The assumption of the rationality of the agents is a consensus on neoclassical economics and other schools, that is often badly translated to real world examples by economists, due to the difficulty of measuring utility.
The dictator game translates this idea quite clearly: game theoristsgame theorists some economists claim humans often act “irrationally” when rejecting options that aremaking offers larger than zero. Their mistake is to assume the utility outcome of the player is equal to the monetary outcome.