Give Emily some credit. If economists generate "models" of human behavior based on the idea that humans will act rationally, then it's splitting hairs to say that their model of human nature is basically rational. That is, unless the economist is explicitly trying to model something that is, for some reason, deliberately at odds with "in the world".

Economists DO tend (not every single one, but I do see a tendency) to lack curiosity about the cognitive machinery that makes the decisions which drive economic events. That is, the cognitive component is surely defined by some sort of functionality...but according to what parameters does it function? Maximizing utility (does that mean money or happiness)? Risk aversion? Reproductive success? Or is the functionality not even designed in such a way that it maximizes anything in particular under novel conditions? Here's an idea that is foreign to most economists, but that I think quite accurately describes human adaptations (cognitive, economic, or otherwise)...humans are designed to make the decisions that have an evolutionary history of leading to a higher probability of greater reproductive success.

That is why humans act at odds with any account of utility maximization as defined by the rules of even ONE-SHOT, anonymous ultimatum (or even dictator!) games.