course, good old short-sightedness and selfishness.
(Really, that's all.)
P.S.: I imagine someone might object that people who favor Nordhaus against Stern in the debate over (long-term) SDR adopt the so-called the positive or descriptivist approach where, instead of using the Ramsey Formula, we use as an SDR the long-term risk-neutral rate of interest (or, in a similar vein, credit markets interest rates). This way, they totally bypass the (normative) debate over "pure time preference." However, I believe there's a catch here: market rates reflect our individual economic decisions on investment and consumption, and thus do not reflect our judgments on the value of the far future - and that is expected, since we have limited lifespans (as my explanation of the cryonics example above emphasized) and ability to process information. And these individual judgments express some sort of pure time preference (or, as I claim, a preference for times you live in). But why should judgments about the value of the future incorporate these biases, since societies (and humanity itself) have longer lifespans (and, I'd like to say, more computing power)?
(Damn, I said I wouldn’t argue about SDRs, but so be it…)
I noticed there are two plausible usual defenses of this positive approach:
- efficiency: this rate incorporates the opportunity costs of investing on future projects, and a different SDR could be exploitable through arbitrage (or crowd-out private investment). However, there are two general objections:
- bite the bullet, but keep things apart: Fleurbaey and Zuber (2013) claim this argument mixes subjects that should be separated: matters concerning the SDR (i.e., comparing present and future consumption and welfare) and the profile of possible investments / decisions (that’s what opportunity costs are about); so, according to this rationale, if market returns are so high, we should save for future investment instead of consuming resources now. Patient philanthropists show how to bite this bullet.
- market failure: Intergenerational affairs and global public goods are a paradigmatic case of market failure (even of government failure), so I don’t quite see what the arbitrage / crowd-out arguments are doing here. It’s not like big-tech companies would be investing on the long-term future, like starting-up space exploration, to reap profits in the next decade – instead, they eventually do it because it’s awesome.
b. collective choice: the positive SDR is a result of decentralized individual decisions, instead of a central authority. Ok, this is not that plausible; I’m sort of surprised by this argument, but Nordhaus actually mentions it, and I’ve seen others follow him. I think the other objections above also apply: long-term future is a market failure, partly caused by biases explainable by our short lifespans. Besides that, it is not clear to me why our judgments on the value of the future should, at the social level, be aggregated like prices in financial markets. However, this suggests that these judgments could instead be made through democratic politics… ideally, I’d agree with that, and I’d really like to see more research on how citizens would respond to something like “how to choose our SDR for intergenerational projects…” but, except for the cases cited in this Future Perfect, I'm not very optimistic about it. I think fiscal and monetary policy often pose analogous intertemporal dilemmas, with a track record of outcomes biased towards the short-term.
 Of course, you might say that a cupcake tomorrow (t+1) will have for you now (t) the same utility that a cupcake on t+1+10^6 will have for you on t+10^6; along this reasoning, cryonics could be assessed as rational by your future self. But this does not solve the dilemma: our reference frame for the evaluation is now – if you have pure time preferences, then your cupcake on t+1+10^6 has negligible value now, because your present self has no relevant interest in the experiences that will happen after t+10^6.
In some way, I agree with that: since I don't think personal identity is particularly relevant from a moral point of view (at least from what we could call a Theory of Good), I don't care (much) more about what would happen to that guy than to some other person living after t+10^6. I'm just following Parfit here. I guess that's likely the best objection one could offer against cryonics (well, not against cryonics per se: death is still bad and a waste).
 Are my intuitions wrong here? I always say that a modus tollens for someone might be a modus ponens for others. I mean, someone could just bite the bullet and say "yeah, future-me would be less valuable because they will only exist in 2300." The reasoning in the reductio is still valid. For this person, from an "objective" POV, cryonics would be a waste; even if maybe not from a subjective POV - i.e., for the person who just woke up from cryo-sleep, only seconds have passed.
 “suppose that a particular person – Sarah, say – could live either in this century or the next. Consider two states of affairs that differ over when Sarah lives. Suppose that Sarah’s well-being is slightly better in the state of affairs in which she lives later, while everyone else’s well-being is unchanged. Then according to the Pareto principle, the ‘Sarah lives later’ state of affairs is better. But according to a value function whose rate of pure time preference is positive, this state of affairs may be worse. Thus δ ≠ 0 is inconsistent with the Pareto principle.”
 Again, no originality here: Phil Goetz brought it up in the LW debate about cryonics 11y ago. It’s another idea that often seems evident for LW and EA people, but it should be explicitly stated in order to dissolve some of the intuitions in favor of pure time preference.