How would you act differently even if we assume that your whole life merely exists inside a simulation? You still have to live the life you've been given - it's not like you can break out of the simulation and go take your real life back. Your actions in the simulation still have their usual effect on the life in the simulation. The only case where it matters is if the simulator wants you to behave certain ways and will reward you accordingly(either real-you or by moving you to a nicer simulation), but that's just a different way to talk about religion.
Although I can't think of any way that I personally would behave differently based on a belief that I exist in a simulation, Nick Bostrom suggests a pretty interesting reason why an AI might, in chapter 9 of Superintelligence (in Box 8). Specifically, an AI that assigns a non-zero probability to the belief that it might exist in a simulated universe might choose not to "escape from the box" out of a concern that whoever is running the simulation might shut down the simulation if an AI within the simulation escapes from the box or otherwise exhibi...
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