Something that seems difficult to me about timeless decision theory is how to reason in a world where most people are not TDTists. In such a world, it seems like the subjunctive dependence between you and others gets weaker and weaker the more TDT influences your decision process.
Suppose you are deciding whether or not to vote. You think through all of the standard arguments you know of: your single vote is virtually guaranteed to not swing the election, so the causal effect of your vote is essentially nothing; the cost to you of voting is tiny, or maybe even positive if you go with a friend and show off your “I Voted” sticker all day; if you vote, you might be able to persuade others to vote as well; etc. At the end of your pondering, you decide that it’s overall not worth it to vote.
Now a TDTist pops up behind your shoulder and says to you: “Look, think about all the other people out there reasoning similarly to you. If you end up not voting as a result of this reasoning, then it’s pretty likely that they’ll all not vote as well. On the other hand, if you do end up voting, then they probably will vote too! So instead of treating your decision as if it only makes the world 1 vote different, you should treat it is if it influences all the votes of those sufficiently similar to you.”
Maybe you instantly find this convincing, and decide to go to the voting booth right away. But the problem is that in taking into account this extra argument, you have radically reduced the set of people whose overall reasoning process is similar to you!
This set was initially everybody that had thought through all the similar arguments and felt similarly to you about them, and most of these people ended up not voting. But as soon as the TDTist popped up and presented their argument, the set of people that were subjunctively dependent upon you shrunk to just those in the initial set that had also heard this argument.
In a world in which only a single person ever had thought about subjunctive dependence, and this person was not going to vote before thinking about it, the evidential effect of not voting is basically zero. Given this, the argument would have no sway on them.
This seems like it would weaken the TDTist’s case that TDTists do better in real world problems. At the same time, it seems actually right. In a case where very few people follow the same reasoning processes as you, your decisions tell you very little about the decisions of others, for the same reason that a highly neuro-atypical person should be hesitant to generalize information about their brain to other people.
Another conclusion of this is that timeless decision theory is most powerful in a community where there is homogeneity of thought and information. Propagation of the idea of timeless decision theory would amplify the coordination-inducing power of the procedure.
I’m not sure if this implies that a TDTist is motivated to spread the idea and homogenize their society, as doing so increases subjunctive dependence and thus enhances their influence. I’d guess that they would only reason this way if they thought themselves to be above average in decision-making, or to have information that others don’t, so that the expected utility of them having increased decision-making ability would outweigh the costs of homogeneity.