Evidential Cooperation in Large worlds (ECL) refers to the idea that, since the universe may be very large, it likely contains many agents very similar to oneself, and that by taking actions that are good for these agents, we learn that they take actions that are good for us. The idea of ECL was introduced in Oesterheld, 2017 as MSR (Multiverse-wide superationality).
Further Reading
Caspar Oesterheld (2017): Multiverse-wide Cooperation via Correlated Decision Making
Caspar Oesterheld (2017): Multiverse wide Cooperation via Correlated Decision Making. Talk at EA Global London.
Johannes Treutlein (2023): Modeling Evidential Cooperation in Large Worlds
Lukas Finnveden (2023): ECL with AI
Various writings: https://longtermrisk.org/ecl