I'm sure game theorists have studied this problem extensively so I could be very wrong and just showing off my ignorance, but I would think that the rational/ut

I'm sure game theorists have studied this problem extensively so I could be very wrong and just showing off my ignorance, but I would think that the rational/utilitarian thing in a situation where a large number of actors are acting selfishly is to also act selfishly, assuming you have no effective method of influencing those other actors.