Rational egoism is the position that humans always act rationally and to further the actor's self-interest. But ever since Carl Jung psychologists have pretty much been in agreement that humans do not always act rationally.
[Jung] himself insistently stressed the reality and value of the “irrational” and did not avoid a respectful and unprejudiced consideration of “irrational” procedures and events.
Richard Thaler actually won a Nobel Prize for proving that humans do not act rationally even in an economic setting.
He has paid special attention to three psychological factors: the tendency to not behave completely rationally, notions of fairness and reasonableness, and lack of self-control.
How do rational egoists defend their position?
There are a number of modern Libertarians who interpret rational egoism as descriptive, rather than normative. For example, these two groups argue that because government welfare for the unemployed is greater than the amount a person could earn working at a entry level job, unemployed people will act rationally and in their self-interest by choosing to not work.