By the way, other commenters have argued these “known last round” or “known single round” cases are an artefact of current conditions, and wouldn’t have occurred in ancestral conditions, which strikes me as an ad hoc response. It’s not hard to see such interactions happening in an ancestral context too, such as one-off trades with a nomadic clan, passing through. We probably would trade, rather than just steal from the nomads (and risk them staying and fighting, which is strictly irrational from their point of view, but rather likely to happen). Or consider finding a tribal colleague alone in a desert with a very-lethal-looking wound (missing legs, blood everywhere) crying in pain and asking for some water. Very safe to walk away in that case, since very high chance that no-one would ever know. But we wouldn’t do it.
The point isn’t that one-shot PDs never arose, or that it was always rational to cooperate. The point is that one-shot PDs were rare, compared to other interactions, and it was often rational to cooperate (because our ancestors evolved adaptations and developed a social structure which turned potential one-shot PDs into something else). And since the mechanisms that influence human behavior (natural selection, emotions, heuristics, reinforcement learning, etc.) don’t have perfect fine-grained control which allows them to optimize every single individual decision, we often wind up with cooperation even in the cases that are true one-shot PDs.
The point isn’t that one-shot PDs never arose, or that it was always rational to cooperate. The point is that one-shot PDs were rare, compared to other interactions, and it was often rational to cooperate (because our ancestors evolved adaptations and developed a social structure which turned potential one-shot PDs into something else). And since the mechanisms that influence human behavior (natural selection, emotions, heuristics, reinforcement learning, etc.) don’t have perfect fine-grained control which allows them to optimize every single individual decision, we often wind up with cooperation even in the cases that are true one-shot PDs.