The question is ambiguous.
You could be asking ‘in what respect is our belief that we are rational decision-makers false?’
And you could also be asking ‘in what respect is the very concept of utter rational in decision-making an incoherent one?’
The answer to the first question is: ‘We are reasonably rational at certain junctures, but we tend to be more emotional than we think we are.”
The answer to the second question is: “If one refused to act except on the condition that one’s motivation was completely rational, one would never be in a constant state of analysis paralysis and would therefore never be able to act, which would defeat the purpose of one’s rationality.”