An accountant learns the truth about game theory

There was an expert accountant who was well versed in game theory. 

He heard that his intelligent niece, who was five years old, always took a 50p piece, when a choice between a 50p piece and a pound coin was offered to her. 

He went to see his niece and offered her just such a choice. She took the 50p and said "Thank you Uncle". 

The accountant tried to explain to his niece "You must understand, a pound coin is twice as valuable as a 50p piece, so you should always choose the pound coin." 

The niece replied "Uncle, but then people will not offer me any money."