EA - Sensitive assumptions in longtermist modeling by Owen Murphy

The Nonlinear Library

Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Sensitive assumptions in longtermist modeling, published by Owen Murphy on September 18, 2024 on The Effective Altruism Forum.
{Epistemic Status: Repeating critiques from David Thorstad's excellent papers (link, link) and blog, with some additions of my own. The list is not intended to be representative and/or comprehensive for either critiques or rebuttals. Unattributed graphs are my own and more likely to contain errors.}
I am someone generally sympathetic to philosophical longtermism and total utilitarianism, but like many effective altruists, I have often been skeptical about the relative value of actual longtermism-inspired interventions. Unfortunately, though, for a long time I was unable to express any specific, legible critiques of longtermism other than a semi-incredulous stare.
Luckily, this condition has changed in the last several months since I started reading David Thorstad's excellent blog (and papers) critiquing longtermism.[1] His points cover a wide range of issues, but in this post, I would like to focus on a couple of crucial and plausibly incorrect modeling assumptions Thorstad notes in analyses of existential risk reduction, explain a few more critiques of my own, and cover some relevant counterarguments.
Model assumptions noted by Thorstad
1. Baseline risk (blog post)
When estimating the value of reducing existential risk, one essential - but non-obvious - component is the 'baseline risk', i.e., the total existential risk, including risks from sources not being intervened on.[2]
To understand this, let's start with an equation for the expected life-years E[L] in the future, parameterized by a period existential risk (r), and fill it with respectable values:[3]
Now, to understand the importance of baseline risk, let's start by examining an estimated E[L] under different levels of risk (without considering interventions):
Here we can observe that the expected life-years in the future drops off substantially as the period existential risk (r) increases and that the decline (slope) is greater for smaller period risks than for larger ones. This finding might not seem especially significant, but if we use this same analysis to estimate the value of reducing period existential risk, we find that the value drops off in exactly the same way as baseline risk increases.
Indeed, if we examine the graph above, we can see that differences in baseline risk (0.2% vs. 1.2%) can potentially dominate tenfold (1% vs. 0.1%) differences in absolute period existential risk (r) reduction.
Takeaways from this:
(1) There's less point in saving the world if it's just going to end anyway. Which is to say that pessimism about existential risk (i.e. higher risk) decreases the value of existential risk reduction because the saved future is riskier and therefore less valuable.
(2) Individual existential risks cannot be evaluated in isolation. The value of existential risk reduction in one area (e.g., engineered pathogens) is substantially impacted by all other estimated sources of risk (e.g. asteroids, nuclear war, etc.). It is also potentially affected by any unknown risks, which seems especially concerning.
2. Future Population (blog post)
When calculating the benefits of reduced existential risk, another key parameter choice is the estimate of future population size. In our model above, we used a superficially conservative estimate of 10 billion for the total future population every century.
This might seem like a reasonable baseline given that the current global population is approximately 8 billion, but once we account for current and projected declines in global fertility, this assumption shifts from appearing conservative to appearing optimistic.
United Nations modeling currently projects that global fertility will fall below replacement rate around 2050 and continue d...